Evaluation of protocols to determine urine output and urinary urea nitrogen excretion in dairy cows with and without dietary salt supplementation.

Urine output and urinary urea-N excretion (UUNe) excretion are critical measures to accurately evaluate N metabolism in lactating dairy cows and environmental concerns related to manure N. The objectives of this study were: (1) to compare estimates of UUNe, urine output, and related variables from 3 pre-established measurement protocols (bladder catheterization, external collection cup, and spot sampling) and from dietary salt supplementation, (2) to study temporal variation in UUNe, urine output, and related variables as affected by measurement protocol, and (3) to evaluate urine specific gravity as a predictor of urine output. Twelve multiparous Holstein cows were used in a split-plot, Latin square design. Cows were randomly assigned to a diet (main plot) containing either 0.7% or 1.6% NaCl (DM basis) and then assigned to a sequence of 3 protocols (subplot) in a balanced 3 × 3 Latin square with 14-d period. For each protocol, measurements were conducted every 4 h for 3 consecutive days. Urine output was determined gravimetrically for bladder catheterization and external collection cup or based on measured cow BW, measured urinary creatinine concentration, and the assumed creatinine excretion of 29 mg/kg BW per day for spot sampling. Urine specific gravity was measured by refractometry. When averaged over a 3-d measurement period and compared with bladder catheterization, spot sampling underestimated urine output (6.8 kg/d; 20%) and UUNe (26 g/d; 13%) but exhibited greater concentration of urinary urea-N (+58 mg/dL; 10%). There were no differences in any measurements determined via bladder catheterization or external cup device protocols, except for urine output that tended to be 3.7 kg/d lower for collection cup compared with bladder catheterization. The 2 gravimetric protocols yielded lower urinary creatinine concentration than spot sampling (64.7 vs. 88.1 mg/dL) and lower creatinine excretion (25.3 mg/kg BW per day) than the value of 29 mg/kg BW per day generally assumed in the spot sampling protocol. Salt supplementation tended to increase urine output (+5.2 kg/d) and decrease urinary urea-N concentration (-93 mg/dL), urinary creatinine concentration (-9.5 mg/dL), milk protein concentration (-0.19 percentage unit) and milk protein yield (-70 g/d). There was greater temporal variation of urine output when measured via the collection cup compared with bladder catheterization in the first 2 d but not the third day of sampling, suggesting that an extended period of adaptation might have improved data quality of the collection cup protocol. The R2 of the linear regression to predict urine output with urine specific gravity was 67%, 73%, and 32% for bladder catheterization, collection cup, and spot sampling, respectively. In this study, spot sampling underestimated both urine output and UUNe, but UUNe determination did not differ between external collection cup and bladder catheterization. However, our data suggested the need to investigate the adaptation protocol, required days of measurements and the conversion of urine mass to urine volume to improve accuracy and precision of urine collection protocols.

Crude protein oscillation in diets adequate and deficient in metabolizable protein: Effects on nutrient digestibility, nitrogen balance, plasma amino acids, and greenhouse gas emissions.

Reducing dietary CP is a well-established means to improve N use efficiency. Yet, few studies have considered if transient restrictions in dietary CP could reduce the environmental footprint of late-lactation cows. We hypothesized that the effects of CP feeding pattern on digestibility and environmental outputs would be amplified at lower dietary CP. We tested CP levels below and near predicted requirements (low protein [LP], 13.8%; high protein [HP], 15.5%) offered in 2 feeding patterns: where diets alternated ±1.8 percentage units CP every 2 d (oscillating [OF]) or remained static. Our study used a 2 × 2 factorial design with 16 mid- to late-lactation Holsteins (mean = 128, SD = 12 DIM), divided into rumen-cannulated (n = 8) and noncannulated subsets (n = 8). For each 28-d experimental period, we recorded feed intake and milk production and took samples of orts (1×/d) and milk (2×/d) for 4 d. For the cannulated subset, we measured and sampled from the total mass of feces and urine production and collected plasma 2×/d across 4 d. For the noncannulated subset, we sampled carbon dioxide and methane emissions 3×/d for 4 d. For each subset, we fit linear mixed models with fixed effects for CP level, CP feeding pattern, the interaction of CP level and CP feeding pattern, period, and a random effect for cow. For plasma and urinary urea-N, we conducted time series analysis. Contrary to our hypothesis, we found no evidence that dietary CP level and CP feeding pattern interacted to influence N balance, nutrient digestibility, or gas emissions. Results showed HP resulted in similar milk N but increased manure N, reducing N use efficiency (milk true protein N/intake N) relative to LP. For OF, urea-N in urine and plasma peaked 46 to 52 h after the first higher-CP phase feeding. Nutrient digestibility and gas emissions were similar across treatments, except CO2 production was greater for OF-HP. In summary, measured variables were minimally affected by dietary CP alternating ±1.8 percentage units every 48 h, even when average dietary CP was fed below predicted requirements (LP). Although our findings suggest that mid- to late-lactation cows are resilient to oscillation in dietary CP, oscillating CP neither reduced the environmental footprint by improving nutrient use efficiencies nor reduced the potential for direct and indirect greenhouse gas emissions.

Dynamic lactation responses to dietary crude protein oscillation in diets adequate and deficient in metabolizable protein in Holstein cows.

Limited research has examined the interaction between dietary crude protein (CP) level and CP feeding pattern. We tested CP level (low protein [LP], 13.8%; high protein [HP], 15.5% CP, dry matter [DM] basis) and CP feeding pattern (OF = oscillating, SF = static) using a 2 × 2 factorial in 16 mid- to late-lactation Holsteins (initially 128 ± 12 d in milk; mean ± SD). Cows ate total mixed rations formulated by exchanging soy hulls and ground corn with solvent soybean meal to keep constant ratios of neutral detergent fiber to starch (1.18:1), rumen-degradable protein to CP (0.61:1), and forage-to-concentrate (1.5:1) in DM. The OF treatments alternated diets every 48 h to vary CP above and below the mean CP level (OF-LP = 13.8% ± 1.8%; OF-HP = 15.5% ± 1.8% CP [DM basis]) whereas diets were constant in SF (SF-LP = 13.8%; SF-HP = 15.5% CP [DM basis]). In four 28-d periods, 8 rumen-cannulated and 8 noncannulated cows formed 2 Latin rectangles. On d 25 to 28 of each period, each cow's feed intake and milk production were recorded, and samples were taken of orts (1×/d) and milk (2×/d). We fit linear mixed models with fixed CP level, CP feeding pattern, and period effects, and a random intercept for cow, computing least squares means and standard errors. Neither CP level, CP feeding pattern, nor the interaction affected DM intake, feed efficiency, or production of milk, fat- and protein-corrected milk (FPCM), fat, true protein, or lactose. Milk urea-N (MUN) yield was lesser for LP. The LP and OF conditions decreased MUN concentration. The CP level tended to interact with CP feeding pattern so that milk protein concentration was greatest for OF-HP. The OF and LP conditions increased the ratio of true protein to MUN yield. Within OF, cosinor mixed models of selected variables showed that cows maintained production of FPCM across dietary changes, but MUN followed a wave-pattern at a 2-d delay relative to dietary changes. A tendency for lesser MUN with OF contradicted prior research and suggested potential differences in urea-N metabolism between OF and SF. Results showed that cows maintained production of economically-relevant components regardless of CP feeding pattern and CP level. Contrary to our hypothesis, the effects of 48-h oscillating CP were mostly consistent across CP levels, suggesting that productivity is resilient to patterned variation in dietary CP over time even when average CP supply is low (13.8% of DM) and despite 48 h restrictions at 12.2% CP.

Production performance of Holstein cows at 4 stages of lactation fed 4 dietary crude protein concentrations.

Dairy cow responses to dietary crude protein (CP) may depend on stage of lactation. The primary objective of this study was to evaluate the effects of 4 concentrations of dietary CP on dry matter intake (DMI), production performance, net energy for lactation (NEL) output in milk, feed efficiency (FE: milk NEL/DMI), and nitrogen use efficiency (100 × milk protein-N/N intake) when fed to cows grouped as early, mid-early, mid-late, and late lactation. Our secondary objective was to determine the range of CP concentration at which production responses were not negatively affected across days in milk (DIM). Multiparous Holstein cows (n = 64) were stratified by DIM [initial average ± standard deviation: 86 ± 14.9 (early), 119 ± 10.0 (mid-early), 167 ± 22.2 (mid-late), and 239 ± 11.1 (late)] and then randomly assigned within DIM group to receive 1 of 4 total mixed rations containing 13.6, 15.2, 16.7, and 18.3% CP (dry matter basis) according to a 4 × 4 factorial arrangement of treatments. Cows were individually fed a covariate diet for 14 d, followed by 56 d of treatment diets. Milk yield and DMI were recorded daily and milk components were analyzed weekly for 2 consecutive days at 3 daily milkings. Data were analyzed using a categorical mixed-effect model to evaluate the effects of CP concentration and DIM using linear, quadratic, and cubic contrasts, and their interactions. Additionally, a mixed-effect cubic regression model was fit with DIM, dietary CP concentration, and their interaction as continuous independent variables. Dietary CP concentration deemed optimal across DIM was determined as the range of CP for which the dependent responses did not differ from the predicted maximum. With advancing stage of lactation, DMI, milk NEL output, and FE decreased linearly (from 30.4 to 28.4 kg/d for DMI, from 33.2 to 23.3 Mcal/d for NEL output, and from 1.09 to 0.82 Mcal milk NEL/kg DMI for FE for early and late lactation cows, respectively). Responses to dietary CP concentration were linear, quadratic, and cubic with the greatest values observed when cows were fed the 16.7% CP diet across DIM (30.8 kg/d, 31.0 Mcal/d, and 1.01 Mcal/kg for DMI, milk NEL output, and FE, respectively). There was an interaction between dietary CP concentration and stage of lactation for DMI, milk NEL output, milk component yield, and FE, which was due to the decline in response to additional CP as lactation progressed. Compared with the 16.7% CP diet, feeding the 18.3% CP diet decreased milk NEL 0.81 and 5.3 Mcal/d for early and late lactation cows, respectively, indicating that feeding a higher CP concentration in late lactation had a negative effect on cow performance. Nitrogen use efficiency declined linearly with increasing CP concentration and DIM. Regression analysis suggested that dietary CP ranging from 16.3 to 17.4% maintained production in early and mid-early lactation. However, dietary CP could be reduced to between 15.7 and 17.1% in late lactation. This research suggested that there are distinct ranges of dietary CP concentrations that maintain cow performance at each stage of lactation.

Substitution of cane molasses for corn grain at two levels of degradable protein. I. Lactating cow performance, nutrition model predictions, and potential basis for butterfat and intake responses.

Little data is presently available on our ability to predict the combined effect of modifying diets with feeds rich in sugars or starch (ST) and rumen-degradable protein (RDP) on the performance of high-producing dairy cows. The objective of this study was to compare responses of 59 lactating Holstein cows to substitution of cane molasses (Mol) for dry corn grain (CG) at 3 levels of Mol and 2 levels of RDP (+RDP or -RDP) in a randomized complete block design with a 3 × 2 factorial arrangement of treatments. Also, lactation responses predicted by 2 nutritional models were compared with observed responses, with Mol composition entered so that nonnutritive materials in Mol were not counted as potentially digestible carbohydrate. We hypothesized that dry matter (DM) intake and milk fat percentage responses would increase with increasing Mol and would potentially be greater with +RDP. For evaluation of the nutritional models, we adopted the null hypothesis that observed and predicted lactation performance would not differ. Cows were individually fed a common diet during a 2-wk covariate period followed by 8 wk on experimental diets. Diets were formulated to be isonitrogenous and provide similar amounts of ST and water-soluble carbohydrates. Experimental diets contained, on a DM basis, 35% corn silage, 20% alfalfa silage, and 16.6% crude protein. The 0, 5.25, and 10.5% Mol diets respectively contained 19.0, 14.5, and 10.0% CG; 28, 25, and 22% ST; and 5.5, 8.5, and 11.5% water-soluble carbohydrates. At 10 wk on study, cows averaged 45.5 kg of energy-corrected milk (ECM). The DM intake (DMI), and yields of milk, milk protein, and ECM, and milk N/intake N declined linearly with increasing Mol. Differences among diets were not detected for milk fat yield and ECM/DMI. No RDP or interaction effects were detected for these measures. That milk production efficiency did not differ across diets suggests that DMI was a primary driver of performance. The similar ECM/DMI and maintenance of milk fat yield would not have been predicted based on Mol and CG composition but may relate to differences in fermentation rates and products. As explanation for these results, we hypothesize that more rapid ruminal evolution of volatile fatty acids post-ingestion with Mol compared with CG may have provided masses of acetate and butyrate in excess of existing energy and synthetic needs that were shunted to milk fat production, and of propionate that depressed intake. The 2001 Dairy National Research Council model and the Cornell Net Carbohydrate and Protein System 6.55 in Nutritional Dynamic System Professional (2021) estimates of metabolizable protein-allowable ECM underestimated actual ECM for +RDP diets by 4.5 and 2.3 kg, respectively, and came close or overestimated for -RDP diets by 0.25 and 5.0 kg, respectively. Prediction discrepancies suggest issues with valuation of dietary protein based on degradability. Improved understanding of factors mediating these results would likely enhance our ability to predict animal responses.

Substitution of molasses for corn grain at two levels of degradable protein. II. Effects on ruminal fermentation, digestion, and nitrogen metabolism.

Our objective was to evaluate cow N metabolism and ruminal measures with diets containing 3 different levels of molasses or finely ground dry corn grain with 2 levels of ruminally degradable protein (RDP). Twelve lactating, ruminally cannulated Holstein cows (parity 2.25 ± 0.62; 185 ± 56 DIM; 41.3 ± 6.3 kg of milk initially) were individually fed in an experiment designed as a split-plot, replicated 3 × 3 Latin square, where each period lasted 28 d. Six diets were formulated according to a 2 × 3 factorial arrangement of treatments, where 2 levels of RDP (+RDP and -RDP) were fed throughout the experiment as the whole plot, and 3 levels of molasses (0, 5.25, or 10.50% of dry matter replacing finely ground dry corn grain) were fed in sequences of the Latin squares. Dry matter intake did not differ by diet, although ash intake increased linearly with increasing molasses. Ruminal pH, organic acid concentration, and ammonia concentration were not affected by diet. Molar percentages of ruminal acetate decreased and butyrate increased linearly with increasing levels of molasses. Ruminal free amino acid concentration was greater for +RDP, whereas branched-chain volatile fatty acids declined linearly with increasing molasses. Rumen content mass, ruminal liquid, and ruminal acetate pool size was greater for -RDP, although ruminal lactate pool size tended to be greater with +RDP. Increased ruminal lactate when increasing molasses with high RDP should be explored further, to optimize microbial efficiency and rumen health. Total-tract apparent dry matter digestibility based on spot sampling was not affected by diet; however, ash digestibility increased linearly with increasing levels of molasses. Calculated urine output was greater for cows fed diets with increasing levels of molasses and for cows fed +RDP. Grams of N distributed to excretion pools were not different across diets, although, as molasses increased, a lower proportion of N intake was excreted in urine. Overall, the results from this experiment showed that dairy cows used dietary carbohydrates differently during ruminal fermentation, with increasing molasses resulting in increased butyrate molar proportions at the expense of acetate. Additionally, RDP tended to modify the effects of carbohydrate fermentation, resulting in a tendency for increasing lactate molar pool size only in diets with greater RDP, although this did not ultimately affect ruminal pH.

Plasma essential amino acid concentration and profile are associated with performance of lactating dairy cows as revealed through meta-analysis and hierarchical clustering.

Our aim was to explore whether changes in plasma essential AA (EAA) concentration ([EAA]p) or profile (defined here as the molar proportion of individual [EAA]p relative to the total [EAA]p) may serve as an indicator of the EAA status of a cow. We undertook a meta-analysis with the objectives to determine if different plasma EAA profiles exist among cows and to explore the association of [EAA]p or the profile of EAA with lactating cow performance and measures of N utilization. We hypothesized the existence of differences in [EAA]p and different plasma EAA profile for cows with greater milk output, feed efficiency, and greater N use efficiency (NUE; milk true protein-N:N intake) compared with cows with lower milk output, feed efficiency, and lower NUE. The data set included 22 feeding trials and 96 dietary treatments. First, a mixed-effect model analysis was used to predict [EAA]p in response to the categorical fixed effect of EAA, continuous fixed effect of National Research Council model-predicted metabolizable protein (MP) supply, continuous fixed effect of body weight, the fixed effect of EAA and MP supply interaction, the fixed effect of EAA and body weight interaction, and the random effect of study. Then, residuals of the model were standardized based on Z-score and clustered using the hierarchical method (Euclidean distance and Ward's minimum variance method) resulting in 2 clusters. Finally, a fixed-effect model was used to evaluate the significance with which clusters were associated with [EAA]p, cow performance, feed efficiency, and NUE. The total concentration of [EAA]p was lower (784 vs. 983 µM) and the concentration of each EAA was on average 22 µM lower for cows in cluster 1 compared with cluster 2 with the smallest and greatest difference found for Met (4 µM) and Val (59 µM), respectively. The percentage difference in [EAA]p was the smallest for Thr (-5.3%) and the greatest for Leu (-37.1%). There was no difference between clusters for Arg, His, and Met molar proportions; however, cows in cluster 1 had a lower molar proportion of Leu and a tendency for lower molar proportion of Val compared with cows in cluster 2. Additionally, cows in cluster 1 had greater molar proportions of Ile, Lys, and Thr and a tendency for greater molar proportion of Phe compared with cows in cluster 2. The fixed-effect model analysis indicated that cows in cluster 1 had higher milk energy output (+3.2 Mcal/d), true protein yield (+87 g/d) and fat yield (+236 g/d), feed efficiency (milk Mcal:dry matter intake; +8% unit), and a tendency for greater MP efficiency (Milk true protein/MP supply; +2.3% unit) than cows in cluster 2. These results suggested greater use of EAA by the mammary gland (as reflected by greater milk protein synthesis) and lower hepatic catabolism of AA (as reflected by a tendency to greater MP efficiency) in cows of cluster 1 compared with cluster 2. Our findings should be evaluated further, including whether the relative molar proportions of plasma EAA might serve as a holistic indicator of the EAA status of cows as related to their productivity, feed efficiency and N utilization.

Effect of 2-hydroxy-4-(methylthio)butanoate (HMTBa) on milk fat, rumen environment and biohydrogenation, and rumen protozoa in lactating cows fed diets with increased risk for milk fat depression.

Biohydrogenation-induced milk fat depression (MFD) is a reduction in milk fat synthesis caused by bioactive fatty acids (FA) produced during altered ruminal microbial metabolism of unsaturated FA. The methionine analog 2-hydroxy-4-(methylthio)butanoate (HMTBa) has been shown to reduce the shift to the alternate biohydrogenation pathway and maintain higher milk fat yield in high-producing cows fed diets lower in fiber and higher in unsaturated FA. The objective of this experiment was to verify the effect of HMTBa on biohydrogenation-induced MFD and investigate associated changes in rumen environment and fermentation. Twenty-two rumen cannulated high-producing Holstein cows [168 ± 66 d in milk; 42 ± 7 kg of milk/d (mean ± standard deviation)] were used in a randomized design performed in 2 blocks (1 = 14 cows, 2 = 8 cows). Treatments were control (corn carrier) and HMTBa (0.1% of diet dry matter). The experiment included a 7-d covariate period followed by 3 phases that fed diets with increasing risk of MFD. The diet during the covariate and low-risk phase (7 d) was 32% neutral detergent fiber with no additional oil. The diet during the moderate-risk phase (17 d) was 29% neutral detergent fiber with 0.75% soybean oil. Soybean oil was increased to 1.5% for the last 4 d. The statistical model included the random effect of block and time course data were analyzed with repeated measures including the random effect of cow and tested the interaction of treatment and time. There was no effect of block or interaction of block and treatment or time. There was no overall effect of treatment or treatment by time interaction for dry matter intake, milk yield, and milk protein concentration and yield. Overall, HMTBa increased milk fat percent (3.2 vs. 3.6%) and yield (1,342 vs. 1,543 g/d) and there was no interaction of treatment and dietary phase. Additionally, HMTBa decreased the concentration of trans-10 18:1 in milk fat and rumen digesta. Average total ruminal concentration of volatile FA across the day and total-tract dry matter and fiber digestibility were not affected by HMTBa, but HMTBa increased average rumen butyrate and decreased propionate concentration and increased total protozoa abundance. Additionally, HMTBa increased the fractional rate of α-linoleic acid clearance from the rumen following a bolus predominantly driven by a difference in the first 30 min. Plasma insulin was decreased by HMTBa. In conclusion, HMTBa prevented the increase in trans FA in milk fat associated with MFD through a mechanism that is independent of total volatile FA concentration, but involves modification of rumen biohydrogenation. Decreased propionate and increased butyrate and ruminal protozoa may also have functional roles in the mechanism.

Relative availability of metabolizable methionine from 2 ruminally protected sources of methionine fed to lactating dairy cattle.

Our objective was to evaluate the lactational responses of dairy cows to methionine provided from 2 ruminally protected sources of methionine activity. Twenty-one Holstein dairy cows [11 primiparous (634 kg of body weight, 140 d in milk) and 10 second-parity (670 kg of body weight, 142 d in milk)] were assigned to a treatment sequence in 4 replicated 5 × 5 Latin squares plus 1 cow, with 14-d periods. Treatments were as follows: control; 7.5 or 15 g/d of a ruminally protected product of 2-hydoxy-4-methylthio-butyric acid (NTP-1401; Novus International Inc., St. Charles, MO); or 7.5 or 15 g/d of a ruminally protected dl-methionine product (Smartamine M; Adisseo, Alpharetta, GA). The diet was predicted to meet metabolizable protein and energy requirements. Diets contained 16.1% crude protein, and the control diet was predicted to be deficient in metabolizable methionine (1.85% of metabolizable protein) but sufficient in lysine (6.8% of metabolizable protein). Feed intake and milk yield were measured on d 11 to 14. Blood was collected on d 14. Dry matter intake, milk yield, energy-corrected milk, milk fat yield and percentage, and efficiencies of milk and energy-corrected milk yield were not affected by treatment. Milk protein percentage and milk protein yield increased linearly with supplementation, without differences between methionine sources or interactions between source and level. Linear regressions of milk protein percentage and milk protein yield against supplement amount within source led to slope ratios (NTP-1401:Smartamine M) of 95% for protein percentage and 84% for protein yield, with no differences between sources for increasing milk protein. Plasma methionine concentrations were increased linearly by methionine supplementation; the increase was greater for Smartamine M than for NTP-1401. Plasma d-methionine was increased only by Smartamine M. Plasma 2-hydoxy-4-methylthio-butyric acid was increased only by NTP-1401. Our data demonstrated that supplementation with these methionine sources can improve milk protein percentage and yield, and the 2 methionine sources did not differ in their effect on lactation performance or milk composition.

Physiological effects of starter-induced ruminal acidosis in calves before, during, and after weaning.

The objectives were to nutritionally induce or blunt ruminal acidosis in young calves and to compare indicators of rumen and systemic health. Ten bull calves (n = 5/diet) were ruminally cannulated at 3 wk of age and received milk replacer and 1 of 2 calf starter diets that were designed to cause (AC; pelleted, 42.7% starch, 15.1% neutral detergent fiber, 57.8% nonfiber carbohydrates) or blunt (BL; texturized, 35.3% starch, 25.3% neutral detergent fiber, 48.1% nonfiber carbohydrates) ruminal acidosis. Mean birth weight was 38.7 ± 1.3 kg. Body weight and calf starter intake were measured weekly. Rumen contents were sampled at -8, -4, 0, 2, 4, 8, 12, and 24 h relative to starter feeding during wk 6, 8, 10, 12, 14, and 16 of age. Blood was collected from the jugular vein during the same weeks for complete blood cell count, blood pH, and partial pressures of oxygen and carbon dioxide. Rate of starter consumption was assessed during wk 16. Marker systems were used to estimate liquid passage and volatile fatty acid absorption rates. Calves were slaughtered at 17 wk, and rumen tissue was collected and assessed for papillae length, width, and degree of tissue degradation. Mean ruminal pH ± standard error was 5.37 ± 0.24 and 5.63 ± 0.24 for AC and BL calves, respectively. Lowest pH values were observed the week after weaning. Total ruminal volatile fatty acid concentrations were 131.5 and 124.8 ± 2.4 mM in AC and BL calves, respectively, and increased with age and time after feeding. Dry matter intake was lower in AC calves at wk 4 and remained lower through wk 16. Rate of starter consumption was also lower in AC calves at wk 16. Body weight also was also lower for AC calves from wk 5 through 16. Blood hemoglobin and hematocrit were lower in AC calves, but other blood characteristics were not different. Rumen volume increased with age and tended to be greater in BL calves. Passage rate and papillae length and width were not different between diets, but AC calves experienced a greater degree of tissue degradation. Ruminal acidosis symptoms in calves appear similar to those in adult cattle, and the etiology of the disease seems to follow similar mechanisms. It is clear from this study that symptoms can be moderated by diet, but further research is needed to determine whether symptoms can be nutritionally prevented or whether calves that experience ruminal acidosis are more susceptible to the disease as adults.

The effects of 2-hydroxy-4-methylthio-butanoic acid supplementation on the rumen microbial population and duodenal flow of microbial nitrogen.

Four multiparous, lactating Holstein cows (average DIM 169.5 ± 20.5 d), fitted with ruminal and duodenal cannulas, were used in a 4 × 4 Latin square with a 2 × 2 factorial arrangement of treatments to investigate the effects of 2-hydroxy-4-methylthio-butanoic acid (HMTBA) when fed with diets differing in metabolizable protein (MP) supply and equal levels of crude protein on milk production and composition, rumen microbial activity, duodenal protein flow, and rumen bacterial community composition in vivo and in vitro. Experimental periods were 28 d in length. Cows were housed in individual tie stalls and were randomly assigned to 4 dietary treatments: low MP or high MP, supplemented with or without 25 g of HMTBA, which was top-dressed once daily at 0930 h. No interactions were observed between HMTBA and level of dietary MP, with the exception of ruminal acetate-to-propionate ratio. Milk yield was not affected by treatment and averaged 23.8 ± 2.06 kg/d. There was a tendency for increased milk protein percent in cows receiving low MP diets, averaging 3.30 ± 0.09% and 3.21 ± 0.09% for low MP and high MP, respectively. The total-tract apparent digestibility of organic matter, neutral detergent fiber, and nitrogen were greater in cows consuming the low MP diet. Rumen pH was lower in cows consuming high MP diets as well as in those consuming HMTBA. Rumen ammonia concentrations tended to be greater in cows consuming HMTBA, and volatile fatty acid concentrations were greater in cows consuming HMTBA. Duodenal dry matter flow, nitrogen flow, and microbial nitrogen flow did not differ between treatments. The bacterial community structure of cows receiving HMTBA was not affected at the phylum level. The relative abundance of bacterial phyla in vivo differed when compared with in vitro conditions for Firmicutes, Bacteroidetes, Proteobacteria, TM7, Tenericutes, Spirochaetes, SR1, and Verrucomicrobia.

Effect of experimental design on responses to 2 concentrations of metabolizable protein in multiparous dairy cows.

The objective of this research was to characterize the implications of changing between diets formulated to be adequate (ADMP) or low (LOMP) in metabolizable protein in a Latin square (LSq) design or of feeding the same diets continuously in a randomized complete block experimental design (RCBD). Fifty-four multiparous early-lactation cows (initial average ± SD; parity 2.8 ± 0.9, 85.8 ± 31 d in milk, 715 ± 63 kg of body weight, 29.1 ± 2.7 kg of dry matter intake/d, and 57.7 ± 5.7 kg of milk yield/d) were blocked by parity and days in milk and were then randomly assigned to experimental design, with 16 cows assigned to LSq and 38 cows assigned to RCBD. Cows within blocks in LSq were randomly assigned to sequence in a 4-sequence, 4-period, 2-treatment LSq balanced for the effects of previous treatment carryover. Cows within blocks in RCBD were randomly assigned to dietary treatment, which was fed over the same four 28-d periods as the cows in LSq. Treatment diets were formulated to be similar in composition with the exception of exchanging an equal quantity of expeller soybean meal from ADMP (16.5% crude protein; 28.4% ash-free, amylase-treated neutral detergent fiber organic matter) for soybean hulls in LOMP (14.6% crude protein; 31.1% ash-free, amylase-treated neutral detergent fiber organic matter). Cows were individually fed treatment diets in a tiestall barn once daily for ad libitum consumption, milked 3 times daily, and administered recombinant bovine somatotropin every 14 d. Milk yield and feed offered and refused were measured daily; BW was recorded on 2 consecutive days each week; milk composition was measured at 6 consecutive milkings each week; and spot samples of feces, urine, and blood were collected during the last week of each period and a covariate period. Experimental designs were analyzed separately using results from wk 4 of each period with mixed effects modeling. Dry matter intake and milk fat yield were not affected by diet in either design, whereas milk and protein yields were greater for cows fed ADMP in both designs. Milk fat and protein percentage responses and milk energy output inferences were different between designs. Milk fat yield and percentage responses were affected by previous treatment carryover in LSq. Metabolic and digestibility inferences were very similar between designs. Under the conditions of this experiment, inferences on N metabolism and the majority of production measurements were not affected by experimental design, with the principal exceptions of milk fat and protein percentage and milk energy output.

Ruminal in situ disappearance and whole-tract digestion of starter feeds in calves before, during, and after weaning.

Ten bull calves (n = 5/diet) were cannulated at 3 wk of age and used in a 2 × 2 factorial design with repeated measures over time to compare rumen and whole-tract degradability of 2 calf starter diets and to describe an in situ technique for estimating ruminal degradability of diets in calves at different ages. Calves received milk replacer and 1 of 2 starter diets through wk 7. Mean birth weight was 38.7 ± 1.3 kg. Weaning occurred in wk 8, and calves received only starter (up to 4,500 g/d) through wk 15. Starter diets were a complete pellet (PEL; 42% starch, 13% neutral detergent fiber, NDF) or texturized feed (TEX; 31% starch, 22% NDF). Portions of each diet were dried and ground through a 2-mm screen, and 1.25 g was inserted into concentrate in situ bags (5 cm × 10 cm, 50-µm porosity). Each calf received duplicate bags of each diet for a total of 8 bags/calf (2 diets × 2 time points). All bags were inserted at the time of starter feeding. Half of the bags were removed at 9 h, and the other half were removed at 24 h. After removal from the rumen, bags were rinsed, dried (55°C), and composited by diet and by calf within week for NDF, nitrogen (N), and starch analyses. This process was repeated over 3 d during wk 5, 7, 9, 11, 13, and 15. Daily starter intake and total fecal excretion were recorded during the same 3-d periods. Diets, refusals, and feces were subsampled, dried, ground, composited by calf by week, and analyzed for NDF, N, and starch content. Apparent digestibility coefficients, total intake, and fecal excretion were calculated and analyzed with a mixed models procedure. Intake and fecal excretion of all measured nutrients increased from wk 5 through wk 15 of age and were greater for calves fed TEX, whereas the proportion of dry matter (DM), N, and starch apparently digested through the total tract decreased from wk 5 to 15 and was greater in calves fed PEL. Ruminal disappearance of DM, N, and starch after 9-h incubations increased linearly with age. Likewise, DM, NDF, and N disappearance after 24-h incubations also increased. Ruminal disappearance of DM and NDF was greater for PEL than for TEX. Ruminal disappearance was estimable for DM, NDF, N, and starch. In addition, changes over time and changes due to rumen environment were clearly demonstrated. Based on these data, there is potential to design specific rations and feed processing methods for calves based on their ability to utilize nutrients.

Effect of 2-hydroxy-4-(methylthio)butanoate (HMTBa) on risk of biohydrogenation-induced milk fat depression.

Diet-induced milk fat depression (MFD) is a multifactorial condition resulting from the interaction of numerous risk factors, including diet fermentability and unsaturated fatty acids concentration, feed additives, and individual cow effects. 2-Hydroxy-4-(methylthio)butanoate (HMTBa) is a methionine analog that has been observed to increase milk fat in some cases, and interactions with MFD risk factors may exist. The objective was to evaluate the effect of HMTBa supplementation on milk fat synthesis in cows with different levels of milk production and fed diets with increasing risk of biohydrogenation-induced MFD. Sixteen high-producing cows (44.1 ± 4.5 kg of milk/d; mean ± SD) and 14 low-producing (31.4 ± 4.3 kg of milk/d) were used in a randomized block design. Treatments were unsupplemented control and HMTBa fed at 0.1% of diet dry matter (25 g/d at 25 kg of dry matter intake). The experiment was 70 d and included a 14-d covariate period followed by 3 phases whereby diets were fed with increasing risk of MFD to determine the interaction of treatment and diet-induced MFD. During the low-risk phase, the base diet was balanced to 33.5% neutral detergent fiber (NDF) and had no exogenous oil (28 d); during the moderate-risk phase, the diet was balanced to 31% NDF and contained 0.75% soybean oil (14 d); and, during the high-risk phase, the diet was balanced to 28.5% NDF and contained 1.5% soybean oil (14 d). An interaction of treatment, production-level, and dietary phase was observed. Low producing cows neither experienced substantial biohydrogenation-induced MFD nor a response in milk fat to HMTBa supplementation. In high-producing cows, HMTBa maintained higher milk fat concentration during the moderate- (2.94 vs. 3.49%) and high-risk (2.38 vs. 3.11%) phases. High-producing cows receiving HMTBa also had greater milk fat yield (0.94 vs. 1.16 kg/d) and lower trans-10 C18:1 (6.11 vs. 1.50) during the high-risk phase. In conclusion, HMTBa increased milk fat in situations with a high risk of biohydrogenation-induced MFD by decreasing absorption of alternate biohydrogenation intermediates.

Effects of replacing soybean meal with canola meal or treated canola meal on ruminal digestion, omasal nutrient flow, and performance in lactating dairy cows.

Extrusion treated canola meal (TCM) was produced in an attempt to increase the rumen-undegraded protein fraction of canola meal (CM). The objective of this study was to evaluate the effects of replacing soybean meal (SBM) with CM or TCM on ruminal digestion, omasal nutrient flow, and performance in lactating dairy cows. To assess performance, 30 multiparous Holstein cows averaging (mean ± SD) 119 ± 23 d in milk and 44 ± 7 kg of milk/d and 15 primiparous cows averaging 121 ± 19 d in milk and 34 ± 6 kg of milk/d were blocked in a randomized complete block design with a 2-wk covariate period and 12-wk experimental period (experiment 1). Dietary ingredients differed only in protein supplements, which were SBM, CM, or TCM. All diets were formulated to contain (dry matter basis) 30% alfalfa silage, 30% corn silage, 4% soy hulls, 2.4% mineral-vitamin premix, and 16% CP. The SBM diet contained 25% high-moisture shelled corn and 8.6% SBM; the canola diets contained 22% high-moisture shelled corn and either 11.2% CM or 11.4% TCM. To assess ruminal digestion and omasal nutrient flow, 6 rumen-cannulated cows were blocked into 2 squares of 3 cows and randomly assigned within blocks to the same 3 dietary treatments as in experiment 1 in a replicated 3 × 3 Latin square design (experiment 2). Data were analyzed using the MIXED procedure of SAS (SAS Institute, Cary, NC). Orthogonal contrasts were used to compare effects of different protein supplements: SBM versus CM + TCM and CM versus TCM. In experiment 1, compared with SBM, apparent total-tract digestibilities of dry matter and nutrients were greater in cows fed both CM diets, and there was a tendency for nutrient digestibilities to be higher in cows fed CM compared with TCM. Diets did not affect milk yield and milk components; however, both canola diets decreased urinary urea N (% of total urinary N), fecal N (% of total N intake), and milk urea N concentration. In experiment 2, compared with SBM, both canola diets increased N intake and tended to increase rumen-degraded protein supply (kg/d) and N truly digested in the rumen (kg/d). Diets did not affect ruminal digestibility, efficiency of microbial protein synthesis, and rumen-undegraded protein flow among diets. Results from this experiment indicate that replacing SBM with CM or TCM in diets of lactating cows improved digestibility and may reduce environmental impact. Moreover, under the conditions of the present study, treating CM by extrusion did not improve CM utilization.

Short communication: Glucose kinetics in dairy heifers limit-fed a low- or high-forage ration at 4 levels of nitrogen intake.

The objective of this research was to evaluate the effect of level of forage and N intake on glucose kinetics in growing dairy heifers. Eight Holstein heifers (beginning at 362 ± 7 kg of body weight and 12.3 ± 0.4 mo of age) were fed 8 rations according to a split-plot, 4 × 4 Latin square design with rations containing either high (75% forage dry matter) or low (25% forage dry matter) levels of forage and 4 levels of N intake (0.94, 1.62, 2.30, 2.96 g of N/kg of metabolic body weight per day). Diets were limit-fed to maintain equal predicted metabolizable energy intake over the four 28-d periods; dietary N was increased through the substitution of high-N ingredients for corn. Blood samples were collected from all heifers at times throughout d 18 to 19 to characterize glucose concentration over the course of a day, and glucose tolerance tests were conducted over the last 8 d of each period (1 heifer/d) at 4 h before feeding. Glucose concentration transiently declined after feeding for all dietary groups, but we found no evidence of a differential response over time that could be attributed to diet. When averaged over a day, glucose concentration was affected by an interaction between level of forage and N intake; however, this response appeared related more to the level of starch in the diet than to the effects of either forage or N intake per se. Early-phase kinetic response of glucose disposal after an intravenous glucose bolus was not affected by dietary treatment, which is consistent with no difference in area under the curve through 30 min. Area under the curve through 120 min tended to linearly decrease with decreasing dietary N intake. This response corresponded to the kinetic analysis, in which heifers consuming higher N intake had an attenuated return to baseline glucose levels. Additionally, heifers consuming lower N intake maintained a period of glucose concentration below baseline before returning to baseline. We concluded that the response to an intravenous glucose bolus differs in dairy heifers fed diets differing in N intake, whereas forage level did not affect this response. This should be considered when formulating rations for low N intake by replacing high-N for high-starch feedstuffs when limit-feeding dairy heifers.

Urinary purine derivatives as a tool to estimate dry matter intake in cattle: A meta-analysis.

The objectives of this study were to investigate the relationship between dry matter intake (DMI) and urinary purine derivative (PD) excretion, to develop equations to predict DMI and to determine the endogenous excretion of PD for beef and dairy cattle using a meta-analytical approach. To develop the models, 62 published studies for both dairy (45 studies) and beef cattle (17 studies) were compiled. Twenty models were tested using DMI (kg/d) and digestible DMI (dDMI, kg/d) as response variables and PD:creatinine (linear term: PD:C, and quadratic term: PD:C2), allantoin:creatinine (linear term: ALLA:C, and quadratic term: ALLA:C2), metabolic body weight (BW0.75, kg), milk yield (MY, kg/d), and their combination as explanatory variables for dairy and beef (except for MY) cattle. The models developed to predict DMI for dairy cattle were validated using an independent data set from 2 research trials carried out at the University of Wisconsin (trial 1: n = 45; trial 2: n = 50). A second set of models was developed to estimate the endogenous PD excretion. In all evaluated models, the effect of PD (either as PD:C or ALLA:C) was significant, supporting our hypothesis that PD are in fact correlated with DMI. Despite the BW-independent relationship between PD and DMI, the inclusion of BW0.75 in the models with PD:C and ALLA:C as predictors slightly decreased the values of root mean square error (RMSE) and Akaike information criterion for the models of DMI. Our models suggest that both DMI and dDMI can be equally well predicted by PD-related variables; however, predicting DMI seems more useful from a practical and experimental standpoint. The inclusion of MY into the dairy models substantially decreased RMSE and Akaike information criterion values, and further increased the precision of the equations. The model including PD:C, BW0.75, and MY presented greater concordance correlation coefficient (0.93 and 0.63 for trials 1 and 2, respectively) and lower RMSE of prediction (1.90 and 3.35 kg/d for trials 1 and 2, respectively) when tested in the validation data set, emerging as a potentially useful estimator of nutrient intake in dairy cows. Endogenous PD excretion was estimated by the intercept of the linear regression between DMI (g/kg of BW0.75) and PD excretion (mmol/kg of BW0.75) for beef (0.404 mmol/kg of BW0.75) and dairy cattle (0.651 mmol/kg of BW0.75). Based on the very close agreement between our results for beef cattle and the literature, the linear regression appears to be an adequate method to estimate endogenous PD excretion.

A 100-Year Review: A century of dairy heifer research.

The years 1917 to 2017 saw many advances in research related to the dairy heifer, and the Journal of Dairy Science currently publishes more than 20 articles per year focused on heifers. In general, nutrition and management changes made in rearing the dairy heifer have been tremendous in the past century. The earliest literature on the growing heifer identified costs of feeding and implications of growth on future productivity as major concepts requiring further study to improve the overall sustainability of the dairy herd. Research into growth rates and standards for body size and stature have been instrumental in developing rearing programs that provide heifers with adequate nutrients to support growth and improve milk production in first lactation. Nutrient requirements, most notably for protein but also for energy, minerals, and vitamins, have been researched extensively. Scientific evaluation of heifer programs also encouraged a dramatic shift toward a lower average age at first calving over the past 30 yr. Calving at 22 to 24 mo best balances the cost of growing heifers with their production and lifetime income potential. Increasingly, farms have become more progressive in adopting management practices based on the physiology and nutrient needs of the heifer while refining key economic strategies to be successful. Research published in the Journal of Dairy Science has an integral role in the progress of dairy heifer programs around the world.

Analysis of production responses to changing crude protein levels in lactating dairy cow diets when evaluated in continuous or change-over experimental designs.

The objective of this study was to evaluate by meta-analysis the effect of experimental design on the production response functions obtained when changing crude protein (CP) levels in lactating dairy cow diets. The final database of studies meeting the selection criteria contained 55 publications with 23 classified as using a continuous (80 diets) and 34 classified as using a change-over (173 diets) experimental design (2 publications reported results from trials using both designs). Mixed model, weighted analysis of covariance was conducted on production measures in response to CP including the continuous covariates year of publication and average days in milk (DIM) and the discrete classification covariate of experimental design. The model was evaluated for curvilinearity in the response to CP, variance heterogeneity, and coincidence in the response between experimental designs, with P<0.05 indicating significant effects and P<0.15 indicating trends. On average, cows in experiments using continuous designs had a lower initial DIM, were on treatment longer, and produced a greater yield of milk and milk protein than cows in experiments using change-over designs. Production responses to increasing CP were increased dry matter intake (DMI), milk and component yield, and feed efficiency (milk Mcal/kg of DMI), and decreased milk N:intake N. Response in milk yield and feed efficiency to increased CP interacted with experimental design where continuous experiments had greater milk yield and feed efficiency response at higher levels of CP. Interaction between CP and design effects on yield of milk protein and fat or milk N:intake N did not approach significance. The database is limited by the lower number of continuous studies and the differences in average DIM between designs; nevertheless, it is concluded that DMI, milk protein yield, milk fat yield, and milk N:intake N responses to CP did not depend on experimental design. Response of milk yield and feed efficiency to CP interacted with experimental design; however, prospective research on the influence of experimental design is required to test these results.

Efficiency and rumen responses in younger and older Holstein heifers limit-fed diets of differing energy density.

The objective of this study was to evaluate the effects of limit feeding diets of different predicted energy density on the efficiency of utilization of feed and nitrogen and rumen responses in younger and older Holstein heifers. Eight rumen-cannulated Holstein heifers (4 heifers beginning at 257 ± 7 d, hereafter "young," and 4 heifers beginning at 610 ± 16 d, hereafter "old") were limit-fed high [HED; 2.64 Mcal/kg of dry matter (DM), 15.31% crude protein (CP)] or low (LED; 2.42 Mcal/kg of DM, 14.15% CP) energy density diets according to a 4-period, split-plot Latin square design with 28-d periods. Diets were limit-fed to provide isonitrogenous and isoenergetic intake on a rumen empty body weight (BW) basis at a level predicted to support approximately 800 g/d of average daily gain. During the last 7d of each period, rumen contents were subsampled over a 24-h period, rumen contents were completely evacuated, and total collection of feces and urine was made over 4d. Intakes of DM and water were greater for heifers fed LED, although, by design, calculated intake of metabolizable energy did not differ between age groups or diets when expressed relative to rumen empty BW. Rumen pH was lower, ammonia (NH3-N) concentration tended to be higher, and volatile fatty acids (VFA) concentration was not different for HED compared with LED and was unaffected by age group. Rumen content mass was greater for heifers fed LED and for old heifers, so when expressing rumen fermentation responses corrected for this difference in pool size, NH3-N pool size was not different between diets and total moles of VFA in the rumen were greater for heifers fed LED, whereas these pool sizes were greater for old heifers. Total-tract digestibility of potentially digestible neutral detergent fiber (NDF) was greater in heifers fed LED and for young heifers, whereas the fractional rate of ruminal passage and digestion of NDF were both greater in heifers fed LED. Digestibility of N was greater for heifers fed HED, but was unaffected by age group, whereas the efficiency of N retention was greater for heifers fed HED and for young heifers. Manure output was reduced in heifers fed HED, but the effect was largest in old heifers. Results confirm previous studies in which young heifers utilize N more efficiently than old heifers, primarily through greater efficiency of postabsorptive metabolism. Results also support the concept of limit feeding HED diets as a potential means to reduce manure excretion and increase nitrogen efficiency.