Differential response to stocking rates and feeding by two genotypes of Holstein-Friesian cows in a pasture-based automatic milking system.

The throughput of automatic milking systems (AMS) is likely affected by differential traffic behavior and subsequent effects on the milking frequency and milk production of cows. This study investigated the effect of increasing stocking rate and partial mixed ration (PMR) on the milk production, dry matter intake (DMI), feed conversion efficiency (FCE) and use of AMS by two genotypes of Holstein-Friesian cows in mid-lactation. The study lasted 8 weeks and consisted in a factorial arrangement of two genotypes of dairy cattle, United States Holstein (USH) or New Zealand Friesian (NZF), and two pasture-based feeding treatments, a low stocking rate system (2 cows/ha) fed temperate pasture and concentrate, or a high stocking rate system (HSR; 3 cows/ha) fed same pasture and concentrate plus PMR. A total of 28 cows, 14 USH and 14 NZF, were used for comparisons, with 12 cows, six USH and six NZF, also used for tracking of animal movements. Data were analyzed by repeated measure mixed models for a completely randomized design. No differences (P>0.05) in pre- or post-grazing herbage mass, DMI and FCE were detected in response to increases in stocking rate and PMR feeding in HSR. However, there was a significant (P<0.05) grazing treatment×genotype×week interaction on milk production, explained by differential responses of genotypes to changes in herbage mass over time (P<0.001). A reduction (P<0.01) in hours spent on pasture was detected in response to PMR supplementation in HSR; this reduction was greater (P=0.01) for USH than NZF cows (6 v. 2 h, respectively). Regardless of the grazing treatment, USH cows had greater (P=0.02) milking frequency (2.51 v. 2.26±0.08 milkings/day) and greater (P<0.01) milk yield (27.3 v. 16.0±1.2 kg/day), energy-corrected milk (24.8 v. 16.5±1.0 kg/day), DMI (22.1 v. 16.6±0.8 kg/day) and FCE (1.25 v. 1.01±0.06 kg/kg) than NZF cows. There was also a different distribution of milkings/h between genotypes (P<0.001), with patterns of milkings/h shifting (P<0.001) as a consequence of PMR feeding in HSR. Results confirmed the improved FCE of grazing dairy cows with greater milk production and suggested the potential use of PMR feeding as a tactical decision to managing HSR and milkings/day in AMS farms.

Response profiles of enteric methane emissions and lactational performance during habituation to dietary coconut oil in dairy cows.

Dietary coconut oil (CNO) can reduce dry matter intake (DMI), enteric methane (eCH(4)) emissions, and milk fat yield of lactating cows. The goals of this research were to examine responses to different CNO concentrations during the habituation period (34-d) and to evaluate temporal patterns of DMI, eCH(4), and milk fat yield. Treatment diets contained (dry basis): 0.0% (CNO0), 1.3% (CNO1.3), 2.7% (CNO2.7), 3.3% (CNO3.3), or 4.0% CNO (CNO4). In experiment 1, 12 primi- or small secundiparous cows were housed in individual, environmentally controlled rooms and fed CNO0, CNO1.3, CNO2.7, or CNO4. Measurements included DMI, eCH(4), and milk yield and composition. Due to a precipitous drop in DMI (26%), cows fed CNO4 were replaced with cows fed CNO3.3 following d 10. Dietary CNO of 2.7% or more reduced eCH(4) emissions. Reduction was greater with increased CNO and during the first than the second half of the day. Simultaneously, decline in DMI of cows fed CNO2.7, CNO3.3, or CNO4 was increasingly precipitous with increased CNO concentration. Total-tract neutral detergent fiber (NDF) digestibility during wk 5 was reduced in cows fed CNO2.7 or CNO3.3, which in part explained concomitantly reduced eCH(4)/DMI. In addition, milk fat yield was depressed at an increasing rate in cows fed CNO2.7, CNO3.3, and CNO4. In experiment 2, DMI was measured individually in 12 multiparous cows during habituation to CNO0, CNO1.3, CNO2.7, or CNO3.3 for 21 d before relocation to individual, environmentally controlled rooms. Dietary CNO2.7 or CNO3.3 reduced DMI by d 4 and total-tract NDF digestibility during wk 5. Relocation to individual rooms was associated with a 15% reduction in DMI, which was not affected by treatment. Results showed that 2.7% or more dietary CNO reduced eCH(4) and DMI, caused milk fat depression, and decreased NDF digestibility.

Enteric methane emissions and lactational performance of Holstein cows fed different concentrations of coconut oil.

To determine if dietary medium-chain fatty acids (FA; C(8) to C(14)) may mitigate enteric methane emissions, 24 cows were blocked by body size (n=2) and randomly assigned to 1 sequence of dietary treatments. Diets were fed for 35 d each in 2 consecutive periods. Diets differed in concentrations of coconut oil (CNO; ~75% medium-chain FA): 0.0 (control) or 1.3, 2.7, or 3.3% CNO, dry matter basis. The control diet contained 50% forage (74% from corn silage), 16.5% crude protein (60% from rumen-degradable protein), 34% neutral detergent fiber (NDF; 71% from forage), and 28% starch, dry matter basis. Data and sample collections were from d 29 to 35 in environmentally controlled rooms to measure methane (CH(4)) production. Methane emitted was computed from the difference in concentrations of inlet and outlet air and flux as measured 8 times per day. Control cows emitted 464 g of CH(4)/d, consumed 22.9 kg of DM/d, and produced 34.8 kg of solids-corrected milk/d and 1.3 kg of milk fat/d. Treatment with 1.3, 2.7, or 3.3% dietary CNO reduced CH(4) (449, 291, and 253 g/d, respectively), but concomitantly depressed dry matter intake (21.4, 17.9, and 16.2 kg/d, respectively), solids-corrected milk yield (36.3, 28.4, and 26.8 kg/d, respectively), and milk fat yield (1.4, 0.9, and 0.9 kg/d, respectively). The amount of NDF digested in the total tract decreased with increased dietary CNO concentrations; thus, CH(4) emitted per unit of NDF digested rose from 118 to 128, 153, and 166 g/kg across CNO treatments. Dietary CNO did not significantly affect apparent digestibility of CP but increased apparent starch digestibility from 92 to 95%. No FA C(10) or shorter were detected in feces, and apparent digestibility decreased with increasing FA chain length. Coconut oil concentrations of 2.7 or 3.3% decreased yields of milk FA C(14). The highest milk fat concentration (3.69%; 1.3% CNO) was due to the greatest yields of C(12) to C(16) milk FA. Milk FA concentrations of C(18:2 trans-10,cis-12) were related to increased dietary CNO concentrations and presumably to depressed ruminal NDF digestion. Moderate dietary CNO concentrations (e.g., 1.3%) may benefit lactational performance; however, CNO concentrations greater than or equal to 2.7% depressed dry matter intake, milk yield, milk fat yield, and NDF utilization. If mitigation of enteric CH(4) emissions is due to decreased digestion of dietary NDF, then this will lessen a major advantage of ruminants compared with nonruminants in food-production systems. Thus, CNO has limited use for enteric CH(4) mitigation in lactating dairy cows.

Comparison of effects of dietary coconut oil and animal fat blend on lactational performance of Holstein cows fed a high-starch diet.

Dietary medium-chain fatty acids (C(8:0) through C(12:0)) are researched for their potential to reduce enteric methane emissions and to increase N utilization efficiency in ruminants. We aimed to 1) compare coconut oil (CNO; ~60% medium-chain fatty acids) with a source of long-chain fatty acids (animal fat blend; AFB) on lactational responses in a high-starch diet and 2) determine the effect of different dietary concentrations of CNO on dry matter intake (DMI). In experiment 1, the control diet (CTRL) contained (dry basis) 40% forage (71% corn silage, and alfalfa hay and haylage), 26% NDF, and 35% starch. Isonitrogenous treatment diets contained 5.0% of AFB (5%-AFB), CNO (5%-CNO), or a 1-to-1 mixture of AFB and CNO (5%-AFB-CNO) and 0.8% corn gluten meal in place of corn grain. Thirty-two multiparous dairy cows (201 ± 46 d postpartum; 42.0 ± 5.5 kg/d 3.5% fat-corrected milk yield) were adapted to CTRL, blocked by milk yield, and randomly assigned to 1 of 4 treatment diets for 21 d with samples and data collected from d 15 through 21. Treatment 5%-CNO decreased DMI markedly and precipitously and was discontinued after d 5. In wk 3, 5%-AFB and especially 5%-AFB-CNO lowered total-tract NDF digested vs. CTRL (2.6 vs. 1.8 vs. 3.1 kg/d, respectively), likely because fat treatments reduced DMI and 5%-AFB-CNO impaired total-tract NDF digestibility. Milk fat concentrations were 3.10% (CTRL), 2.51% (5%-AFB), and 1.97% (5%-AFB-CNO) and correlated negatively to concentrations of C(18:2 trans-10,cis-12) in milk fat. Additionally, 5%-AFB and 5%-AFB-CNO tended to lower milk yield and decreased yields of solids-corrected milk and milk protein compared with CTRL. Fat treatments decreased milk lactose concentration, but increased milk citrate concentration. Moreover, cows fed 5%-AFB-CNO produced less solids-corrected milk than did cows fed 5%-AFB. In experiment 2, diets similar to CTRL contained 2.0, 3.0, or 4.0% CNO. Fifteen multiparous cows (219 ± 42 d postpartum; 42.1 ± 7.0 kg milk yield; mean ± SD) were blocked by DMI and randomly assigned to 1 of 3 treatment diets for an 8-d evaluation. Dietary concentration of CNO affected DMI, with the greatest depression at 4.0% CNO. Overall, dietary CNO depressed DMI and NDF digestibility of a high-starch diet compared with AFB. Feeding CNO to lactating cows equal to or greater than 2.5% decreased lactational performance or DMI.

Evaluating estimates of phosphorus maintenance requirement of lactating Holstein cows with different dry matter intakes.

The objective was to evaluate estimates of the inevitable fecal loss component of the P maintenance requirement of lactating Holstein cows consuming differing amounts of a low-P diet. The maintenance requirement for P is the sum of inevitable (e.g., unavoidable) endogenous fecal P plus endogenous urinary P when an animal is fed near its true P requirement (i.e., zero P balance). Urinary excretion of P is normally very low in healthy cattle. Inevitable fecal P is the main part of the total P maintenance requirement; it can be expressed as grams of fecal P/kilogram of dry matter intake (DMI). Twenty-one multiparous lactating Holstein cows (55 to 253 +/- 6 d in milk, range +/- SD; 0 to 171 +/- 64 d pregnant) with a wide range of pretrial milk yields (25.3 to 47.3 +/- 1.23 kg/cow per day) were selected to achieve a range in DMI and assigned to treatment groups of low, medium, and high DMI. To obtain an even greater range in DMI, rations fed to cows in the low and medium treatment groups were restricted to 75 and 50% of their pretrial ad libitum intakes, respectively. Dry matter intakes during the experiment averaged 11.3 (low), 15.3 (medium), and 25.1 (high) kg/cow per d, respectively. All cows were fed the same low-P diet (0.26% P, dry basis) throughout the experiment. Phosphorus balances of cows in all treatments were not different from zero and unaffected by DMI. Average daily total inevitable fecal P excretion was 15.3, 18.2, and 26.3 g/cow for low, medium, and high DMI, respectively. Inevitable fecal P excretion was 1.36, 1.19, and 1.04 g/kg of DMI for low, medium, and high and decreased linearly with increasing DMI. The regression equation to estimate inevitable fecal P excretion across the range of DMI was: (g/d) = [0.85 +/- 0.070 (g/d)] x DMI (kg/d) + [5.30 +/- 1.224 (g/d)]; (R(2) = 0.90). This equation can be used to estimate the inevitable fecal P component of the total P maintenance requirement of lactating Holstein cows.

Periparturient responses of multiparous Holstein cows fed different dietary phosphorus concentrations prepartum.

Our objective was to compare the effects of different prepartum dietary phosphorus concentrations on periparturient metabolism and performance. Forty-two late pregnant multiparous Holstein cows were fed 0.21, 0.31, or 0.44% P (dry basis) for 4 wk before expected calving. After parturition, all cows were fed a common lactation diet (0.40% P). In the prepartum period, cows fed 0.21% P had lower blood serum P concentrations compared with cows fed 0.31 or 0.44% P. However, serum P concentrations of all cows were within the normal range (4 to 8 mg/dL) until the day of calving when average concentrations dropped below 4 mg/dL. From 3 to 14 d postpartum, serum P of cows fed 0.21% P was greater than that of cows fed 0.31 or 0.44% P. No cows presented with or were treated for clinical hypophosphatemia in the periparturient period. Total serum Ca was lower before calving through 2 d postpartum for cows fed 0.44% P compared with those fed 0.21 or 0.31%. Prepartum dietary P treatments did not alter blood osteocalcin, hydroxyproline, and deoxypyridinoline, indicators of bone metabolism, or concentrations of parathyroid hormone or 1,25-dihydroxyvitamin D3. Energy-corrected milk yield and milk composition (first 28 d of lactation) were not affected by prepartum dietary P concentrations. It is concluded that feeding 0.21% P (34 g of P/cow daily) prepartum is adequate for periparturient multiparous Holstein cows with high metabolic demands and genetic potential for milk production. No adverse effects on periparturient health, dry matter intake, or 28-d lactation performance resulted.

Peripartum responses of dairy cows to partial substitution of corn silage with corn grain in diets fed during the late dry period.

We randomly assigned 189 cows in a commercial dairy farm to dietary treatments with supplemental corn grain (SC) or without supplemental corn grain (NC) approximately 3 wk before expected parturition. Diets formulated were similar except that dry ground corn replaced 21% of the corn silage in one diet. Cows fed SC had reduced plasma beta-hydroxybutyrate and tended to have increased plasma insulin concentrations prepartum compared with cows fed NC. Treatment did not affect nonesterified fatty acid concentrations prepartum, any blood variables postpartum, or incidences of health disorders. Effects of treatment on production responses were highly dependent on parity as indicated by parity x treatment x time interactions for milk and protein yields. Primiparous cows fed SC had lower milk protein yield, higher somatic cell count and days open compared with cows fed NC. The SC diet resulted in lower milk yields in early lactation and increased somatic cell count and days open for cows in second parity. However, cows in third parity or greater fed the SC diet yielded more milk and protein in early lactation, and had decreased somatic cell counts and days open. Increasing the corn grain concentration of the diet fed prepartum was advantageous to third and greater parity cows in this experiment, but showed no benefits during lactation for cows in first or second parities.

Effects of altering dietary cation-anion difference on calcium and energy metabolism in peripartum cows.

Our objective was to determine the effects of varying dietary cation-anion differences (DCAD: meq[(Na + K) - (Cl + S)]/100 g of dry matter) in prepartum diets on Ca, energy, and endocrine status prepartum and postpartum. Holstein cows (n = 21) and heifers (n = 34) were fed diets with varying amounts of CaCl2, CaSO4, and MgSO4 to achieve a DCAD of +15 (control), 0, or -15 meq/100 g of dry matter for the last 24 d before expected calving. Dietary Ca concentration was increased (by CaCO3 supplementation) with decreasing DCAD. Plasma ionized Ca concentrations prepartum and at calving in both cows and heifers increased with reduced DCAD in the diet. At calving, plasma ionized Ca concentration was 3.67, 3.85, and 4.35 for cows and 4.44, 4.57, and 4.62 mg/dl for heifers fed diets containing +15, 0, and -15 DCAD, respectively. All heifers had normal concentrations of plasma ionized Ca (>4 mg/dl) at calving. Also at calving, plasma concentrations ofparathyroid hormone and calcitriol were less in cows and heifers fed diets containing reduced DCAD, but the plasma concentration of hydroxyproline was not affected by diet. Prepartum dry matter intake, energy balance, and body weight gains were lower and concentration of liver triglyceride was higher for heifers but not cows fed the -15 DCAD diet. Also, nonesterified fatty acids the last week prepartum were positively correlated with liver triglyceride for heifers but not cows. Feeding of anionic salts plus CaCO3 to reduce DCAD to -15 and increase Ca in prepartum diets prevents hypocalcemia at calving in cows, but decreases prepartum dry matter intake and increases the concentration of liver triglyceride in heifers. That heifers maintained calcium homeostasis at calving regardless of diet but ate less when fed the -15 DCAD diet suggests that they should not be fed anionic salts before calving.

Responses of lactating dairy cows to copper source, supplementation rate, and dietary antagonist (iron).

Forty-eight lactating Holstein cows were fed low-Cu diets with 500 mg of supplemental Fe/kg of dry matter (DM), a Cu antagonist, for a 30-d Cu-depletion period. After depletion, two Fe treatments (0 and 500 mg of Fe/kg of dietary DM) and five Cu treatments (2 x 5 factorial arrangement) were compared over 83 d. The Cu treatments were control (basal diet containing 8 mg of Cu/kg of DM) and either 15 or 30 mg of supplemental Cu/kg of dietary DM from either CuSO4 or Cu-lysine. Feeding 500 mg of supplemental Fe/kg of DM (in addition to basal dietary concentration of 140 mg Fe/kg) depressed liver Cu in the absence of Cu supplementation. Apparent Cu retention, estimated from Cu intake minus fecal Cu, was increased greatly by Cu supplementation immediately after the depletion period but declined to very low net retention by d 45 of the 83-d experiment. There were no differences detected between CuSO4 and Cu-lysine except a tendency over time for Cu-lysine to maintain higher plasma Cu, especially in the absence of the Fe antagonist.

Effects of ammonium chloride and sulfate on acid-base status and calcium metabolism of dry Jersey cows.

Eight nonlactating, nonpregnant Jersey cows were used in a crossover experiment with two 28-d periods. The control diet consisted of corn silage plus a concentrate mix (68:32, DM basis). The treatment diet was the same, except that NH4Cl and (NH4)2SO4 (98 g of each/d per cow) were added to the concentrate. Cows fed the treatment diet had lower blood pH, higher ionized Ca in blood, and more urinary excretion of Ca, titratable acid, and ammonium than cows fed the control diet. For cows fed the treatment diet, ionized Ca in blood was greater after equal amounts of Na2-EDTA were infused to both treatment groups, and treatment cows recovered faster after infusion of Na2-EDTA was stopped than did control cows. The treatment diet induced mild metabolic acidosis and increased the cows' ability to maintain normal blood Ca concentrations; it potentially could reduce incidence of milk fever.

Effects of diet magnesium on acid-base status and calcium metabolism of dry cows fed acidogenic salts.

The objective was to study effects of dietary Mg on acid-base status and Ca metabolism of Holstein cows fed acidogenic diets with relatively high Ca concentrations. Eight nonlactating, nonpregnant Holstein cows were used in a switchback experiment with three 28-d periods. The normal Mg (.2%, dry basis) diet consisted of corn silage plus a concentrate mix supplemented with NH4Cl (126 g/d per cow) and (NH4)2SO4 (126 g/d per cow). The high Mg (.37%, dry basis) diet had MgSO4 substituted for an equivalent amount of S supplied by (NH4)2SO4 in the normal Mg diet. Cation-anion differences of the two diets were -302 (normal Mg) and -289 (high Mg) meq/kg of dietary DM. Compared with cows fed the normal Mg diet, those fed high Mg tended to have higher blood pH and plasma concentrations of total Ca but lower plasma concentrations of P and lower urinary excretion of ammonium and net acid. Cows fed the high Mg diet also tended to increase Mg excretion with a decrease in urinary excretion of Ca. Metabolic responses to intravenous infusion of Na2-EDTA were similar among cows fed either diet. Results indicate that increasing Mg intake of cows fed acidogenic salts was of no advantage with regard to Ca metabolism.

Mineral and water nutrition.

In providing minerals to dairy cattle it is important to distinguish between dietary requirements and feeding recommendations. The requirement is the absolute amount of an element needed to meet the animal's metabolic needs for maintenance, growth, pregnancy, and lactation divided by the coefficient of absorption; this is estimated by the factorial method. Actual estimates of requirements for lactating dairy cattle have been determined for Ca and P. The major difficulties in relying on the requirement estimate are that dry matter intake varies and the true absorption coefficient of the mixture of feeds in the ration generally is unknown. Therefore, feeding recommendations, based on feeding graded concentrations of an element, often offer more applicable information. With the exception of Ca and P, the current feeding recommendations for the other macrominerals, Mg, Na, K, Cl and S, have resulted from feeding trials. With certain environmental and physiologic situations the feeding recommendations may vary. For example, during heat stress the dietary K recommendation for the lactating cow should be higher than in cool weather because of increased sweating and decreased feed intake. Another example may be that the source of supplemental Mg may affect what dietary inclusion rate will yield optimal performance and should be recommended. An important consideration in dairy ration formulation in the future will address the interrelationships of the various macrominerals. There is accumulating evidence that shows that different concentrations of Na, Cl, and K may interrelate and affect lactational performance. Many times the naturally occurring concentrations of one or more of these elements may have to be associated with varying concentrations of the others in order to optimize animal performance and health. Much experimentation likely will examine these interrelationships in the future. Supplementation of trace elements in diets of dairy cattle is common practice. This is because the quantities found in common feedstuffs are relatively low relative to the animal's needs and vary considerably among and within feeds. Supplementation provides a safety margin against potential deficiency. The reader is cautioned to consult other publications regarding potential interactions among the trace elements and their interactions with other nutrients. Potential oversupplementation is a real concern. Maximum safe tolerance concentrations for most trace elements are available. Water is the most important indispensable nutrient. Water requirements are affected by physiologic state and rate of milk yield, body weight, rate of dry matter intake, Na intake, and average minimum environmental temperature. Practically, offering a fresh, abundant supply of easily accessible drinking water at all times is essential.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of supplemental protein on acid-base status and calcium metabolism of nonlactating Jersey cows.

The objective was to study effects of 11, 15, and 19% dietary CP on acid-base status, Ca balance, and metabolic responses to intravenous infusion of disodium EDTA. Dietary protein content was increased by supplementation of hydrolyzed feather meal and distillers dried grains with solubles to a concentrate combined with cottonseed hulls (40:60). Six nonlactating, nonpregnant multiparous Jersey cows (average 6.7 yr old) were used in two balanced 3 x 3 Latin squares with 24-d periods. Increasing supplemental CP decreased blood base excess, urinary titratable base, and net base excretion, but increased urinary ammonium excretion. Calcium excretion and balance were not affected by supplemental CP. Analysis to detect heterogeneity of regression showed that response of plasma EDTA-free Ca to EDTA infusion over time was not different among treatments. Increasing supplemental protein induced mild acidosis but did not affect Ca balance or responses to Ca removal from blood (via EDTA infusion) of non-lactating cows.

Lactational responses to and in vitro ruminal solubility of magnesium oxide or magnesium chelate.

Thirty-six midlactation Holstein cows were used in a randomized incomplete block design to evaluate lactational responses to varying dietary concentrations of Mg supplemented by MgO or Mg chelate. Basal diet was 41:4:55 corn silage:cottonseed hulls:concentrate (.21% total Mg). Magnesium oxide was added to the basal diet to give .32, .37, and .43% total dietary Mg, and Mg chelate was added to provide .23, .25, and .27% Mg, DM basis. Dietary treatments were formulated to supply equal concentrations of bioavailable Mg from either Mg source. Dry matter intake and milk yield were greater by cows fed MgO-supplemented than Mg chelate-supplemented treatments. Milk fat percentages were not affected. Milk protein percentages increased with Mg chelate compared with protein percentages with MgO. Treatments did not affect gross efficiency (4% FCM/DM intake) or body weight change. Lack of response to Mg chelate suggested that either the bioavailability was not as high as assumed or that sufficient total bioavailable Mg was not provided in those treatments. A companion in vitro experiment showed that MgO-supplemented concentrates, with more total supplemental Mg, supplied two to three times more soluble Mg than Mg chelate-supplemented concentrates.

Effects of a new multielement buffer on production, ruminal environment, and blood minerals of lactating dairy cows.

Twelve multiparous Holstein cows were used to evaluate the capacity of a multielement compound consisting mainly of northupite and sylvite to alleviate low milk fat percent. Possible mechanisms of action were assessed. Cows were arranged in a 4 X 4 Latin square design replicated three times. Basal diet was 55% concentrate:45% forage fed ad libitum. Dietary treatments were control, NaHCO3 at 1% of diet DM, and multielement buffer at 1 and 3% of diet DM. Feed intake and milk production were similar for all treatments. Milk fat percentages for the four treatments were 2.97, 3.21, 3.43, and 3.67%, respectively. A shift toward a higher molar percent of ruminal acetate and a lower molar percent of valerate appeared to coincide with changes in milk fat percentage. Milk protein percentage also was increased by supplemental multielement buffer. Ruminal fluid acidity was reduced by NaHCO3 and multielement buffer. Extent of in situ digestion of forage DM and cellulose was improved when cows consumed a buffering agent. Rate of corn silage digestion tended toward improvement. As mineral buffer was consumed, concentrations of Mg and K increased in ruminal fluid and blood. As dietary Na intake increased, ruminal and plasma Cl concentrations were depressed and plasma S and Ca were elevated.

Lactational responses to dietary magnesium, potassium, and sodium during winter in Florida.

Forty-eight midlactation Holstein cows were used to evaluate dietary treatments arranged as a 4 x 2 x 2 factorial: .26, .38, .48, or .60% Mg, .24 or .62% Na, and 1.14 or 1.59% K. Supplemental Mg, K, and Na were supplied by feed-grade magnesium phosphate, potassium bicarbonate, or sodium chloride. All dietary treatments were equal in Ca and P. There were no effects of dietary Na or K on feed intake or milk production. Feed intakes were equal with .26, .38, and .48% Mg but declined 4.9% with .60% Mg. Milk yields responded curvilinearly to dietary Mg. Similarly, 4% FCM yields responded curvilinearly, increasing 7% with .48% Mg compared with .26% Mg then declining with .60% Mg. Milk fat percentages were unaffected by dietary Mg concentration, but milk fat yields responded curvilinearly. Milk protein percentages declined linearly as dietary Mg increased. Plasma Mg concentrations increased linearly from 2.52 to 2.68 mg/dl as dietary Mg increased. Renal fractional excretions of Ca responded curvilinearly as dietary Mg increased and decreased as dietary K increased. Results of this experiment suggested that current recommendations for dietary Mg do not maximize lactational performance. A companion laboratory experiment showed that feed-grade magnesium phosphate had less alkalizing capacity than two MgO sources.

Effects of supplemental potassium and sodium chloride salts on ruminal turnover rates, acid-base and mineral status of lactating dairy cows during heat stress.

Effects of added dietary sodium and potassium chloride salts on ruminal turnover rates, acid-base balance and mineral status of lactating dairy cows experiencing a nycterohemeral cycle of heat stress were examined. Black globe-humidity index in the chambers averaged 94 during the daytime and 68 during the nighttime. Four ruminally cannulated multiparous Holstein cows in mid-lactation were confined in climatic chambers for a single-reversal experiment consisting of two 17-d periods. To the basal diet (50% corn silage: 50% concentrate, which contained .97% potassium, .19% sodium and .20% chloride), 1.25% sodium chloride plus 1.85% potassium chloride were added, making the high mineral treatment (1.93% potassium, 68% sodium and 1.85% chloride). Liquid dilution rates from the rumen were measured by chromium-ethylenediaminetetraacetate disappearance. Turnover rates of solids were determined by appearance of ytterbium in feces. Ruminal contents, arterial blood and urine were collected hourly for 26 h. Grab samples of feces were sampled over 6 d. Dry matter intakes and milk yields were not affected by the diets (averaging 17.8 and 21.1 kg/d, respectively). Cows fed the high mineral diet drank 17% more water (P less than .01). Tests for homogeneity of regression were utilized to compare chromium disappearance and ytterbium appearance data, which were best described by second-order polynomial functions. Increased ruminal chromium disappearance (P less than .01) and decreased total volatile fatty acid concentrations (P less than .01) suggested faster liquid dilution rates with high mineral diet, but turnover rates of solids were not affected. Urinary potassium secretion compensated for the high potassium content of the high mineral diet without an alkalogenic effect on acid-base status. Lower urine pH and higher urine ammonium concentrations during cool hours suggested that the high chloride content of the high mineral diet had an acidogenic effect. The results are interpreted to indicate that high level inclusion of sodium and potassium chloride salts altered digestive, acid-base and mineral status of heat-stressed lactating cows.

Responses of lactating cows to dietary sodium source and quantity and potassium quantity during heat stress.

Effects of heat stress and added dietary sodium bicarbonate (0 or 1.0% of dry matter), sodium chloride (0 or .73% of dry matter), and total dietary potassium (1.3 or 1.8% of dry matter) on acid-base status, production, and mineral metabolism of lactating Holstein cows were evaluated. Design was split-plot with 24 cows in shade or no shade environments; dietary treatments were arranged as a 2 X 2 X 2 factorial within each environment. Basal diet (38% corn silage:62% concentrate) contained .18% sodium; sodium bicarbonate and sodium chloride treatments were in addition. All dietary treatments were equal in chloride content. Cows in no shade exhibited signs of respiratory alkalosis during the hot part of the day. Daily feed intake was lower in no shade than shade but milk yield and percent milk fat were not affected by environment. Sodium bicarbonate addition increased actual and 4% fat-corrected milk yields and percent milk fat. Sodium chloride addition increased actual and 4% fat-corrected milk yield when adjusted for amount of feed intake, and 1.8% dietary potassium increased feed intake and actual milk yield. Increasing total dietary sodium from .18 to .55%, from either supplemental source, enhanced 4% fat-corrected milk production, but combination of sources (.88% total sodium) showed no additional benefit over .55%.

Interaction of protein percent with caloric density and protein source for lactating cows.

Experiment 1 was to test effect of three ratios of energy to protein in complete mixed diets for 36 lactating cows in three, 28-d periods. Energy was varied with cottonseed hulls, pelleted ground corrugated boxes, and a mixture of the two. Crude protein was varied with soybean meal to give energy:crude protein of 5.7, 5.0, and 4.6 for each energy amount. Cottonseed meal was compared with soybean meal in corrugated box diets. Feed intake was much higher with cottonseed hulls, and appreciable feedlot bloat resulted from pelleted ground corrugated box diets. Data adjusted to equal feed intake showed significant effect of energy to crude protein ratio on milk yield and improved digestion of organic matter with soybean meal vs. cottonseed meal. Experiment 2 tested the hypothesis that lactating cows consuming high-protein alfalfa may benefit from supplemental protein. Diets were 50% forage. Six diets were 14 or 18% crude protein in three ratios of alfalfa hay to corn silage (0:100, 50:50, 100:0). Additional corn silage diets were to compare: 14 versus 18% protein from distiller's dried grains with solubles only and with .5 or .9% urea (four diets); two 14% protein diets compared .6% added potassium chloride with or without .5% urea. Thirty-six Holstein cows in early lactation received one of the 12 diets in each of three 28-d periods. Distiller's grains with solubles markedly depressed milk yield (2.2 kg/d) and milk protein (.22%); heat damage of distiller's grains was evident. Protein interacted with alfalfa so gain in milk from 18 versus 14% increased from .55 to 1.36 to 2.66 kg/d as alfalfa changed from 0 to 50 to 100%. Thus, crude protein of alfalfa was not as effective as that from soybean meal in supporting milk yield.

Production and physiological responses of dairy cows to varying dietary potassium during heat stress.

Objectives were to study influences of heat stress and dietary potassium content on production and physiological responses of 8 Jersey and 10 Holstein cows blocked by breed and assigned randomly to no shade or shade environments. Each cow received a different dietary potassium treatment (.66, 1.08, and 1.64% of dry matter) in each of three 30-day periods. Rates of potassium loss from skin were almost five times greater for no shade as for shade cows during the hottest part of the day (1300 to 1500 h). Overall, cows with no shade ate 56% less during the daytime (0800 to 1600 h), 19% more during nighttime (1600 to 0800 h), and 13% less total feed than cows with shade. Interactions of environment and breed with dietary potassium treatment suggest differences in feed intake and milk yield responses to increasing dietary potassium content. Total daily feed intake and milk yield of cows with no shade responded in curvilinear fashion to increasing dietary potassium, whereas responses in shade were small. Largest responses in no shade were as dietary potassium increased from .66 to 1.08%. Milk yield of Holsteins increased with increasing dietary potassium, but yield of Jerseys did not. Combined effects of elevated potassium loss from skin and reduced potassium and dry matter intake during heat stress suggested that lactating dairy cows may benefit from supplemental potassium.