The logistic curve as a tool to describe the daily ruminal pH pattern and its link with milk fatty acids.

Daily ruminal pH variation can be summarized by a cumulative logistic curve based on the amount of time below multiple pH points and characterized by 2 parameters (ß(0) and ß(1)). Moreover, rumen pH variation affects the rumen microbiome as well as the biohydrogenation pathways resulting in a modified secretion of milk fatty acids (FA). The aims of this study were to assess the shifts in milk FA due to rumen pH changes and to estimate the relationship between milk FA and the 2 parameters of the logistic curve. The data consisted of milk samples of 2 experiments. In experiment 1, 3 cows were subjected to 5 treatments in which the type and amount of concentrate were changed during 33 d: (1) control diet 1, (2) stepwise replacement of a standard concentrate (CONC) by a CONC rich in rapidly fermentable carbohydrates, (3) increase in the total amount of CONC, (4) treatment with a buffer solution, and (5) control diet 2. A 3×3 Latin square design with 3 cows was used in the second experiment. During the first 14 d of each period, the cows received a control diet with a standard CONC, whereas in the last 7 d the standard CONC was replaced step-by-step by a CONC rich in rapidly fermentable carbohydrates and the amount of CONC was increased. During each period, a different buffer treatment was added to the diet. Milk FA and pH reacted similarly in both experiments: decreasing proportions of iso FA and increasing proportions of odd-chain FA were observed. However, an abrupt change to a 76% CONC diet as for one cow of experiment 1 led to almost a 10-fold increase in C18:1 trans-10 (0.79 vs. 6.75 g/100g of FA). In experiment 2, the stepwise approach of adding CONC and the continuous supplementation of buffer led to minimal increases in C18:1 trans-10 and decreases in rumen pH compared with the diet with standard CONC only. Fatty acid proportions were influenced by the level of rumen pH (ß(1)) or the rumen pH variation (ß(0)), or both. High proportions of C18:1 trans-10 (above 4 g/100g of FA) occurred with low and largely fluctuating pH (low ß(1), low ß(0)), whereas situations with low, stable pH (low ß(1), great ß(0)) did not induce a shift toward the secondary biohydrogenation pathway. C18:1 trans-11 and C18:2 cis-9, trans-11 were only influenced by the pH variation and not by the average pH, whereas iso C14:0 and iso C16:0 FA were only dependent on the average pH and not influenced by diurnal pH variation. Overall, milk FA changes were related to pH changes; however, this relationship is not straightforward and needs further research.

Rotational and continuous grazing of sheep in the Inner Mongolian steppe of China.

The aim of this study was to compare the effects of rotational and continuous grazing on herbage mass (HM), organic matter digestibility (dOM) and intake (OMI) and live weight gain (LWG) of sheep grazing on the inner Mongolian steppe, China at a stocking rate of 4.5 sheep/ha during the growing season. In the years 2005 and 2006, four 2-ha plots were used of which two were divided into four 0.5 ha paddocks each for rotational grazing, where sheep were moved each 10 days to the next paddock. The dOM was estimated from faecal crude protein concentration and OMI by oral administration of titanium dioxide. Herbage mass was similar in both grazing systems and dOM and OMI were higher (p < 0.05) at continuous grazing than at rotational grazing, but LWG did not differ probably because of extra energy expenditure for grazing and walking in a larger area. The dOM and OMI decreased (p < 0.05) with progress of the growing season and differed between years. Since precipitation during the growing season in both years was lower than the 30 years average which was probably the reason that positive effects of non-grazing periods on herbage regrowth and quality at rotational grazing could not occur, further studies are required in years with average precipitations before a final evaluation of these grazing systems can be made. Moreover, it seems necessary to quantify energy expenditure for physical activity of animals in grazing studies.

Effects of perennial ryegrass cultivars on milk yield and nitrogen utilization in grazing dairy cows.

The effects of 4 diploid perennial ryegrass cultivars that differed in water-soluble carbohydrate (WSC) concentrations on milk yield and nitrogen (N) utilization in dairy cows were evaluated in a 2-yr grazing experiment. Twelve lactating dairy cows were assigned to 1 cultivar for a 2-wk period in a 4 x 4 Latin square design with 3 replicates. Each year, the experiment lasted 8 wk. Swards were in a vegetative stage throughout the experiment. Herbage constituents were determined, and DM intake was estimated with the n-alkane technique. Nitrogen utilization was calculated as N excreted in milk divided by N intake, assuming a zero N retention. Two cultivars had consistently higher WSC concentrations and slightly lower neutral detergent fiber concentrations than the other 2 cultivars. The ranking of the cultivars in chemical composition traits in both years was rather consistent. Cows grazing the cultivar with the lowest concentration of WSC had the lowest herbage DM intake, N intake, milk yield, and milk N yield in 2002, but with a similar difference in WSC concentration, no differences among cultivars were found in 2003. In both years, milk urea N concentration was slightly higher for cows grazing the cultivar with the lowest WSC concentration, although it was significant only in 2003. Nitrogen utilization (N milk:N intake, g/g) varied between 0.241 and 0.246 in 2002 and between 0.190 and 0.209 in 2003, and in both years there was no effect of cultivar. At relatively high N concentrations in grass and only small differences among cultivars in neutral detergent fiber concentrations, cultivars with an elevated WSC concentration did not increase N utilization in grazing dairy cows.

Effects of perennial ryegrass cultivars on intake, digestibility, and milk yield in dairy cows.

The effects of 8 diploid perennial ryegrass (Lolium perenne L.) cultivars on dry matter (DM) intake, DM digestibility, and milk yield (MY) of dairy cows were evaluated in the summer of 2000 and 2001. Each summer, herbage was harvested daily and stall-fed to 12 dairy cows during six 2-wk periods. Six cultivars were fed in 3 periods (1, 3, and 5) according to a double 3 x 3 Latin square design. In the other periods (2, 4, and 6), 2 cultivars were fed in a repeated measurement design. Herbage mass and leaf blades in the sward canopy varied among cultivars, but differences were not consistent between years. The largest differences in herbage composition were found in water-soluble carbohydrate content, followed by crude protein content. only small differences were found in the neutral detergent fiber (NDF) content. A higher water-soluble carbohydrate content was found in 2 cultivars in both years, whereas ranking of cultivars in crude protein and NDF content was not consistent with years. Dry matter intake and MY were not affected by cultivar. In both years, DM digestiblity was high (>77%), with very small differences among cultivars in 2000 (<0.5%) and larger differences in 2001 (up to 4%). This was associated with a delayed heading date in 2001, resulting in larger differences in leaf blades and NDF content among cultivars. It may be concluded that the 8 cultivars used in our experiments do not provide grass breeders with encouraging evidence to include selection criteria for an increased DM intake, DM digestibility, and MY in their grass breeding schemes.

Comparison of techniques for estimating herbage intake of grazing dairy cows.

For estimating herbage intake during grazing, the traditional sward cutting technique was compared in grazing experiments in 2002 and 2003 with the recently developed n-alkanes technique and with the net energy method. The first method estimates herbage intake by the difference between the herbage mass before and after grazing and the regrowth between the 2 points in time. The second technique estimates herbage intake by the ratio of a dosed even-chain synthetic n-alkane (C32) and a naturally occurring odd-chain n-alkane (C31 or C33) in the herbage and feces. The third technique calculated the intake from the animal's energy requirements for milk production and maintenance. The sward cutting technique estimated herbage intake with the highest coefficient of variation and had different results in the 2 experimental years. The n-alkanes method yielded less variable results, whereas the net energy method gave the least variable results. In 2002, the estimates of the alkane ratio C32:C33 were best related with estimations of the net energy method. In 2003, the estimates of the alkane ratio C32:C31 were best related. The estimate based on the alkane ratio C32:C33 had a lower coefficient of variation than the one based on the alkane ratio C32:C31. Therefore, the C32:C33 alkane method was considered to be a better direct estimator for herbage intake by grazing lactating dairy cows.

Intake regulation and grazing behavior of dairy cows under continuous stocking.

This experiment aimed at studying the behavioral strategies grazing dairy cows use to satisfy their nutritional needs as the day progresses and the possible role of rumen fill in controlling these behavioral strategies. The day was divided into 3 main periods (0600 to 1200 h, 1200 to 1800 h, and 1800 to 2400 h), where the 3 main grazing bouts (dawn, afternoon, and dusk) of dairy cows usually occur. Four late lactating rumen-cannulated dairy cows were used in a repeated measures design; grazing bout was the within-subjects factor. Cows had access to a 1-ha grass sward under a continuous stocking system. To estimate dry matter intake, bite rate, bite mass (BM), and intake rate at the 3 grazing bouts, cows were rumen-evacuated at 0600, 1200, 1800, and 2330 h, and the jaw recorders were fitted to the cows between these time points. Time spent eating by dairy cows at the dusk grazing bout was much longer than that at the other 2 grazing bouts and composed about 40% of the daily total eating time. Total grazing jaw movement (TGJM) rate was constant during the day at around 75/min. Bite rate, BM, and, hence, intake rate increased, but chewing rate decreased as the day progressed. The increase in BM was mainly due to the increase in dry matter content of the grass at dusk rather than increased bite dimensions. Therefore, it could be concluded that the main behavioral strategies dairy cows use to satisfy their nutritional needs under continuous stocking include manipulating their eating time, biting rate, and chewing rate, with little control over TGJM rate and BM. Dairy cows interrupted the first 2 grazing bouts (dawn and afternoon) long before reaching their maximal rumen capacity, indicating that rumen fill is less likely to play a significant role in signaling the termination of these 2 grazing bouts. However, rumen pool sizes were always maximal at the time when the dusk grazing bout ceased, indicating that rumen fill is more likely to play a major role in signaling the termination of the dusk grazing bout.