1Department of Animal Science,University of Tennessee,2506 River Drive,Brehm Animal Science,Knoxville,TN,37996 USA.
2Division of Animal and Nutritional Sciences,West Virginia University, 333 Evansdale Drive, Agricultural Sciences Building, Morgantown,WV,26505 USA.
Animal. 2017 Dec;11(12):2220-2228. doi: 10.1017/S1751731117001057. Epub 2017 May 18.
Poor utilization of amino acids contributes to losses of milk protein yield in dairy cows exposed to heat stress (HS). Our objective was to test the effect of essential amino acids on milk production in lactating dairy cows exposed to short-term HS conditions. To achieve this objective, 12 multiparous, lactating Holstein cows were assigned to two environments (thermoneutral (THN) or HS) from days 1 to 14 in a split-plot type cross-over design. All cows received 0 g/day of essential amino acids from days 1 to 7 (negative control (NC)) followed by an intravenous infusion of l-methionine (12 g/day), l-lysine (21 g/day), l-leucine (35 g/day), l-isoleucine (15 g/day) and l-valine (15 g/day, methionine, lysine and branched-chain amino acids (ML+BCAA)) from days 8 to 14. The basal diet was composed of ryegrass silage and hay, and a concentrate mix. This diet supplied 44 g of methionine, 125 g of lysine, 167 g of leucine, 98 g of isoleucine and 109 g of valine per day to the small intestine of THN cows. Temperature-humidity index was maintained below 66 for the THN environment, whereas the index was maintained above 68, peaking at 76, for 14 continuous h/day for the HS environment. Heat stress conditioning increased the udder temperature from 37.0°C to 39.6°C. Cows that received the ML+BCAA treatment had greater p.m. rectal and vaginal temperatures (0.50°C and 0.40°C, respectively), and respiration rate (8 breaths/min) compared with those on the NC treatment and exposed to a HS environment. However, neither NC nor ML+BCAA affected rectal or vaginal temperatures and respiration rates in the THN environment. Compared with THN, the HS environment reduced dry matter intake (1.48 kg/day), milk yield (2.82 kg/day) and milk protein yield (0.11 kg/day). However, compared with NC, the ML+BCAA treatment increased milk protein percent by 0.07 points. For the THN environment, the ML+BCAA treatment increased concentrations of milk urea nitrogen. For the HS environment, the ML+BCAA treatment decreased plasma concentrations of arginine, ornithine and citrulline; however, differences were not observed for the THN environment. In summary, HS elicited expected changes in production; however, infusions of ML+BCAA failed to increase milk protein yield. Lower dry matter intake and greater heat load in response to ML+BCAA contributed to the lack of response in milk production in HS cows. The ML+BCAA treatment may have reduced the breakdown of muscle protein in heat-stressed cows.
在遭受热应激（HS）的奶牛中，氨基酸利用率低会导致牛奶蛋白产量下降。我们的目的是测试必需氨基酸对泌乳奶牛短期 HS 条件下产奶量的影响。为了实现这一目标，将 12 头经产泌乳荷斯坦奶牛在分块型交叉设计中从第 1 天到第 14 天分配到两个环境（常温（THN）或 HS）中。所有奶牛从第 1 天到第 7 天（阴性对照（NC））每天接受 0 g 的必需氨基酸，然后从第 8 天到第 14 天每天静脉输注 l-蛋氨酸（12 g/天）、l-赖氨酸（21 g/天）、l-亮氨酸（35 g/天）、l-异亮氨酸（15 g/天）和 l-缬氨酸（15 g/天，蛋氨酸、赖氨酸和支链氨基酸（ML+BCAA））。基础日粮由黑麦草青贮料和干草以及精饲料混合物组成。该日粮每天向 THN 奶牛的小肠提供 44 g 蛋氨酸、125 g 赖氨酸、167 g 亮氨酸、98 g 异亮氨酸和 109 g 缬氨酸。THN 环境的温度-湿度指数保持在 66 以下，而 HS 环境的温度-湿度指数保持在 68 以上，每天连续 14 小时保持在 76 以上。热应激条件使奶牛的乳房温度从 37.0°C 升高到 39.6°C。与 NC 处理组和 HS 环境相比，接受 ML+BCAA 治疗的奶牛的下午直肠温度和阴道温度（分别为 0.50°C 和 0.40°C）和呼吸频率（8 次/分钟）更高。然而，无论是 NC 还是 ML+BCAA 都没有影响 THN 环境中的直肠或阴道温度和呼吸率。与 THN 相比，HS 环境降低了干物质采食量（1.48 kg/天）、产奶量（2.82 kg/天）和牛奶蛋白产量（0.11 kg/天）。然而，与 NC 相比，ML+BCAA 处理使牛奶蛋白百分比增加了 0.07 个百分点。对于 THN 环境，ML+BCAA 处理增加了牛奶尿素氮的浓度。对于 HS 环境，ML+BCAA 处理降低了血浆精氨酸、鸟氨酸和瓜氨酸的浓度；然而，THN 环境没有观察到差异。总之，HS 引起了预期的生产变化；然而，ML+BCAA 的输注未能增加牛奶蛋白产量。对 ML+BCAA 的更高干物质采食量和热负荷导致 HS 奶牛产奶量没有反应。ML+BCAA 处理可能减少了热应激奶牛肌肉蛋白的分解。
