Milk yield, content, and composition are altered by heat stress. Thirty-two multiparous, lactating Holstein cows [balanced by days in milk (mean ± standard deviation; 184 ± 59); body surface area (5.84 ± 0.34 m2)] were randomly assigned to 1 of 2 dietary treatments [total mixed ration with rumen-protected Met (RPM; Smartamine M; Adisseo Inc., Antony, France; 1.05 g of RPM/kg of dry matter intake) or total mixed ration without RPM (CON)], and within each dietary treatment group cows were randomly assigned to 1 of 2 environmental treatment groups in a split-plot crossover design. The study was divided into 2 periods with 2 phases per period. In phase 1 (9 d), all cows were in thermoneutral conditions and fed ad libitum. In phase 2 (9 d), group 1 (n = 16) was exposed to a heat stress challenge (HSC) using electric heat blankets. Group 2 (n = 16) remained in thermoneutral conditions but was pair-fed (PFTN) to HSC counterparts. After a 21-d washout period, the study was repeated (period 2) and the environmental treatments were inverted relative to treatments from phase 2 of period 1, whereas dietary treatments (RPM or CON) remained the same for each cow. Cows were milked 3× per day and samples were taken on d 1, 5, and 9 of each phase. Vaginal temperature was measured every 10 min, rectal temperature and skin temperature were measured 3× per day, and respiration rate and heart rate were recorded once per day. Cow activity was measured using an accelerometer. Paired difference values were calculated for each cow for each period based on the difference between phase 1 baseline means and phase 2 values for each variable. Cows in HSC had a greater increase in vaginal temperature and respiration rate (+0.2°C and +13.7 breaths/min, respectively) compared with cows in PFTN (0.0°C and -1.6 breaths/min, respectively). Cows in PFTN had a greater decrease in dry matter intake and milk yield (-3.9 and -2.6 kg/d, respectively) compared with cows in HSC (-3.2 and -0.9 kg/d, respectively). Cows in CON had a greater decrease in milk protein concentration for PFTN (-0.10 percentage units) and HSC (-0.06 percentage units) compared with cows in RPM for PFTN (0.00 percentage units) and HSC (-0.02 percentage units). Cows in CON for HSC had greater decrease in milk fat concentration compared with cows in RPM for HSC (-0.10 and +0.12 percentage units, respectively). In conclusion, HSC altered physiological and production parameters of cows. Additionally, RPM helped maintain milk protein and fat concentration during HSC, whereas dry matter intake, milk yield, and feed efficiencies were not affected by RPM.

中文翻译：

---

瘤胃保护蛋氨酸对泌乳荷斯坦奶牛热应激过程中泌乳性能和生理变量的影响。

牛奶的产量，含量和组成会因热应激而改变。三十二头产乳的荷斯坦奶牛[在奶中的天数保持平衡（平均值±标准差； 184±59）；体表面积（5.84±0.34 m2）]随机分配给2种饮食治疗中的一种[瘤胃保护的Met（RPM； Smartamine M； Adisseo Inc.，法国安东尼；法国）； 1.05 g RPM / kg干重物质摄入量）或不使用RPM（CON）的总混合日粮]，并且在每个饮食处理组中，将牛随机分配到两个环境处理组中的一个，进行分块交叉设计。该研究分为两个时期，每个时期分为两个阶段。在第1阶段（9天），所有母牛处于热中性条件并随意喂养。在阶段2（9 d）中，使用电热毯将组1（n = 16）暴露于热应力挑战（HSC）。第2组（n = 16）保持热中性状态，但与HSC对应者配对喂养（PFTN）。经过21天的清除期后，重复进行研究（第2期），相对于第1期第2阶段的处理，环境处理被倒置，而每头母牛的饮食处理（RPM或CON）保持不变。奶牛每天挤奶3次，并在每个阶段的第1、5和9天取样。每隔10分钟测量一次阴道温度，每天测量3次直肠温度和皮肤温度，每天记录一次呼吸频率和心率。使用加速度计测量母牛的活动。根据每个变量的阶段1基线均值和阶段2值之间的差异，为每个时期的每头母牛计算成对的差异值。与PFTN中的母牛相比，HSC中的母牛的阴道温度和呼吸速率增加幅度更大（分别为+ 0.2°C和+13.7次呼吸/分钟）（分别为0.0°C和-1.6次呼吸/分钟）。与HSC奶牛（分别为-3.2和-0.9 kg / d）相比，PFTN奶牛的干物质摄入量和产奶量下降幅度更大（分别为-3.9和-2.6 kg / d）。与PFTN（0.00％单位）和HSC（-0.02％单位）的RPM奶牛相比，CON奶牛的PFTN（-0.10个百分点）和HSC（-0.06个百分点）的牛奶蛋白浓度下降幅度更大。与采用HSC的RPM的母牛相比，采用CON进行HSC的母牛的乳脂浓度降低幅度更大（分别为-0.10和+0.12个百分点单位）。总之，HSC改变了母牛的生理和生产参数。另外，