Effects of rumen-protected methionine on lactation performance and physiological variables during a heat stress challenge in lactating Holstein cows

摘要

Milk yield, content, and composition are altered byheat stress. Thirty-two multiparous, lactating Holsteincows [balanced by days in milk (mean ± standard deviation;184 ± 59); body surface area (5.84 ± 0.34 m~2)]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 gof RPM/kg of dry matter intake) or total mixed rationwithout RPM (CON)], and within each dietarytreatment group cows were randomly assigned to 1 of2 environmental treatment groups in a split-plot crossoverdesign. The study was divided into 2 periods with2 phases per period. In phase 1 (9 d), all cows were inthermoneutral conditions and fed ad libitum. In phase2 (9 d), group 1 (n = 16) was exposed to a heat stresschallenge (HSC) using electric heat blankets. Group 2(n = 16) remained in thermoneutral conditions but waspair-fed (PFTN) to HSC counterparts. After a 21-dwashout period, the study was repeated (period 2) andthe environmental treatments were inverted relative totreatments from phase 2 of period 1, whereas dietarytreatments (RPM or CON) remained the same for eachcow. Cows were milked 3× per day and samples weretaken on d 1, 5, and 9 of each phase. Vaginal temperaturewas measured every 10 min, rectal temperatureand skin temperature were measured 3× per day, andrespiration rate and heart rate were recorded once perday. Cow activity was measured using an accelerometer.Paired difference values were calculated for each cowfor each period based on the difference between phase1 baseline means and phase 2 values for each variable.Cows in HSC had a greater increase in vaginal temperatureand respiration rate (+0.2°C and +13.7 breaths/min, respectively) compared with cows in PFTN (0.0°Cand −1.6 breaths/min, respectively). Cows in PFTNhad a greater decrease in dry matter intake and milkyield (−3.9 and −2.6 kg/d, respectively) compared withcows in HSC (−3.2 and −0.9 kg/d, respectively). Cowsin CON had a greater decrease in milk protein concentrationfor PFTN (−0.10 percentage units) and HSC(−0.06 percentage units) compared with cows in RPMfor PFTN (0.00 percentage units) and HSC (−0.02percentage units). Cows in CON for HSC had greaterdecrease in milk fat concentration compared with cowsin RPM for HSC (−0.10 and +0.12 percentage units,respectively). In conclusion, HSC altered physiologicaland production parameters of cows. Additionally, RPMhelped maintain milk protein and fat concentrationduring HSC, whereas dry matter intake, milk yield, andfeed efficiencies were not affected by RPM.