Late-gestation heat stress abatement in dairy heifers promotes thermoregulation and improves productivity

摘要

Multiparous, nonlactating pregnant cows are negativelyaffected by heat stress, but the effect of heatstress on more thermotolerant pregnant heifers hasreceived less attention. Our objective was to characterizethe effect of late-gestation heat abatement on thermoregulatoryresponses and subsequent milk productionof nulliparous Holstein heifers. Pregnant heifers,blocked by body condition score (BCS) and predictedtransmitting ability (PTA) for milk, were enrolled ineither heat stress (HT, shade of freestall barn; n =16) or cooling (CL, shade of freestall barn, water soakers,and fans; n = 15) environments during the last 60d of pregnancy (～8 weeks). Rectal temperature (RT;thermometer), respiration rate (RR; breaths/min),sweating rate (SR; VapoMeter, Delfin Technologies,Kuopio, Finland), and skin temperature (ST; infraredthermometer) were measured thrice weekly from enrollmentto calving. Vaginal temperature (VT; i-buttonintravaginal device) was measured every 10 min for 7consecutive days at wk −8, −6, −4, and −2 relative tocalving and averaged hourly. Daily thermoregulatorypatterns assessed by SR and ST, were measured every4 h over a 36-h time interval at wk −6, −4, and −2relative to calving. Upon calving, milk, protein, and fatyields were recorded twice daily for 15 wk. The averagetemperature-humidity index (Hobo Pro temperatureprobe, Onset Computer Corporation, Pocasset, MA) inthe barn during the precalving period was 77 (minimumof 72, maximum of 82). Only heifers that gave birth toa female calf (CL = 12, HT = 14) were included in thestatistical analysis. In the precalving period, CL heifershad lower RR (44.3 vs. 60.0 ± 1.6 breaths/min), RT(38.7 vs. 38.8 ± 0.04°C), unshaved ST (34.7 vs. 35.3 ±0.17°C), and unshaved SR (19.0 vs. 35.2 ± 1.9 g/m2h),relative to HT heifers. Additionally, VT was lower inCL heifers during wk −4, and −2, specifically duringearly morning and early afternoon hours. When measuredover a 36-h time interval, ST and SR were lowerin CL heifers, when compared with HT heifers for allweeks. Notably, ST was reduced overnight and SR wasreduced during the daytime. Cooled heifers had highermilk yield (35.8 vs. 31.9 ± 1.4 kg/d), when comparedwith HT heifers. Similar to multiparous cows, our dataindicate that actively cooling heifers in late pregnancyis effective in promoting thermoregulation and resultsin elevated milk production postcalving.