ABSTRACT
For an evaluation of the effects of heat stress during pregnancy on calf birth weight and postpartum maternal milk yield, 21 cows and 10 heifers were assigned to shade (S; n = 16) or no shade (NS; n = 15) treatments during the last trimester of pregnancy in June 1978. At parturition, all cows were removed from treatment and uniformly managed in the milking herd. At 4-d intervals from d 199 of pregnancy to parturition, respiration rates, rectal temperature, heart rate and Black Globe temperature were recorded and blood samples obtained via tail venipuncture between 1300 and 1600 h. Weekly prepartum body weights of dam, birth weight of calf and subsequent daily milk yield were recorded. Black Globe temperature, rectal temperature and respiration rates were higher in cows given NS. Calf birth weight was lower in the NS group. Milk yield was correlated in a linear manner with calf birth weight, and cows in group NS exhibited reduced lactation performance after calving. Plasma progestin concentrations were higher in heat-stressed cows (6.0 vs 5.1 ng/ml). Estrone-sulfate concentrations were reduced in plasma of NS cows (2,505 vs 4,433 pg/ml). Thus, hormone concentrations of maternal and fetal origin were altered by environment. Plasma thyroxine concentrations were lower in NS cows (51.2 vs 66.4 ng/ml), while plasma triiodothyronine concentrations were elevated (1.8 vs 1.5 ng/ml), indicating altered thyroid hormone metabolism in heat-stressed cows. Heat stress altered endocrine dynamics during pregnancy and reduced calf birth weight and may have indirectly altered subsequent milk yield.
