Effects of late gestation heat stress on postpartum milk production and reproduction in dairy cattle.

Carry-over effects of late gestation heat stress on postpartum productive and reproductive traits were estimated from DHI records using 341 lactations from six sites in Mississippi. Climatological data were gathered from records of weather stations near the sites. Using multiple linear regression analyses, predictor variables for lactations were age at calving, lactation number, maximum degree-days (above 32.2 degrees C) during the periods 30 and 60 d prepartum, and precipitation 30 and 60 d prepartum. Months and sites were indicator variables. Dependent variables included milk and fat production during early, mid, and late lactation; days to peak lactation; days open; services per conception; and body weight. Age at calving affected milk and fat production in mid and late lactation and services per conception. Degree-days for 60 d prepartum had the greatest negative influence on production variables; its statistical significance was shown in predictions of milk and fat production in early and midlactation. Days open were higher for July than for cows calving in August or September. Sites had effects on many milk and fat measurements and some reproductive traits. These results indicate that heat stress in the last 60 d of gestation has negative effects on some production variables.