The effects of ambient temperature and humidity on pregnancy rate in Beefmaster cows in a subtropical environment of Mexico

Pregnancy rates of a Beefmaster herd with improved management in the east coast of Mexico under intense heat load during most part of the year (THI range = 56–102) were evaluated, using the GENMOD procedure of SAS, with respect to temperature–humidity index (THI) prior or after artificial inseminations (AI; n = 2039 inseminations). There was a departure from linearity (P < 0.05) between THI and conception rates, as maximum THI exceeded 92 units pregnancy rate decreased more rapidly, although conception rate changed only five percent points over the THI range found in this study. When heat stress was as it maximum the day of AI (THI = 88–96) lower or higher THI one or three days prior or after AI did not affect conception rates (47–56%). With lower heat load at AI (THI = 80–88) higher THI before or after neither modify conception rates (48–58%). Pregnancy rates of cows differed among seasons (P < 0.01), peaking in the fall (62%) and decreasing in spring (44%). Inseminations per pregnancy were lower in summer and fall (1.5) than in other seasons (1.9–2.0; P < 0.01). There was a weak association between days open (average 242 days for 14,910 calvings) and monthly average THI during the study period, with longer intervals between parturition and pregnancy in cows calving in the fall. The main conclusions of this study were that pregnancy rate was mildly compromised by extreme heat load in Beefmaster cows in this subtropical environment, and that cooler or warmer temperatures prior or after AI bear no consequences for pregnancy rate during the warmest seasons for this breed of cattle.