Quantitative relationship between climatic conditions and the conception rate of Japanese Black cattle in commercial cow–calf operations

Heat stress has a major effect on mammalian reproductive function. To clarify the relationship between heat stress and reproduction, it is important to investigate the quantitative relationship between the conception rate (CR) of beef cattle and climatic conditions. Therefore, the present study aimed to investigate the quantitative relationship between the CR of beef cattle and climatic conditions, as measured by the maximum temperature (MAX) and the temperature-humidity index (THI), in commercial cow–calf operations. Data were collected from 10,599 farms in Miyazaki, Japan, including records for 131,837 services of 79,265 heifers and 821,807 services of 147,464 cows between January 2005 and December 2014. Climate data in the vicinity of the farms were collected from the Japan Meteorological Agency. THI was calculated as 0.8 × meanT + (meanRH/100) × (meanT - 14.4) + 46.4; where T was the temperature, and RH was the relative humidity. The observational unit was an artificial insemination (AI) record, and the dependent variable was conception (1 or 0). Climatic conditions were calculated as the average value during the 21 days prior to the AI date, and each climatic condition was separately entered into the model for analysis. The mean CR in heifers was 57.4%. The CR in heifers was associated with climatic conditions (MAX and THI) and the AI number (P < 0.05), but not with the age at first AI. There were no interactions between factors regarding the CR of heifers. The CR of heifers increased with increasing MAX (odds ratio, 1.002) and THI (odds ratio, 1.001). The mean CR in cows was 54.9%. In a model using the MAX, the CR was associated with the MAX, parity, interval from calving to first AI, and AI number (P < 0.05). Significant interactions were found between the MAX and the interval from calving to first AI, and between the quadratic effect of MAX and the AI number. In a model using the THI, the CR was associated with the THI, parity, interval from calving to first AI, and AI number (P < 0.05). Significant interactions were found between the THI and the interval from calving to first AI, and between the THI and the AI number. In summary, low environmental temperature was associated with low CR in Japanese Black cattle.