117 Did embryo transfer improve dairy fertility during the recent warming?

Heat stress in summer is a major contributing factor to reduced reproductive performance among lactating dairy cattle. This global problem poses serious welfare concerns and results in economic losses. The World Meteorological Organization officially certified that the global average temperature in 2023 was the highest ever recorded. Thus, the effects of heat stress on the reproductive performance of dairy cows is an issue that needs to be addressed. In this study, the relationship between barn temperature–humidity index (THI) and conception rates of AI and embryo transfer (ET) was determined in Holstein dairy cows in Japan from 2021 to 2024. Temperature and humidity sensors were installed in 31 dairy barns throughout Japan, and these parameters were measured on an hourly basis. Daily average, minimum, and maximum THI values were calculated using the obtained values. Reproductive performance data for each farm were acquired from dairy herd milk records. A total of 13 315 cows mated or transferred between June 2021 and March 2024 (AI, 11 650 cows; ET, 1665 cows) were examined to elucidate the association between THI in the barn on the day of mating or transfer and conception rate. Embryos used for ET were collected and frozen in seasons other than summer. The chi-squared test was used to compare the conception rates of AI and ET among groups according to THI. The conception rates of AI and ET in multiparous cows throughout the study period were 33.9% and 40.6%, respectively, indicating significant differences (P < 0.05). In heifers, the conception rates exceeded 60% for both AI and ET, which were significantly higher than those in multiparous cows (P < 0.05). However, there was no relationship between THI in the barn on the day of mating or transfer and conception rates of AI or ET in heifers. This finding suggests that the heat environment on the day of mating or transfer did not affect the conception rate in heifers. The conception rate of AI in multiparous cows decreased progressively as the minimum daily THI on the day of insemination increased, with a significant decrease at THI ≥ 65, and was reduced to 18.7% at THI ≥ 75 (P < 0.05). In contrast, the conception rate of ET remained stable at less than ~40% at all THI values, with no decrease in the conception rate at higher THI. At THI ≥ 65, the conception rate of ET (47.4%) was significantly higher than that of AI (28.7%; P < 0.01). In comparisons of the conception rates of AI in terms of milk yields of <25, 25–35, and ≥ 35 kg per day, the conception rate decreased with increasing THI values at all milk yield levels. However, the conception rate of ET did not decrease with increasing THI values, although the baseline rates were different. The conception rate of AI in multiparous cows during the summer of 2023, when the highest temperatures were recorded, remained low even in September, when the THI had already decreased and the weather was cooler, unlike the trend in the summer of 2021 and 2022. In conclusion, it is evident that elevated THI in the barn was responsible for the reduced conception rate of AI in multiparous cows. Furthermore, the carryover effect of the 2023 heat wave on the conception rate of AI was severe. Conversely, the conception rates of ET did not decrease with increasing THI values. These findings indicate ET has potential as a reproductive strategy for dairy cows during future warming.