Effect of Reduction in Dietary Protein on Fertility Performance of Lactating Holstein Cows during Heat Stress

The aim of this research was to study the Effect of reduction in dietary protein on fertility performance of lactating Holstein cows during heat stress. For this we used 63 Holstein dairy cows (71 ± 16. 5 DIM) after voluntary waiting period and has been confirmed that the health reproduction based on randomized complete block design. Randomly، cows enrolled to the one of three experimental rations (18. 5، 17. 5، 16. 5% CP). All cows were inseminated according to Heat-Synch protocol. Ovarian structure was defined by ultrasound twice in protocol. The number of small (< 5 mm)، medium 2 (5 to 9. 9 mm)، and large (> 10 mm) follicles were counted by ultrasonography. Cumulative conception rate to first and second service was 47. 7، 52. 4 and 76. 2 % for treatments 1، 2 and 3 respectively. Numbers of Service per conception were 2. 07، 2. 03 and 1. 53. Days open 131. 7±26، 132. 5±18 and 121. 16±22 d for treatments 1، 2 and 3 respectively. In conclusion increase in the level of crude protein of diet above recommended NRC (2001) because of increase in blood urea nitrogen and harmful effects on uterine health and decrease in fertility during heat stress not recommended.