Author: Longfei Han

Publisher:

Published: 2022

Total Pages: 0

ISBN-13:

Improving heat-stressed animal health and survival through management, nutrition, and genetics is critical to profitability and sustainability on a dairy farm. The objectives of this dissertation were to estimate genetic parameters of heat tolerance in dairy cattle for the health and survival traits; to investigate the long-term impact of heifer development; and to examine the potential role of ghrelin in thermoregulation during heat stress. Five experiments were conducted and summarized. The first experiment investigated the effect of heifer development (measured as body weight at first calving, BWFC) on milk production and survival. First lactation 305-d milk yield and 24-month milk yield were analyzed with a linear model. Survival analysis was performed to estimate the impact of BWFC on survival. Heifers in the top 60% of body weight at first calving had significantly higher first lactation 305-d milk yield than lighter heifers, but no relationship of BWFC and first lactation 305-d milk yield existed within the top 60%. Relationships between BWFC and 24-month milk yield were not significant. Heifers with higher BWFC lost significantly more body weight in early lactation. While BWFC was a significant predictor of first lactation 305-d milk yield, it accounted for 3% of variation, suggesting that BWFC is not a primary contributor to variation in milk yield. Compared to the lightest heifers, heaviest heifers were 49% more likely to be culled at any given time. These data indicated that, among heifers managed similarly, heavier heifers produced more milk in first lactation than lighter heifers, but lost more BW, faced a higher risk of being culled, and did not produce more milk long-term. The second experiment examined 1), the effect of climatic conditions at birth on organic calf stayability to 365 d of age (STAY) and respiratory disease (RESP) by 365 d of age, and 2), the genetic parameters of organic Holstein calf heat tolerance (HT) for STAY and RESP. HT was defined as the rate of change in STAY and RESP when a calf was born under heat stress. The final dataset included 77,090 calves from 16 USDA certified organic farms and included 35,520 Holsteins for genetic analyses. Phenotypic data were combined with climatic data obtained from local weather stations based on the zip code. Phenotypic analyses were performed with a model that included THI, breed, diurnal THI variation (daily maximum THI -- daily minimum THI; DTV), contemporary group, breed, and genetic effects. Estimated breeding values were calculated for Holstein sires with more than 10 daughters in the dataset. By 365 d of age, 80% of the calves stayed in the herd, and 7% of the calves recorded at least one RESP. Calf STAY decreased and RESP increased when calves were born in THI over 65. The additive genetic HT effect was significant for STAY but not RESP. The heritability for STAY and RESP were 0.100 and 0.025, respectively. In conclusion, calf STAY, RESP, and HT for STAY were heritable in organic herds and can be improved through genetic selection. The third experiment investigated the genetic and environmental effect of heat stress at calving on organic cow health and survival. Health trait were mastitis, reproductive disease, disease during the transition period (TRAN); and stayability to 30 DIM (1 = stayed in the herd, 0 = culled or died by 30 DIM; STAY30). The final dataset included 83,394 lactations from 40,712 cows of 6 different breeds of which Holstein (32,007 lactations, 15,938 cows) were most frequent and were used for genetic analysis. Weather condition at calving was defined as cool (average THI