1. Effect of temperature and humidity on gestation length H.D. Norman, J.R. Wright,* and J.B. Cole Animal Improvement Programs Laboratory, Agricultural Research Service, USDA, Beltsville, MD 20705-2350 Abstr. M38 INTRODUCTION High temperature and humidity have been shown to affect production traits but their effects on other traits has not been examined. More accurate prediction of gestation length (GL) could aid herd managers in assuring that appropriate nutrition and health actions are given to pregnant cows at the optimal time. OBJECTIVES Determine if monthly differences in GL are caused by temperature and humidity. Determine how various measures of temperature and humidity account for differences in GL. DATA & METHODS (cont.) Edits:  Inseminations initiating gestation must have been after February 1998 and subsequent calving parturition must have been before January 2006  Holstein with >86% breed purity based on AIPL crossbred database  Matings to identified AI sires only  10% sample selected for analysis (477,068 conceptions) Model: GL =calf conception year (4 heifer groups, 8 cow groups) + calf conception herd-year (absorbed) + calf conception month (Jan., Feb., etc.) + calf parturition code (8 groups based on multiple birth status and gender) + age within parity at conception (5 heifer groups, 17 cow groups) + lactation length (7 groups) + milk yield (5 groups) + THI (16 groups) + residual (random) Analysis:  Use SAS GLM to evaluate the following 16 measures of THI: –Day of calving –Individual days 1 to 13 prior to calving –Mean THI of calving day and 6 days prior to calving (7 days) –Mean THI of calving day and 13 days prior to calving (14 days) DATA & METHODS Lactation, reproductive, and dystocia records from the national dairy database at AIPL as supplied by dairy records processing centers, breed associations, and the National Association of Animal Breeders. Meteorological data from 238 weather stations supplied via subscription to the Southern Regional Climate Center, Louisiana State University. Meteorological data matched to herd ZIP code by choosing closest weather station based on distance between latitude/longitude of center of ZIP codes using Haversine formula to approximate distance. Trait definition: Temperature-Humidity Index (THI) = (1.8T + 32) – (0.55 – 0.0055H) (1.8T – 28) where T = temperature in Celsius and H = relative humidity expressed as a percentage. RESULTS Frequency of records by THI level: Relationship of THI with GL: Comparison of various THI measures: A=conception year, conception herd-year, calf parturi- tion code, age within parity, lactation length, milk yield M=month of conception T n =THI of day n prior to calving Model effect(s)R 2 A.1071A,M,T 7.1143 A,M.1125A,M,T 8.1141 A,T 6.1097A,M,T 9.1140 A,M,T 0.1139A,M,T 10.1139 A,M,T 1.1140A,M,T 11.1139 A,M,T 2.1140A,M,T 12.1139 A,M,T 3.1142A,M,T 13.1138 A,M,T 4.1143A,M,T 0-6.1154 A,M,T 5.1145A,M,T 0-13.1164 A,M,T 6.1145 http://aipl.arsusda.gov/ RESULTS (cont.)  Adding THI to the model including conception month increased prediction of GL only slightly.  Only small differences between were found between various THI measures.  THI measured at days 5 and 6 were slightly more effective than other single-day measures.  THI mean of 7 days or 14 days was more useful in helping to predict GL than single-day measures. Solutions by THI group for model including all effects, conception month, and THI at 6 days prior to calving: Solutions by THI group for model including all effects, conception month, and THI at 6 days prior to calving: RESULTS (cont.)  Adding THI to the model including conception month increased prediction of GL only slightly.  Only small differences were found between various THI measures.  THI measured at days 5 and 6 were slightly more effective than other single-day measures.  THI mean of 7 days or 14 days was more useful in helping to predict GL than single-day measures. Solutions by THI group for model including all effects, conception month, and THI at 6 days prior to calving: Solutions by THI group for model including all effects, conception month, and THI at 6 days prior to calving: CONCLUSIONS THI is useful in predicting GL but has limited value when conception month is included in the model. Effectiveness of THI in predicting GL is greater at higher temperatures.