Identification of maintenance energy requirements and estrus in beef cows.

摘要

Scope and Method of Study: The effects of maintenance energy requirements (MR) on rumen temperature, postnatal calf growth, and concentrations of IGF-I, thyroxine (T4), glucose and insulin in plasma were determined in spring calving Angus cows (n = 42). Nonlactating cows were individually fed a complete diet for 8 wk in amounts to supply MR (Level Model 1; NRC 1996). Cows were classified based on MR as low (LMR, n = 13; > 0.5 SD less than mean), moderate ( MMR, n = 11; +/- 0.5 SD of mean) or high (HMR, n = 8; > 0.5 SD greater than mean).;Angus cows, 4 to 8 yr of age, were used to evaluate changes in rumen temperature (RuT) associated with estrus. Temperature boluses (SmartStock, LLC) were placed in the rumen with a balling gun during gestation. Boluses were programmed to transmit RuT hourly. Estrus of cows was synchronized with PGF2alpha at 79 +/- 14 d after calving in May (n = 25) or 85 +/- 22 d after calving in December (Dec, n = 30). The HeatWatch RTM Estrus Detection System (CowChips, LLC) was used to monitor onset of estrus. Increases in mean RuT for any 8 h period ≥ 0.3°C, ≥ 0.5°C or ≥ 0.7°C above the mean for a cow during 12 to 84 h preceding the 8 h increase were used as criteria to predict estrus.;Findings and Conclusions: Constant body weight (BW) was achieved for at least 17 d. Daily MR averaged 90.5 +/- 5.3 Kcal·kg BW -0.75·d-1. There was a 29% difference in the amount of energy required for maintenance between the most and least efficient cows. Calf birth weight (P = 0.89), 205 d adjusted weaning weight (P = 0.58), and ADG from birth to weaning ( P = 0.58) were not influenced by MR. Plasma concentrations of glucose, thyroxine and insulin were not influenced by MR; however IGF-I in plasma and RuT were influenced by MR. A combination of biomarkers may identify cows that require less energy to maintain BW.;Mean RuT during the first 8 h after onset of estrus was greater than RuT during 16 to 32 h before or after estrus in May and Dec (P 0.001). An increase in RuT ≥ 0.3°C or ≥ 0.7°C correctly predicted estrus in 100% and 70% of estrous cows, respectively, in Dec; 100% of estrous cows in May were correctly predicted using either criterion. The use of RuT has potential application for detection of estrus in beef cows.