CASE STUDY: Feed Intake and Performance of Heifers Sired by High- or Low-Residual Feed Intake Angus Bulls

The objective of this project was to investigate the effects of selecting sires for residual feed intake (RFI) on the performance of their daughters. Bulls with low or high estimated breeding values (EBV) for RFI were selected from the Angus Society of Australia sire summary and mated to Angus cross commercial cows at the Kansas State University Cow-Calf Unit in 2005 and 2006. The average EBV of low- and high-RFI bulls were −0.55 and 0.27 kg DM, respectively. Heifers born in 2006 were tested for feed intake in 2 groups (n = 24, n = 26), and heifers born in 2007 (n = 42) were sent to a commercial bull test facility for feed intake and BW gain tests. Body weights were collected every 14 d and used to calculate midtest BW and ADG. Actual feed intake was regressed on midtest metabolic BW and ADG to calculate an expected feed intake for each heifer. Residual feed intake was calculated by subtracting the expected intake from the actual intake. There were no significant differences between heifers sired by lowor high-RFI EBV bulls in RFI, feed intake, G:F, or BW gain (P \u3e 0.05). Heifers in this study were being developed on a less energy-dense diet than the diet used to rank their sires. Genetic differences in RFI calculated in growing bulls may not have been expressed on the lower plane of nutrition of these developing heifers

CASE STUDY: Feed Intake and Performance of Heifers Sired by High- or Low-Residual Feed Intake Angus Bulls

The objective of this project was to investigate the effects of selecting sires for residual feed intake (RFI) on the performance of their daughters. Bulls with low or high estimated breeding values (EBV) for RFI were selected from the Angus Society of Australia sire summary and mated to Angus cross commercial cows at the Kansas State University Cow-Calf Unit in 2005 and 2006. The average EBV of low- and high-RFI bulls were −0.55 and 0.27 kg DM, respectively. Heifers born in 2006 were tested for feed intake in 2 groups (n = 24, n = 26), and heifers born in 2007 (n = 42) were sent to a commercial bull test facility for feed intake and BW gain tests. Body weights were collected every 14 d and used to calculate midtest BW and ADG. Actual feed intake was regressed on midtest metabolic BW and ADG to calculate an expected feed intake for each heifer. Residual feed intake was calculated by subtracting the expected intake from the actual intake. There were no significant differences between heifers sired by lowor high-RFI EBV bulls in RFI, feed intake, G:F, or BW gain (P \u3e 0.05). Heifers in this study were being developed on a less energy-dense diet than the diet used to rank their sires. Genetic differences in RFI calculated in growing bulls may not have been expressed on the lower plane of nutrition of these developing heifers

Pregnancy rate and first-service conception rate in Angus heifers

The objective of this project was to determine the genetic control of conception rate, or pregnancy percentage in Angus beef heifers. Producers from 6 herds in 5 states provided 3,144 heifer records that included breeding dates, breeding contemporary groups, service sires, and pregnancy check information. Two hundred fourteen sires of the heifers were represented; with 104 sires having less than 5 progeny, and 14 sires having greater than 50 progeny. These data were combined with performance and pedigree information, including actual and adjusted birth weights, weaning weights, and yearling weights, from the American Angus Association database. Heifer pregnancy rate varied from 75 to 95% between herds, and from 65 to 100% between sires, with an overall pregnancy rate of 93%, measured as the percentage of heifers pregnant at pregnancy check after the breeding season. Pregnancy was analyzed as a threshold trait with an underlying continuous distribution. A generalized linear animal model, using a relationship matrix, was fitted. This model included the fixed effects of contemporary group, age of dam, and first AI service sire, and the covariates of heifer age at the beginning of breeding, adjusted birth weight, adjusted weaning weight, and adjusted yearling weight. The relationship matrix included 4 generations of pedigree. The heritability of pregnancy and first-service conception rates on the underlying scale was 0.13 ± 0.07 and 0.03 ± 0.03, respectively. Estimated breeding values for pregnancy rate on the observed scale ranged from −0.02 to 0.05 for sires of heifers. Including growth traits with pregnancy rate as 2-trait analyses did not change the heritability of pregnancy rate. As expected for a reproductive trait, the heritability of pregnancy rate was low. Because of its low heritability, genetic improvement in fertility by selection on heifer pregnancy rate would be expected to be slow