A. Yilmaz*,
M. E. Davis*1,
R. C. M. Simmen+
*The Ohio State University Department of Animal Sciences
+University of Florida Department of Animal Science
This study was designed to examine the effects of selection for high vs. low blood serum IGF-I concentration on reproductive performance of spring- and fall-calving Angus cows. Data were obtained from an ongoing experiment involving 100 spring-calving (50 high and 50 low line) and 100 fall-calving (50 high and 50 low line) purebred Angus cows. Effects of year-line-season and age of dam on calving rate were highly significant. Calving rate was slightly higher in high IGF-I line cows (P=0.07), but did not differ between spring- and fall-calving cows (P=0.47) when only natural service matings were considered. Postweaning IGF-I concentrations of cows that had data for calving rate were significantly higher in high line cows, but did not differ between spring- and fall-calving cows. Among those females that had data for calving rate, IGF-I concentrations as heifers averaged 38.5 ± 8.9 (P=0.0001), 42.7 ± 9.9 (P=0.0001), and 34.8 ± 10.2 (P=0.0007) ng/mL more IGF-I for high line than low line at d 28, 42, and 56, respectively, of the postweaning period. Mean IGF-I was 38.6 ± 9.4 (P=0.0001) ng/mL higher in the high line. Therefore, the slightly higher calving rate found in the high line may have been associated with higher IGF-I concentrations. Mean age of heifers at first calving did not differ between the high and low IGF-I line heifers (P=0.54). Spring born heifers were significantly older than fall-born heifers at first calving. Relationships (as indicated by regression coefficients) between age of heifers at first calving and IGF-I concentrations were negative, but nonsignificant, for all measures of IGF-I. Thus, selection for higher blood serum IGF-I concentration improved calving rate of Angus cows and had a small, albeit desirable, effect on age at first calving. Season had a large influence on age of heifers at first calving but not on calving rate.
1For more information contact at: The Ohio State University, 221 Plumb Hall, 2027 Coffey Road, Columbus, OH, 43210; 614-292-4984; fax: 614-292-2929; e-mail: davis.28@osu.edu