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Size and Management Effects on Reproduction in First-, Second- and Third-Parity Cows¹

Iowa State University, Ames 50011

Abstract

The effects of size, management and size x management interactions on reproduction in beef cattle were determined by using 3,126 breeding and calving records from 1980 to 1985. Cows were from three synthetic lines that differed in mature size (small, medium and large). The lines differed also in the percentages of Jersey, Angus and Simmental. Each line was replicated in two herds. One herd (Rhodes) followed a spring calving schedule and weaned calves at about 180 d of age. The other herd (McNay) calved cows in the fall and weaned calves early at about 45 d of age. Cycling rate, conception rate, abortion rate and calving rate were each evaluated for first-, second- and third-parity dams. An important interaction (P < .05) was found between lines and management in heifers. Cycling rate and calving percentage were almost identical for all three lines in the Rhodes herd, but small heifers in the McNay herd had about 20% higher cycling (83.8 vs 63.1%) and calving rates (73.8 vs 53.0%) than large heifers. Line x management interactions were not significant in second- and third-parity dams, but line did remain a significant source of variation in calving rate. Calving rates in second-parity dams were 85.0, 78.1 and 70.7% for small, medium and large cows, respectively. In third-parity dams, small cows had a higher calving rate (P < .05) than either medium or large cows (80.9 vs 66.1 or 68.7%, respectively). In addition, cows in the Rhodes herd had a higher calving rate (P < .05) than cows in the McNay herd (75.3 vs 68.4%). These results suggest that small cows are reproductively more efficient in terms of calving rate than larger cows when managed similarly. Interactions in first-parity dams indicated that differences among lines may be accentuated under less favorable conditions.

¹ Journal Paper No. J-13007 of the Iowa Agric. and Home Econ. Exp. Sta., Ames. Project No. 2000.