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Influence of Season, Sex and Hormonal Growth Stimulants on Feedlot Performance of Beef Cattle¹

University of Arizona, Tucson

Abstract

Four trials were conducted at Yuma, Arizona, to evaluate the influence of season, sex and hormonal growth stimulants on feedlot performance of beef cattle. Two of the trials were conducted during the hot summer months and two during the moderate winter months. Growth stimulants tested were melengestrol acetate (MGA), diethylstilbestrol (DES), Synovex-H and Rapigain–1. Steers, heifers and ovariectomized heifers were involved in trial 1, steers and heifers in trial 2, and heifers in trials 3 and 4. Rations and general management procedures were duplicated as closely as possible for all trials.

Seasonal differences (summer vs. winter) were the most important source of variation in feedlot performance. Gains during the winter trials were 14 and 24% greater than during the summer, and feed requirements were reduced by 7 and 19% during the winter months. None of the growth stimulants tested were effective in improving feedlot performance during the summer months, although significant responses were observed during the moderate winter months. Thus, the stresses imposed by high summer temperatures are major factors limiting feedlot performance of beef cattle.

A significant improvement in daily gain of heifers treated with MGA was observed in one winter trial, although no effect was noted in the second winter study. Steers did not respond to MGA. Treatment with DES improved feed conversion in both steers and heifers. Synovex-H significantly increased rate of gain of heifers in the last winter study. The improvement in gain of heifers treated with Rapigain–1 was not statistically significant.

Sex comparisons indicated that steers gained more rapidly and efficiently than heifers. However, these differences were minimized when animals were exposed to hot summer conditions. Spayed heifers had the poorest performance of any sex group.

¹ Arizona Agricultural Experiment Station Technical Paper No. 1438. These studies were conducted in cooperation with the U.S.D.A. under Western Regional Project W–46.

² Heifers of mixed breeding from the Texas area.

³ Melengestrol acetate supplied by the Upjohn Company, Diethylstilbestrol by Charles Pfizer & Company, Synovex-H and Rapigain–1 by E. R. Squibb & Sons.