Protein and Selenium Levels for Growing and Finishing Beef Cattle

Abstract

Two feedlot studies were conducted with 96 Hereford steers to assess the relationship between selenium adequacy and protein requirements of growing and finishing cattle. The first study was a 98-day growing trial with feeder steers as they gained from 230 to 340 kilograms. Diets contained 55% corn silage (dry matter basis) with shelled corn and were supplemented with soybean meal and sodium selenite to provide three levels of protein, each with basal or added selenium. Two lots of eight cattle each were fed each diet. After the growing study, the cattle were reassigned to a 112-day finishing study (340 to 480 kg) and fed diets including whole shelled corn and 7% corn silage, with basal or added protein as soybean meal or linseed meal with or without supplemental selenium as sodium selenite. In the growing study, cattle fed the three protein levels consumed 676, 858 and 1026 g crude protein/day and gained 1.01, 1.09 and 1.23 kg/day over the 98 days. Feed conversion averaged 5.76, 5.60 and 5.07. Rate (P<.01) and efficiency of growth were improved 23 and 12%, respectively, for cattle fed the 16.5% protein compared to those fed the 11.6% protein diet. Feed intakes were greater (P<.05) for cattle fed 14.1 and 16.5% protein diets than for those fed the 11.6% protein diet. Plasma urea nitrogen increased (P<.01) with protein level (7.8, 10.7, 15.2 mg/100 ml). Cattle fed each diet responded positively to selenium in rate and efficiency of gain, with the greatest response observed for those fed the lowest protein diet. Plasma selenium levels were marginal for cattle fed basal diets, indicating a deficiency, and increased (P<.01) about twofold with supplemental selenium. During the finishing phase, supplemental selenium added inorganically as sodium selenite or organically in linseed meal did not alter performance. Although liver selenium levels increased (P<.01) when either sodium selenite or linseed meal was added, the combination resulted in no further increase above levels observed for cattle fed basal selenium linseed meal diets. Response in daily gain to supplemental protein was significant, and performance with either soybean meal or linseed meal was similar. Protein intake averaged approximately 700 and 880 g/day for basal- and soybean meal-supplemented cattle. Gains of cattle previously fed 11.6, 14.1 and 16.5% protein during the growing study differed (P<.002) during the finishing study, with rates of 1.31, 1.24 and 1.12 kg/day, respectively. The interaction of growing-finishing phase protein levels was not significant. These data indicate that for Hereford cattle growing from 230 to 340 kg and fed corn grain-corn silage diets, the crude protein requirement approaches 1,000 g/day, while for finishing cattle over 340 kg and fed shelled corn, the requirement is largely for maintenance and is between 700 and 880 g/day. These data also indicate that selenium levels are most critical during early stages of growth and when cattle are fed diets marginal or deficient in protein.