Metabolism of Nitrogen in Boran and in Hereford-Boran Crossbred Steers3

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Abstract
IN trial 1, six 18-month-old Boran steers were paired by weight to six 12-month-old ¾ Hereford, ¼ Boran crossbred (HBC) steers and fed a low-protein hay in a comparative digestibility trial over a 28-day period. No supplementary energy or nitrogen (N) was provided, but adequate minerals and vitamins were supplied. Voluntary intake of dry matter (DMI) was 23% higher (P<.01) for the Boran than for the HBC. The coefficients of digestibility of DM, energy and N, as well as the percent retained N in the two breeds were similar. The main differences in the protein metabolism between the Boran and the HBC were the higher (P<.05) urinary N (UN) and five times more (P<.01) urinary urea N (UUN) loss in the Boran as compared to the HBC. The UN of the Boran and the HBC contained 47 and 12% UUN, respectively, indicating an increase in the recycling of urea in the HBC on 7% protein hay. In trial 2, Boran and HBC steers were fed low-protein hay supplemented with concentrates containing urea (U) or cottonseed meal (CS) which were isocaloric and isonitrogenous. There were six Boran and five HBC steers on each concentrate supplement in a 2 × 2 factorial arrangement in each of two 28-day periods. Voluntary DMI was higher (P<.01) for the Boran than for the HBC steers. The coefficients of digestibility for DM, energy, N corrected to an equal N intake and the percent retained N in the two breeds were similar. The retention of N was influenced more (P<.01) by the source of N and level of ingested N than by the breed of cattle. However, the Boran lost more (P<.01) N in the urine than the HBC. In addition, the Boran when provided with extra N above maintenance apparently increased its level of serum urea linearly with ingested N while the increase in the HBC was negligible. Despite similar intakes of N from concentrates U and CS, retained N from urea was 64% (P<.01) of that of cottonseed meal. Copyright © 1972. American Society of Animal Science. Copyright 1972 by American Society of Animal Science.

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