Comparative nitrogen digestibility in Brahman, Brahman × Shorthorn, Africander × Hereford, and Hereford Steers

Abstract

1. Eight nitrogen metabolism experiments, each with sixteen steers representing grade Brahman, Brahman x Shorthorn, Africander x Hereford and Herefords, were carried out. The steers were fed according to body weight to the power 0·73, on rations consisting of natural pasture hay alone or in combination with lucerne hay.2. These experiments showed that the steers with zebu blood were significantly superior to the Herefords with regard to dry-matter digestibility and apparent nitrogen digestibility. The breed difference disappeared when nitrogen digestibility was calculated from the relationship feed nitrogen minus undigested faecal residue nitrogen, divided by feed nitrogen.3. Two further nitrogen metabolism experiments with six Africander x Hereford and four Hereford steers fed natural pasture hay plus lucerne hay, and one experiment with two Brahman x Shorthorn, two Africander x Hereford and two Hereford steers fed chaffed wheat straw were carried out. The steers were fed an equal amount of ration, irrespective of body weight.4. In these three experiments faecal nitrogen was fractionated into undigested residue nitrogen, non-dialysable centrifugable nitrogen, non-dialysable non-centrifugable nitrogen, and dialysable nitrogen. The breed groups differed significantly in the amount of dialysable faecal nitrogen excreted but not in any other component.5. The amount of dialysable faecal nitrogen excreted on equal intake was very significantly negatively correlated with body weight (r = −0·807, P < 0·01).6. It was found that in two experiments the mean difference between Africander x Herefords and Hereford steers, mean body weights 863 and 686 lb., respectively, was 1·81 ± 0·51 g. nitrogen per day. It was calculated that this would represent about 70 ± 20% of the body-weight difference between the breeds if the lost nitrogen was fully utilizable. This agreed well with the estimate of 65% derived from the square of the correlation coefficient.