SUMMARYFood intake, eating rate, weight change and fasting metabolic rate were measured on 12 animals of each of the following breed-types: Hereford × Shorthorn (HS), Brahman × HS (BX) and Africander × HS (AX). Measurements were made on ad libitum and fixed levels of lucerne and ad libitum and fixed levels of low-quality tropical pasture hay (approx. 1·2% N) during feeding periods of about 100 days' duration.The BX and AX were heavier and either gained more weight or lost less weight than the HS on all diets. At feeding levels close to maintenance, which included the ad libitum pasture hay diet, adjustment for differences in either live weight or intake did not alter the unadjusted ranking of the breeds. On ad libitum lucerne, however, adjustment for either weight or intake showed higher gain in the HS than in the BX. The voluntary food intake per kg live weight was consistently and significantly highest for the HS and lowest for the BX. Rates of eating were not significantly different for the different breed-types but they were higher for high-quality lucerne and for the lower level of feeding. Fasting metabolic rate per kg live weight was highest for the HS and lowest for the BX; the difference between the breeds varied with the previous dietary treatment. The ratio of voluntary food intake to fasting metabolism was practically constant between breeds and animals within breeds, though different between diets.Correlations for animals within breeds and years between the various parameters within diets and between diets are presented.

A Brahman x Hereford and a Hereford steer fed on a chaffed tropical pasture hay were infused intravenously and intraruminally with urea at 9–45 g N/day. By comparing the respective increases in rumen ammonia concentration which occurred when urea was infused intravenously and intraruminally, it was possible to estimate the net flow of urea into the rumen resulting from the intravenous administration. In both steers there was a limit to the net amount of urea passing from the blood to the rumen of c. 17–20 g N/day, which was reached at plasma urea concentrations of c. 12 mg N/100 ml. This concentration of plasma urea was achieved in the Brahman x Hereford steer when c. 23 g N/day was infused intravenously but in the Hereford when 32–35 g N/day was infused.

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A comparison is presented of the composition of the faeces, urine, and plasma when Hereford and Brahman x Hereford steers were fed two different quality diets at three levels of intake. On a high quality diet (lucerne hay), the Brahman x Hereford steers produced significantly less faecal dry matter, total nitrogen, and non-dialysable nitrogen than the Hereford steers, but there were no significant differences between the breeds or between animals within a breed in total urinary nitrogen or any of its major constituents. On a low quality diet (blue grass–spear grass hay) the two breeds were not significantly different in faecal dry matter, total nitrogen, or non-dialysable nitrogen. The Brahman x Hereford steers excreted more total nitrogen, urea, and creatinine in their urine, part of which could be attributed to a higher liveweight; and they had significantly lower nitrogen balances. In animals of the same breed, water intake and urine volume were negatively correlated with plasma urea and nitrogen balance and positively correlated with urinary total nitrogen, creatinine, and uric acid.

I. The fasting metabolism of twenty-seven animals, nine Brahmans, nine Africanders and nine Hereford x Shorthorns was measured after fasts of 96 h duration. Urine was collected over successive 24 h intervals and gas exchange was determined 96-103 h after the last meal.2. Urinary urea decreased and creatinine increased throughout the fasting period. There was no difference between breeds in the urinary excretion of nitrogen, urea and creatinine on the 4th day of fasting (72-96 h). Excretion of nitrogen, urea and creatinine increased by 259, 197 and 45 mg/kg increase in fasted weight and by 8.9,7'6 and 0.91 mg/kcal increase in fasting metabolism respectively. Protein oxidation accounted for 25.6, 21.7 and 22.7% of the fasting heat production of Brahman, Africander and Hereford x Shorthorn cattle respectively, values which were not significantly different. 13'7%.