An experiment was conducted using 36 Hereford x Shorthorn bullocks of about 350 kg initial live weight. Winter planes of nutrition 13 December to 23 March were 'low' (hay only) and 'medium' (hay plus 1-8 kg concentrate); the former group of 18 bullocks lost on average 21-5 kg while the latter gained 26-5 kg. Each group was then stocked at 2-5 (low), 4-3 (medium) and 6-2 (high) bullocks per hectare and rotationally grazed on 10 plots, 7 of which contained H.I.-white clover mixture and the remaining 3 contained permanent pasture. Pasture digestibility and intake were determined for 16 weekly periods between early April and the end of September. Organic matter digestibility (OMD) of grazed herbage was determined in vitro on samples obtained by means of two rumen-fistulated bullocks. One gelatin capsule containing about 10 g of chromic oxide was daily administered to each bullock to estimate organic matter output. Winter feeding planes had no significant effect on OMD, and increasing the stocking rate increased herbage OMD only during the final 6 weeks. Significantly more organic matter was ingested during the grazing season by animals which had previously been fed on a low plane of nutrition. Stocking rates also significantly affected OMI. Bullocks fed on a low plane during winter were slightly more efficient subsequently in converting herbage into live-weight gain but this was a reflection of their lower average body weight; when feed efficiency was expressed on the basis of metabolic size, bullocks fed on a medium plane were more efficient in converting feed to liveweight gain. It is concluded that the higher daily gains on pasture of bullocks previously fed on a low plane is largely the result of a significantly higher feed intake by these animals.It has been observed that animals which are fed If increased herbage consumption accounts, at on a low or medium plane of nutrition during winter least in part, for the higher rate of gain full expresoutgain those fed on a high plane when all are sion to this potential should result from ungrazed during the subsequent summer (Meyer & restricted access to good quality pasture. More Clawson, 1964;Lawrence & Pearce, 1964). A full compensation, therefore, is to be expected when explanation for this phenomenon, however, is less animals are stocked at a low rather than a high readily available. Studies have been conducted on intensity and this has been demonstrated by Meyer the influence of winter feeding planes on subse-et al. (1965). The relative efficiency of animals quent responses of beef steers (Bohman, 1955; previously fed on two planes of nutrition, in terms Heinemann & van Keuren, 1956). Detailed experi-of converting herbage into meat, is of particular ments at the University of California (Meyer et al. importance where these animals reach slaughter 1965;Hull et al. 1965) have contributed much to weight at the end of the grazing season, our present understanding of this subject. TheseThe objectives of the present experiment were authors offer two explanations for th...

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SUMMARYNinety-three Kellakui fat-tailed single lambs were assigned after birth to a 2×2×2 factorial experiment; 46 were uncastrated males and 47 were females. About half the lambs of each sex were docked before they were 1 week old. Half the lambs had unrestricted milk from the ewes until 115 days of age (phase 1) and the other half received all the ewes' milk for the first 30 days and then about one-third of the milk until weaning at 60 days. Creep feed was avail-able to all lambs from about 3 weeks of age, in addition to 200 g of alfalfa dry matter per lamb daily. From 115 days of age, all lambs were fed on a standard fattening ration until slaughtered at weights of 46 to 49 kg for males and 35 to 38 kg for females (phase 2).Lambs given unrestricted milk were 7·4 kg heavier than restricted lambs at the end of phase 1. Significantly better gains due to docking were observed only for females during phase 2. There was little indication that docking affected feed conversion efficiency. In docked lambs the fat normally deposited in the tail was partially (less than 50%) relocated as subcutaneous plus intermuscular and internal fat. Lean meat percentages of carcasses were similar for docked and control treatments. There is a need to standardize reporting of results from docking experiments to make valid comparisons.

Two group-feeding and one individual-feeding experiments were made for 112 and 182 days, respectively, with 76 cross-bred and eight Zebu bulls. Simmental, Friesian and Jersey sire breeds were mated to three Ethiopian Zebu breeds, namely Boran, Horro and Barca, while Zebu bulls were included for comparison purposes in Expt 3. The trials were conducted in confinement at Holetta Research Station, located at 2400 m elevation and having a mean maximum temperature range of 18-7-24 °C. In the group-feeding trials (Expts 1 and 2) a common diet was fed, consisting of native hay (30 %) molasses (20 %) and a concentrate supplement. In the third experiment, two diets containing 30 and 50 % native hay were supplemented by concentrate and fed individually to eight Simmental and eight Friesian cross bulls, while eight Zebus served as a control.The average daily gains, feed intake and conversion did not differ markedly between Simmental and Friesian crosses, but lower values were generally found for Jersey crosses. Cross-breds outgained Zebus, consumed more feed and converted it more efficiently to live-weight gain. Performance differences were noted between the Zebu breeds as well as some evidence of sire x dam breed interactions. Significantly higher gains were achieved with the low than with the high roughage diet.

Nitrogen levels of 0, 184 and 368 lb (0, 83-47 and 166-94 kg) were applied, as calcium ammonium nitrate, in six dressings throughout the grazing season to a perennial ryegrass/white clover sward. Herbage samples taken, periodically from each treatment and analysed nitrate for, indicated that the latter increased with increasing levels of applied nitrogen. Herbage nitrate levels weie higher towards the end of the grazing season than at any other time. There was no consistent relationship between applied nitrogen and total plant nitrogen, although the latter tended to run parallel with applied-nitrogen levels during the early part of the year. Sheep performance was significantly increased with the first increment of 184 lb (83-47 kg) of nitrogen; a second increment of 1841b resulted in a further, but non-significant, increase. High-nitrate pastures did not significantly reduce livei vitamin A storage in sheep. It is suggested that approximately 350 Ib (158-79 kg) of nitrogen, applied uniformly throughout the grazing season, results in pasture nitrate levels which have no adverse affect on sheep performance. INTRODUCTION As livestock intensification progresses the need for larger nitrogen applications to grassland will become more apparent. Nitrogen applied as ammonium salts is first oxidized to nitrate by soil bacteria and as such can be absorbed by plant roots. The rate at which absorbed nitrate is formed into protein is related to plant growth rate. With high nitrogen application, plants can absorb more nitrate than they can readily form into protein.Accumulation of nitrate in plants can have two different effects. A toxic level will lead to animal deaths within a matter of days: nitrate is reduced by rumen bacteria to nitrite which combines with blood haemoglobin to form methaemoglobin; the oxygen-carrying capacity of the blood is thereby reduced. Nitrate below toxic levels inhibits the conversion of carotene to vitamin A in ruminants (9,12,26,27).Johnson and Baumann (17) showed that a normal thyroid gland was essential for the conversion of carotene to vitamin A. Since nitrate reduces thyroid activity (34, 35), this suggests 228 that the effect of nitrate, mediated through the thyroid gland, may contribute to the lower in vivo carotene conversion. Sell and Roberts (25) confirmed this finding with chicks.The addition of nitrate reduced liver vitamin A in both sheep and rats (13, 21) and other workers (16, 20, 26) have reported that highnitrate diets interfere with carotene and vitamin A metabolism. The feeding of nitrite, but not nitrate, significantly lowers liver vitamin A storage from the orally-administered preformed vitamin (7). Liver vitamin A storage from carotene is reduced by both nitrate and nitrite, with the greatest effect from the latter.Bacterial reduction of nitrates in silages produces oxides of nitrogen which, in addition to the human health hazard, affect the biological activity of carotene. Gaseous oxides of nitrogen (NO and N2O4) destroy carotene, which as a result has no vitamin A activit...