A closed-loop system is proposed for the control of voluntary food intake in lactating cows, and an expression is deduced relating the response of food intake to changes in milk yield and body-weight gain.

A closed-loop system necessarily involves a delay in the response to changes in production. The rate of increase of food intake is there- fore slower than the rate of increase in milk yield. The consequent deficit in energy during the rising part of the lactation curve is met by the mobilization of body reserves, which are partly accounted for by losses in body weight. During the declining part of the lactation the delay effect leads to an excess of energy intake and to the replacement of body reserves and, consequently, of body weight.

The expression deduced from the model was fitted to four different types of lactation curve corresponding to long and short lactations of Friesians and Jerseys fed ad libitum on a complete diet. The expected food intake based on the control model was contrasted with a linear regression model. The former gave a better account of the variation in food intake in all four types of lactation.

The total change in body weight during lactation was partitioned between changes in weight due to the mobilization and replacement of reserves and gain directly attributable to food intake. There was, in general, good agreement between the observed losses in weight occurring at the beginning of lactation and those predicted from the mobilization of reserves for milk production.

The physiological implications of the model and the values estimated for the parameters are discussed.

Two experiments were made with dairy cows in 1969 and 1970 to compare two grazing systems. Treatment A was a rigid rotational system and B was a system in which cows always grazed on an area which had been cut 4 weeks earlier for conservation. For both treatments the stocking rate was 6·2 cow/ha and fertilizer N was applied at a rate equivalent to 2·5 kg/ha per day.

Individual records of milk yield, milk composition and live weight were kept, the quality of herbage available was estimated weekly, and individual intakes were estimated by faecal index methods at approxi- mately monthly intervals. In 1969 the cows remained on the same treatment throughout the season and in 1970 a reversal design was adopted.

The quantities and quality of herbage available per cow were generally similar under both treatments. There were no significant differences in feed intake, or animal production between the treatments. High yields of 11900 kg milk and 28 000 Meal ME per hectare were recorded.

The data were collected from seven dairy farms in India on Sahiwal × Friesian crosses over a period of 30 years (1937–1966). There were nine distinct genetic groups (grades); Sahiwal, 1/8, 1/4, 3/8, 1/2, 5/8, 3/4 and 7/8 Friesian and Indian Friesians (animals having 15/16 or more Friesian blood).

Least-square analysis of variance was run to examine the effects of grades, farms, periods, seasons, and breed of sire on body weights at birth, 19 weeks, 12 months and 3 years. The effects due to grades, farms and periods were significant for all the traits. The effects due to season and breed of sire were significant for all traits except for weight at 3 years.

The least-square constants showed that, with increase in Friesian inheritance, birth weight increased considerably, while the increase in body weights at 19 weeks, 12 months and 3 years was only marked up to the 1/2-Friesian group.

The effects of dietary treatments designed to increase the incidence of diarrhoea were studied in two experiments with Ayrshire calves. In the first experiment, as a control a normal milk replacer was fed to calves at a rate of 170 g powder in 1·71 litre of water given twice daily. When the amount of powder was increased to 425 g in 1·71 litre of water, faeces consistency and intake of dry concentrates, decreased. When the amount of water was increased to 2·85 litre with 170 g of milk replacer each feed, the consistency of the faeces decreased. Inclusion of 2 % sodium citrate or 50% whey powder in the milk replacer tended to reduce live-weight gain in the period 0 to 32 days but did not increase the incidence of diarrhoea.

In the second experiment, the normal milk replacer and the replacer containing 50% whey powder were both given at 425 g powder in 1·71 litre water each feed without adverse effects upon performance. Clinical diarrhoea was virtually absent in both experiments and it was concluded that the homebred calf reared carefully in a good environment and given antibiotics prophylactically for 3 days is relatively resistant to substantial changes in the composition and methods of feeding of milk replacers.

1. Twenty-four Ayrshire cows were used in a changeover experiment to investigate the effects on milk yield of offering concentrates at three rates of feeding (220, 250 and 290 g SE/kg milk) and two nutrient densities (63·5 and 72·5 SE). Nutrient density had no effect on milk yield but there was a positive relationship between rate of feeding and persistency.

2. A regression equation relating log10 (—log10 persistency) to the rate of feeding (g SE/kg milk) was fitted to the data and accounted for 92·3 % of the variation between treatments.

Eight Border Leicester castrated male lambs (wethers) were fostered on to 8 Merino ewes (MB) at birth, and 10 Merino wethers were fostered on to Border Leicester ewes (BM). Two additional groups of 10 wethers consisted of Border Leicester lambs (BB) and Merino lambs (MM) reared by their natural mothers. The lambs and ewes grazed together and growth rate, and milk and herbage consumption of the lambs were recorded.

Live-weight gains to weaning at 74 days were 275, 245, 204 and 184 g/day for groups BB, MB, BM and MM respectively. Lambs reared by Border Leicester ewes received more milk and consumed less grass than lambs of similar genotype reared by Merinos, but BM lambs consumed less milk than BB lambs. Merino lambs also consumed less grass than Border Leicester lambs.

It was concluded that Merinos grew more slowly than Border Leicester lambs primarily because of their lower voluntary food intake. The lower milk production of Merino ewes was considered to be of secondary significance since if herbage is available ad libitum, lambs with high voluntary intakes will compensate for the lack of milk by consuming more forage.

An attempt to induce ewes to lamb at intervals of 8 months using high levels of artificial daylength applied in a non-light-proofed building was carried out over a period of 2½ years.

Thirty each of Dorset Horn, Clun Forest and Kerry Hill ewes were used. There were two different light regimes, both involving abrupt changes in daylength, all of which occurred at daylengths in excess of natural daylength. In both treatments the artificial daylength was abruptly increased to 22 hr at conception. In one treatment there was an abrupt decrease in daylength at parturition whilst the other treatment was an abrupt decrease in daylength approximately 50 days before parturition. The size of the daylength decrease possible was determined by the natural daylength at the time the decrease was applied and at the estimated time of onset of next oestrus.

The majority of ewes lambed four times in the 2½ year period, parturition occurring in March-April 1969, December 1969-January 1970, September-October 1970 and May 1971. The Clun Forest ewes produced 38, 29, 20 and 22 lambs at the four lambings, whilst the Dorset Horn ewes produced 41, 35, 26 and 25 lambs at the four lambings. Only 11 of the Kerry Hill ewes lambed throughout the whole experiment producing 3, 5,0 and 7 lambs at the four lambings. It is suggested that the changes in daylength at these high levels controlled the breeding activity of the ewes.

Wafers made from dried ryegrass were fed to castrated male lambs in two experiments of 40 and 30 days' duration. In the first experiment, wafers of chopped feed were prepared which varied in unit density from 0·7 g/ml to 1·0 g/ml. In the second experiment, coarsely ground forage was passed through a ram press with a die of 5 cm diameter and the extruded material was then split into quarter and half wafers or left unsplit as whole wafers. Voluntary intake was measured throughout each experiment.

For the wafers with density of 1·0 g/ml the daily intake of organic matter was 75 g/kg LW0·75 as compared with 64 g/kg LW0·75 for the wafers with density of 0·7 g/ml but this difference was not significant. There was a significant (P < 0·05) interaction between wafer density and sub-period of the experiment. The daily intake of low-density wafers, expressed in g/kg LW0·75, increased throughout the experiment, whereas that of the higher density wafers did not change.

There was no overall effect of package size on organic matter intake, but the interaction between package size and sub-period of the experiment was significant (P · 0·001). The intake of the whole wafers was initially lower than that for the other treatments, but increased throughout the experiment, whereas the intake of the quarter and half wafers remained steady.

A study was made of the effects of different levels of nutrition between 3 and 6 months and between 6 and 12 months of age on subsequent growth to mature size, longevity and lifetime production of Scottish Blackface female sheep on a hill farm. Treatment differences between 3 and 6 months were small and resulted in only a 3 kg live-weight advantage for the animals receiving a high level of feeding. Treatment differences between 6 and 12 months were considerable and resulted in a mean 14 kg advantage for animals receiving a high level of feeding. Those animals remained significantly heavier until 42 months of age and significantly larger, as depicted by skeletal measurements, until 54 months of age.

A high level of feeding between 3 and 6 months of age had no significant long-term effect on wool growth ewe survival or lifetime lamb production, whereas a high level of feeding between 6 and 12 months of age had a significant positive effect on the mean number of lambs born per ewe over five lamb crops. This effect was not maintained to weaning, due to an apparently greater lamb mortality. Treatment effects on ewe survival and on flock lifetime production, although considerable, were not statistically different.

It is concluded that any advantages of improved feeding during rearing were largely lost through the inadequacy of the adult nutritional environment and only when the latter was not limiting would higher standards of rearing be justified.

Male Sudan desert sheep and goats of three age-groups, young, yearling and mature, were used to compare some of their carcass characteristics when reared under range conditions and in feedlot. Carcass traits studied were the dressing percentage, the percentage of bone in the carcass and wholesale cuts, chemical composition of meat, and muscle fibre thickness. An organoleptic test was carried out on meat from the fattened animals.

The dressing percentage, percentage of edible meat, chemical fat and muscle fibre thickness were increased by fattening in the feedlot. Increases were also observed with advancing age, but the effect was more marked in fattened than unfattened animals. Goats yielded a higher carcass weight per unit live weight than sheep, the stomach content and skin weight relative to live weight being greater in the sheep than in the goats. Goats were shown to possess less chemical fat and thicker fibres than sheep. Taste panel scores revealed a highly significant (P < 0·005) treatment effect on tenderness and a significant (P = 0·5) treatment effect on juiciness in relation to age and species. Taste panel flavour scores were not associated with animal age or species.

Social organization in the pig is based on the dominance hierarchy. Hitherto this has been assumed to be stable but present studies have shown that spontaneous changes of rank can occur. It has proved almost impossible, however, to alter social ranks artificially. Pig groups also show considerable fluctuations from day to day in the level of aggression at feeding periods. This variation cannot be correlated with the occurrence of sexual cycles in the females or variations in the general level of group activity.

In a Welsh Mountain flock of 104 ewes, 51 had their tails tied aside for the duration of one oestrus. Tail-tying significantly increased the conception rate at this oestrus. It also significantly increased the mean litter size of ewes superovulated with 1000IU PMSG from 1·70 to 2·33. It is suggested that the most likely reason for this increase in the number of multiple births was an increased number of ejaculations per oestrus. Perinatal mortality was high in lambs from multiple births. The effect of PMSG and tail-tying was to increase the lambing percentage from 77 to 120.

Thirty British Friesian steers were given either a pelleted diet of bruised barley and ground barley straw (70:30) containing 9·4% crude protein (N × 6·25) or a similar diet with urea or soya bean meal added to raise the crude protein content to 15·0%. Up to a live weight of 200 kg, the growth rate and feed conversion of steers were improved by supplementary nitrogen and urea provided this as effectively as soya bean meal. Beyond 200 kg there were no differences in growth rate or feed conversion between any of the experimental treatments.

The stage of ossification of various, normally visible, bones were examined by two independent observers on 134 sides of beef from cattle of known dates of birth. These examinations failed to produce sufficiently consistent values to allow the ages of these cattle at slaughter to be determined from their skeletal characteristics within an accuracy of 3 months.