The present value of future genetic improvement in a closed population is taken to be proportional to the sum of discounted changes in the population mean. It is shown that, if T animals are available for selection and R is the discount rate per generation, the selection intensity which maximizes present value is a function of TR. If TR is greater than 15, under 10% of the population should be selected. It is then shown how T can be chosen to maximize profit if the value of genetic improvement and cost of testing are known. The theory is illustrated by a numerical example.

A beef progeny test was organized for 66 British Friesian bulls standing at AI centres in England. Eighty-one progeny groups averaging 8-5 steers each were reared on the farms of various co-operating organizations and their growth and carcass characters were recorded. Heritability of various measurements of gain varied from 0 to 0·34, of carcass conformation measures from 0·24 to 0·50, of muscle and fat percentage in the rib joint was 0·58 and 0·46 respectively, of other measures of fatness varied from 0·04 to 0·18, of weight of feet was 0·4 and of weight of hide 0·5.

Phenotypic correlations among beef characters gave no indication that carcass composition could be predicted from the external carcass measurements or subjective scores used in this study. Genetic correlations suggested that selection for weight for age would produce a fatter and more compact carcass. Genetic correlations were high and negative between milk yield (contemporary comparison) and weight for age, carcass weight/length, and blockiness of leg, and high and positive between milk yield, and carcass depth/length and percentage bone in the rib joint. Their standard errors were also high.

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.

A method of selection for two or more traits is investigated, where the selected individuals have to be the highest performing for any one trait, but not necessarily for several traits. The efficiency of selection of extremes is contrasted with the methods of culling levels and tandem selection. It is shown to be superior to culling level selection when the proportion of individuals retained for breeding exceeds 0·5. At higher selection intensities, the effectiveness of selection of extremes falls between that of culling levels and tandem selection.

1. Intake and digestibility were determined in a trial with 30 British Friesian calves given diets containing straw and either 3·2 (H), 2·9 (M) or 2·6 (L) Meal metabolizable energy per kg dry matter. For one series of diets the straw was chopped and the diets were made into cobs (1·0 × 2·5 × 4·0 cm); for a second series the straw was milled and the diets were made into pellets (1·0 × 1·25 cm)

2. Calves given the H diets containing either chopped or ground straw ate and gained less well than calves given M and L diets but there were no differences in feed intake or growth between calves given M and L diets.

3. The retention of feed residues was greater and the flow of fluid out of the rumen was less for cattle offered the H diet than the L diet.

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.

Based on the Agricultural Research Council's feeding system, equations were developed that allow the calculation of the metabolizable energy requirements for maintenance and weight gain by cattle, separately or combined. A general equation was developed for the straight-forward calculation of the total metabolizable energy requirements of growing and fattening cattle for any combination of body weight, rate of weight gain, age, level of muscular work, and metabolizable energy concentration of the diet. The estimates of energy requirement made by the use of this equation are in excellent agreement with those made by the Agricultural Research Council using an iterative method.

In addition to avoiding the awkward iterative process, the equations proposed are readily adaptable to computer use.

A simple feeding system for British Friesian dairy female calves was examined. It involved once-daily feeding of a fixed amount of milk substitute to calves from 5 to 32 days of age. A comparison was made between low-fat and high-fat milk substitutes each offered at three levels of feeding. Performance was measured for a further 28 days after weaning. There was a greater incidence of nutritional scours in calves given the low-fat diets and also in those at higher levels of feeding. Live-weight gain to weaning and to 60 days was greater on the high-fat diets and at the higher levels of feeding. Intakes of concentrates, hay and water were little affected by type or level of milk-substitute feeding. Total feed costs over the 8-week period were greater for the high-fat diets and for the higher levels of feeding, but feed costs/kg live-weight gain were similar for the low-and high-fat diets. The results indicate that low levels of a high-fat milk substitute should be offered when using a once-daily feeding system in order to ensure a low incidence of nutritional scours and relatively low feed costs.

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.

A study was made of the chemical composition and energy value of the whole, empty body, and of the relative size of certain organs in 11 ponies ranging in full-body weight from 81 to 259 kg. The fatty acid composition of various adipose tissues was examined in three ponies.

Ingesta-free body weight (Y) was found to be highly predictively related (r2 = 0·) to the full-body weight (X) in accordance with the equation, Y = 0·9678X–10·41. The gastrointestinal contents constituted 9·65 ± 3·44% of the full-body weight.

Despite the heterogeneous nature of the population examined and the small range in body fatness (6·6% to 18·9% of ether-extractable lipids in the empty body), 93·3 % of the variation in the concentration of body fat was associated with the variation in the concentration of water. The mean fat-free, empty body contained 70·7 % of water, 22·6 % of protein and 6·0 % of ash. Of the variation in the weights of water, fat, protein, ash and energy, 97·0%, 64·5%, 91·0%, 85·2% and 87·0%, respectively, were ascribable to the variation in empty-body weight.

For the conglomerate body, the calorific values of protein and fat were 5·381 and 9·311 kcal/g, respectively.

The ranges for the mean percentage contents of major individual acids in the fatty acids of five adipose tissues were: oleic, 29·5–33·3; palmitic, 26·1–28·0; linoleic, 17·1–24·0; palmitoleic, 6·2–9·5; and linolenic, 3·8–10·2. In one animal the depot fats contained relatively high concentrations (8·6–12·0 %) of linolenic acid; the concentrations of palmitic and linoleic acids were correspondingly lower than those of the depot fats of the other horses.

Tissue weights of various segments of the gastrointestinal tract were linear functions of body weight. About 87 % and 82 % of the variation in the weights of liver and kidneys respectively, was associated with the variation in full-body weight.

British Friesian dairy female calves were offered once daily either 320 or 480 g of milk substitute reconstituted in warm water. Weaning took place when calves were consistently eating 400, 650 or 900 g/day of concentrates. The number of days taken to reach weaning was greater with calves offered 480 g than with calves offered 320 g of milk substitute per day. Calves with a high initial live weight achieved the required concentrate intake and were therefore weaned sooner than smaller calves. A fall in energy intake occurred immediately after weaning and this was greatest in calves previously offered 480 g/day of milk substitute. The treatments had no significant effect on the total intake of concentrates and hay over the 8 weeks of the experiment. Increasing the amount of milk substitute from 320 to 480 g/day increased total feed costs, and feed costs per kg live-weight gain. Increasing the required level of concentrate intake at weaning also increased total feed costs and feed costs/kg live-weight gain, but to a lesser extent. The results suggest that it is preferable to feed dairy replacement calves only 320 g of milk substitute in a once-daily feeding system. Weaning can be successfully under-taken when the calves are consistently eating 400 g/day of concentrates.

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.

Sixty gilts by Landrace sires out of Large White × Landrace females were allocated on the basis of litter relationship and initial weight to six treatments involving the individual feeding of six dietary protein levels, 15·5%, 17·4%, 20·2%, 22·3%, 25·3% and 27·3% crude protein (CP), at a common energy level of approximately 3500 kcal/kg DE, from 23 to 59 kg live weight. Amino acid balance was maintained relatively constant with synthetic lysine, methionine and tryptophan. There were significant increases in growth rate and efficiency of feed conversion from the 15·5% to the 17·4% CP diets and significant reductions in both from the 25·3 % to the 27·3 % CP diets. Carcass lean by dissection increased linearly (r = 0·976) and fat decreased linearly (r = −0·992) with dietary protein increment over the full range studied. As best growth performance and maximum lean content were attained at different dietary CP levels, a combined measure of daily lean deposition was calculated; this indicated that maximum rate of lean growth and efficiency of feed conversion to lean were reached on the 22·3% CP level. In terms of efficiency of utilization of nutrients, maximum retentions of both DE and DCP in the edible part of the carcass appeared to be attained on the two lowest dietary protein levels.

1. Hormonexa, a bio-active feed supplement, was included at the rate of 3·1 kg per 1000 kg of the concentrate ration for finishing Hereford · Ayrshire steers.

2. Live-weight gain and food conversion were unaffected by the addition of Hormonexa.

3. The killing-out percentage of the supplemented animals was significantly lower and their carcass quality was slightly, but not significantly, lower than that of the control animals.

Forty-eight uncastrated buffalo male calves and 16 male Friesian × Egyptian Native calves were used in two experiments to study the effect of weaning age on the performance of calves fed on a high concentrate ration after the first four months of age. Half of the buffalo calves were weaned at 120 days of age on to 330 kg whole buffalo milk (late-weaned) and the other half were weaned at 45 days of age on to 100 kg whole buffalo milk (early-weaned). Cattle calves were weaned at 31 days of age on to 65 kg whole cow milk. At the age of 7 days the calves were put on a high energy ration of 74% total digestible nutrients and 13·5% digestible protein. After 125 days of age all calves were offered a high concentrate ration (cottonseed cake and rice bran in the ratio of 1:1, plus 2% Ca(OH)2 and 1% common salt). Air-dried roughage was supplied at a level of ½% of the body weight. Buffalo and f-Native calves reached a slaughter weight of 400 kg or more about 540 days of age. Three-quarters Friesian calves reached the same weight at 480 days of age. The high concentrate ration caused a high incidence of bloat in ¾Friesian bulls. The efficiency of feed utilization was better for cow calves than that for buffalo calves. The system of early weaning of buffalo calves proved to be most economical and saved about 26% of the cost per kg live-weight gain. Slaughter experiments indicated that fat deposition was greater in male cattle of 400 kg than in buffalo bulls.

Forty-eight gilts by Landrace sires on Large White × Landrace females were randomly allocated to eight pens and within pens to six treatment groups involving three diets and two levels of feeding from 23 to 59 kg live weight. All diets were formulated to have approximately the same ratio of digestible energy to crude protein (160 kcal DE/unit % CP) but different energy and protein concentrations: (A) 3500 kcal/kg DE and 21 % CP, (B) 3150 kcal/kg DE and 19% CP, and (C) 2800 kcal/kg DE and 17% CP. Amino acid balance was maintained relatively constant with synthetic lysine, methionine and tryptophan. The levels of feeding were such that the lower level of diet A allowed an intake of energy and protein similar to the higher level of diet B, and the lower level of B was similar to the higher level of C.

On the lower level of feeding, growth rate, efficiency of feed conversion and carcass fat content increased linearly with each increment in nutrient concentration; on the higher level of feeding growth rate and EFC increased from diet C to B but not from B to A, whereas carcass fat content increased linearly with diet from the lowest to the highest concentration. There was a non-significant tendency for the higher density diets at a similar level of nutrient intake to give better EFC and fatter carcasses than the lower density diets.

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.

Wafers of dried grass which differed in particle size (modulus of fineness) were prepared and fed ad libitum to cattle and sheep. The response of the two classes of stock to the feeds was similar. The digestibility of organic matter and of cellulose decreased with decrease in modulus of fineness (more fine particles) and this was associated with more rapid passage of the finely-milled material through the alimentary tract. Organic matter intake generally increased with decrease in modulus of fineness, but the pattern of response differed between experiments. In one experiment the difference in voluntary intake between feeds with modulus of fineness of 2·5 and 1·0 was less than 5% and not significant, but in a second experiment the intake of feed with modulus of fineness of 2·1 was 18% lower than that of feed with modulus of fineness of 1·0. In the first experiment the wafers broke down during handling and feeding with the result that the feed of low modulus of fineness was extremely dusty, but in the second experiment a binding agent was included and little breakdown of wafers occurred. It is suggested that dustiness may have restricted the intake of some of the feeds in the first experiment. Intakes of digestible organic matter did not differ significantly between treatments in either experiment.

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.