Genetic responses from selection strategies based on selection among all potential parents of the following year's crop were compared with those based on selection only within progeny crops. In the former group of strategies the duration of stay of an individual in the breeding herd is determined by the estimated breeding value of that individual, whereas in the classic model selected individuals of the same sex all remain in the breeding population for the same period. The former group of strategies, termed ‘parent selection’ strategies, were consistently superior in genetic response to the progeny selection strategies with a very large relative advantage under some conditions. The superiority of strategies that make more efficient use of genetic differences between own or parental age was greater among parent selection strategies than among progeny selection strategies. Similarly, increases in female to male mating ratios, fertility and survival rate generally resulted in larger increases i n genetic response in the more efficient strategies. The effect of changes in number of male and female age groups differs markedly between strategies.

A quantitative model of energy intake and utilization by ewes in various physiological states grazing a pasture of perennial ryegrass is presented. The model consists of two major components: a forage intake component incorporating forage availability and maximum potential intake as affected by age of the grass and physiological state of the ewe; a second component representing energy utilization by the ewe for maintenance, growth, pregnancy, lactation and wool growth.

The behaviour of the model was examined and found to be consistent with published information. Partial validation of the model was accomplished by comparing the model output with actual experimental data not used to construct the model.

In one experiment, 227 female sheep in two age groups were reared on low (107 sheep) and high (120 sheep) levels of nutrition from birth to 12 mo of age after their dams had been given low and high levels of nutrition during the last 6 weeks of pregnancy. From 12 mo, 47 of the low-level sheep and 51 of the high were grazed on poor hill pasture to provide a low-level adult nutritional environment while 46 of the low level and 52 of the high were grazed on good quality pasture to provide a high-level adult nutritional environment. These treatments were continued for 5 production years for one age group and 4 production years for the other.

Live-weight differences associated with rearing treatment persisted throughout the experiment in both adult environments. Lifetime lamb production was significantly affected by both rearing and adult treatments.

The effects of these treatments on reproductive potential were examined in a second experiment by determination of ovulation rate when the sheep were brought into uniform good body condition at mating after the end of the first experiment. Potential ovulation rate was significantly affected by rearing treatment but not by adult treatment, although live weight and body size were affected by both.

It is concluded that while reproductive potential is related to body size it may also be influenced by the pattern of early growth during unidentified developmental stages.

Commercial lamb carcasses were surveyed in 1971/72 as part of the development programme for a national classification scheme for Great Britain. Carcasses of 421 castrated male lambs were evaluated and their left sides dissected. The sample comprised seven groups representing the main breed types in the national population. Within these groups, lambs were selected from three regions on each of three occasions corresponding with the beginning, middle and end of the normal slaughtering season for each group.

The overall means (±SD) for side weight and percentage subcutaneous fat in the side were 8·3 ± 1·8 kg and 12·6 ± 3·2 respectively. When adjusted to the overall mean subcutaneous fat percentage (constant % SF), the breed-type side weight means ranged from 6·4 kg (Welsh Mountain) to 10·8 kg (British Longwool). At constant% SF, lambs slaughtered earlier in the season were lighter than those of the same breed type slaughtered later.

Significant differences (P<0·05) were recorded between breed-type groups in fat distribution and lean/bone ratio. These were reflected as important differences in percentage lean at constant % SF, group means ranging from 54·5 (lambs by Suffolk rams out of Scottish Halfbred ewes) to 56·7 (lambs by British Longwool rams out of hill ewes).

When lambs of the same breed type from different regions were compared at constant % SF and constant side weight, there were few important differences in carcass characteristics.

The results are discussed in relation to the classification scheme which has since been introduced by the Meat and Livestock Commission.

Seventy-two lactating dairy cows were used in a 4x 3 factorial experiment to assess the effects of the protein content of concentrates on the response to concentrate feeding level. Concentrates containing 12,18 and 24% crude protein in the dry matter were given at each of four levels of feeding: 0·25, 0·36, 0·47 and 0·58 kg/kg milk for a 7-week experimental period. Milk yield, milk energy output, liveweight change and the percentage solids-not-fat and protein in the milk were significantly affected by both the level of concentrate feeding and the protein content of the concentrate. Response curves for the effect of level of concentrate feeding on milk output and liveweight change were computed for each of the three protein levels, and although there was no significant interaction between these two main factors the benefit from increased protein in the concentrate was shown to depend partly on the level of feeding at which the concentrate was used. At low feeding levels the response to an increase in protein content of the concentrate tended to be smaller than that obtained at high feeding levels. At no stage was there any marked response to protein content in the concentrate above 18%. The response curves have been used to demonstrate the interchangeability of level of concentrate feeding and protein content of the concentrate as means of supporting a given level of milk production.

In a long-term experiment, the performance of British Friesian, Ayrshire and Jersey cattle and their first and second crosses was evaluated over 526 first and 477 second lactations. Live weight at 18 mo of age and the ratios of lactation yield to live weight were examined as well as milk yield and composition of milk.

Percentage heterosis in each combination of two pure breeds was estimated. For the three first crosses combined, heterosis for 305-day milk yield averaged 6·4% in first and 3·7% in second lactations. Heterosis in live weight at 18 mo averaged 2·9%; none was found in the Ayrshire × British Friesian crosses, but the others averaged 4·7%. Second crosses overall had an average live weight near to that of first crosses, but their milk yields were lower and, in the combined lactations, approximately equalled the purebreds' total.

Components of variance due to breed and environmental factors were calculated. Breed accounted for 50% of variance in live weight at 18 mo but less than 10% of variance in milk yield.

The influence of fertility on heterosis is discussed with reference to earlier findings, and economic implications are considered.

Mixtures (1: 1) of whole-crop dried wheat or barley, with peas or beans as a nitrogen supplement, were fed ad libitum to four groups of 16 6-mo-old male castrate sheep for a period of 42 days in 1971. The mean dry-matter consumptions (kg/day) and live-weight gains (g/day) were: wheat+peas 1·69, 264; wheat+beans 2·05, 288; barley+peas 1·80, 282; and barley+beans 1·81, 229. This experiment was repeated in 1972. The differences between treatments were non-significant.

Dried pelleted whole-crop maize was fed ad libitum to British Friesian steers for 90 days, in conjunction with molasses and one of three protein sources, soya bean meal, urea or whole crop beans. The dry-matter intakes (kg/day) and live-weight gains (kg) were: maize+soya 9·68, 1·26; maize+urea 9·38, 10·4; and maize+beans 9·72, 1·15.

Dried maize in conjunction with urea, whole crop beans and dried grass to supply crude protein levels between 9 and 11% was fed to 48 Hereford × British Friesian steers. The dry-matter intakes (kg/day) and live-weight gains (kg/day) were: whole crop maize, 8·35, 0·87; whole crop maize+urea, 9·55, 1·0; whole crop maize+whole crop beans, 7·44, 0·70; whole crop maize+dried grass, 8·94, 1·15; whole crop maize+rolled barley, 9·30, 1·23; and dried grass + rolled barley, 9·24, 1·21. Although there was no significant difference in live-weight gain between the treatments, two levels of performance were apparent.

One-hundred-and-seventy British Friesian heifers were individually fed from 6 weeks before to 6 weeks after artificial insemination on a diet of 90% maize silage and 10% decorticated groundnut in the dry matter. Three levels of nutrition were compared: 48, 55 and 62 g/day of digestible organic matter/kg live weight0·73, which gave mean growth rates of 0·34, 0·50 and 0·68 kg/day respectively. Pregnancy rates to first service were not significantly different, averaging 67, 69 and 65%. Heifers were given a body condition score at the time of insemination, and pregnancy rates corrected for level of nutrition were 42,72, 70 and 63% for heifers in poor, moderate, good or very good condition. For heifers in poor/moderate condition increasing the level of nutrition increased pregnancy rates and for those in good/very good condition pregnancy rates declined, but these effects were not significant.

Data were available from 11 474 gilts performance-tested on 137 farms. The data were analysed to provide pooled within-farm sire component estimates of h2 and genetic correlations.

Estimates of h2 of 0·43 ± 0·10, 0·44 + 0·09 and 0·49 ±009 were obtained for weight for age, C + K and shoulder fat depths respectively in 3583 gilts in 49 Large White herds. Comparable Landrace values derived from 2810 gilts in 43 herds were 0·44 + 0·11, 0·40 ±0·10 and 0·51 ±0·12 respectively. Estimates were consistent from herd to herd.

Estimates of h2 made on data from 5081 crossbred pigs, of mainly Large White and Landrace parentage, varied with herd from 0·10 to 0·40 for weight for age and from 0·08 to 0·96 for shoulder fat depth. Significant differences between breeds were found for the genetic correlations between weight for age and the fat depths, and between shoulder and mid-backfat depths. The variance was partitioned into various components. For index score, the within-herd variance averaged 73% of the total variance.

1. In a comparative slaughter experiment, 12 female pigs (six at 80 kg and six at 100 kg) were allocated at first oestrus to each of five treatments: Treatment 1, initial slaughter, or Treatments 2, 3 and 4, mated and given 19·5, 25·8 or 32·1 MJ ME/day for the last 100 days of pregnancy, or Treatment 5, not mated (virgin) and given 25·8 MJ ME/day over a similar period. Pigs on Treatments 2, 3, 4 and 5 were given the same amount of protein and were killed about 123 days after first oestrus. Piglets were removed at birth.

2. There was no evidence of any special effect of pregnancy in the stimulation of permanent maternal growth. The average live weight, ingesta-free body and carcass gains of the pair-fed, mated and virgin gilts (±SE of difference) were, respectively, 32·3 and 36·9 + 2·8, 27·5 and 27·6 ± 3·4, and 26·0 and 26·7 + 2·1 kg. There were no statistically significant differences between these two treatments in carcass composition, specific gravity or backfat thickness. The mated pigs had lighter livers (P < 0·01) and heavier reproductive tracts (P < 0·05), and lost about 9 kg within a week of parturition.

3. Increasing energy intake increased piglet birth weights (P<005) but had no effect on the number of piglets born. The pigs that were initially heavier (100 kg v. 80 kg) had 1·5 more piglets, though this was not statistically significant (P<0·1).

4. Although there was no special effect of pregnancy on permanent maternal growth, the conversion of food by the once-mated pig was very efficient if an allowance was made for the food cost of producing the piglets.

Twenty-eight Landrace × (Landrace × Large White) young female pigs (gilts) were allocated to one of four treatment groups at about 75 kg live weight. The gilts were mated at second oestrus and suckled their piglets for 35 days. Gilts were slaughtered 14, 21, 28 or 35 days after weaning. The ‘effective’ food conversion ratios (i.e. total gilt plus litter feed intake (kg) divided by total gilt weight gain plus litter weaning weight) of the four groups were 5·22; 4·70; 4·62; and 4·56 respectively. The differences were not statistically significant.

Neither carcass weights nor individual joint weights were significantly affected by treatment. Samples of 11 middle sections from the 14-, 21- and 28-day slaughter groups were boned and cured. Bacon produced from the streak blocks had poor colour, texture and moisture-holding capacity. It was concluded that streak blocks from gilts that had farrowed were unsuitable for curing as streaky bacon.

In an analysis of data from 211 litters in a Pietrain herd, litter size at birth and at weaning was lower than is generally found in indigenous breeds. Mortality in growing and breeding pigs was high with circulatory failure accounting for 23% and 95% of deaths respectively. Pietrains, fed ad libitum over the live-weight range 27 to 87 kg, grew more slowly by 130 g/day than contemporary Large Whites on the same feeding regime, had higher killing-out percentages (by 3 to 4 units) and larger eye muscles in cross-section (by 9·9 cm2) but tended to have higher feed conversion ratios. There was no breed difference in backfat measurements but Pietrain carcasses were shorter by 83 mm and their muscle quality was markedly inferior to that of the Large Whites.