Twenty-four young female sheep, 12 Galways and 12 Fingalways, were used to study the effects of breed and level of nutrition on pre-puberal plasma luteinizing hormone and progesterone concentrations throughout the oestrous cycle and during the first 35 days of pregnancy. The sheep were spring-born and were housed at the end of July when they weighed approx. 34 kg. Equal numbers of animals from each breed were fed on a concentrate diet either ad libitum or at a restricted level that was slightly above their estimated maintenance requirements. The Fingalways reached puberty 1 month earlier than the Galways (11 October ±4 days v. 10 November ±5 days). The restricted animals tended (P < 0·10) to reach puberty later than those fed ad libitum (1 November ±5 days v. 20 October ±4 days). Breed differences in the mean level of plasma progesterone on any day of the oestrous cycle were not significant. The mean daily plasma progesterone concentration was similar for the animals on the two planes of nutrition throughout the cycle, with the exception of day 11 when those fed ad libitum had a significantly higher level (P < 0·01). Plasma progesterone levels in the pregnant females between 5 and 35 days post mating were generally higher in Fingalways than in Galways and were not influenced by plane of nutrition. Plasma luteinizing hormone levels tended to be higher, during the 7 weeks before puberty, in Fingalways than in Galways and in animals fed ad libitum than in those on the restricted level of feeding.

In an experiment carried out on five farms in 3 years and comprising 1221 annual weight records of adult ewes, an increased level of feedingwas comparedwith feeding according to farming practice. The higher feeding level started 6 weeks before mating andfinishedjust before lambing. Significant effects of feeding level on ewe live weight and body condition, ewe fecundity and the birth and weaning weights of lambs were found.

Ewe fecundity increasedlinearly with ewelive weight at 8 weeks prior t o mating (b=1·05 lambs/kg per 100 ewes), curvilinearly with ewe body condition score at 8 weeks before mating (maximum at score 3·32) and curvilinearly with weight gain from 8 to 2 weeks before mating (maximum at 2·3 kg gain). A 10% range in live weight, condition score and live-weight gain resulted in a predicted fecundity range of 21 lambs per 100 ewes alive at lambing. Changes in weight and condition score during the mating period did not affect ewe fecundity.

(1) Voluntary intake of forage was measured in 31 Scottish Blackface ewes for the last 14 weeks of pregnancy, throughout lactation, and 15 weeks subsequently. During pregnancy 15 ewes were given hay and 16 dried grass (apparent dry-matter digestibilities in vitro were 51 and 69·6% respectively). All ewes were given the same dried grass during lactation and after weaning (digestibilities 75 and 73% respectively). Body fat was estimated from tritiated water space.

(2) Pregnant ewes consumed twice as much digestible dry matter from dried grass (1028 g) as from hay (502 g). Intakes of ewes with twin and single foetuses were similar.

(3) During lactation the mean daily intakes of dried grass were 2278, 2610,2612 and 2722 g dry matter respectively, for ewes that had been given dried grass in pregnancy and had single and twin lambs, and for those that had been given hay and had singles and twins. Differences between dried grass and hay were consistently significant (P < 0·01).

(4) After weaning the intakes declined rapidly but their ranking remained similar.

(5) Lamb birth weights were affected by nutrition during pregnancy, lamb growth rates within twins or singles were not influenced by maternal nutrition during pregnancy or lactation. Differences in ewe intakes during lactation were reflected in ewe body-weight changes.

(6) For all ewes, up to 64% of the variation in intake during lactation could be related to factors prevailing before and at lambing (pregnancy diet, ewe weight and fat content) and during lactation (lamb gain and ewe weight change).

The trial compared 50 purebred Canadian Holstein and 46 British Friesian steers. The cattle were reared on a grass/cereal system designed to achieve an overall target live-weight gain of 0·82 kg per day from weaning to slaughter at 18 months of age. The cattle, born i n September 1973, grazed during the summer of 1974 and were slaughtered out of yards from January to July 1975. Equal numbers of each breed were allocated at random to three slaughter weights (477, 500 and 523 kg live weight). The Meat and Livestock Commission measured and classified all carcasses. In addition, a sample was cut according to commercial specifications. There were no significant differences in slaughter age or live-weight gain between the breeds. A significant difference (P < 0·05) in killing-out percentage was found in favour of the British Friesian as was a small (4%) but highly significant difference (P < 0·001) in carcass gain.

The Canadian Holstein carcasses were considerably longer (P < 0·001) with less eye-muscle area (P < 0·001). The British Friesian carcasses contained 0·7% more saleable meat (P < 0·001) and 1% less bone (P < 0·01). There were no significant differences in fat trim. There were large differences in live appearance and carcass classifications, the British Friesians being of superior beef conformation.

Regressions of progeny growth rate and slaughter age on bull weight for age (400-day) record were estimated from data on 63 Hereford bulls. The bulls had been weight-recorded by the Meat and Livestock Commission and subsequently progeny-tested by the Milk Marketing Board on 1300 crossbred British Friesian steers. Interpretation of the regressions led to estimates of 16 to 35% (± 9 to 17%) for the ‘effective’ heritability of400-day weight. These are lower than the average in the literature and may indicate that farm records with natural rearing of bulls are less reliable than tests with uniform rearing and feeding conditions.

Simple algebraic models of milk yield, live-weight change and food requirement are developed for use in a programmable desk calculator. The unit of time used is a week and of energy a megajoule. An algebraic equation for milk production is used to define potential energy output. An expression using live-weight gain describes the demand for energy for the recovery of weight lost and the requirements of pregnancy. The use of appropriate conversion factors enables the necessary dietary input to be calculated. The model also takes account of restricted feeding by allocating dietary energy to milk production and live-weight gain in proportion to their potential demand. Ten decisions relating to milk yield, live-weight loss and live-weight gain are required by the model.

Recurrence relationships are used to relate breeding values of age-sex classes from different time periods. Their application to single-stage (progeny) and multi-stage (parent) selection is demonstrated. These relationships enable definition of the effect of age structure, initial genetic differences between age groups, and the extent to which allowances are made for these or later genetic differences between age groups. The expressions derived show that, given initial genetic differences between age groups, subsequent progeny means will fluctuate even under completely random selection. Using these means as a basis for measuring responses to selection, it is shown that there can be selective effects where selection is at random within parental age classes. A careful definition of the alternative programme is therefore important in interpreting results of selection experiments and in investment appraisal of selection programmes.

These models were then used to describe economic returns from parent and progeny selection programmes and from programmes in which returns are realized in more than one age group. A further extension of the model accommodates the effects of finite population size on returns through its effects on genetic variance.

By separating the (constant) within- and (fluctuating) between-group components of the selection differential within the recurrence relationships a number of computational problems are overcome.

Genetic analyses were carried out on 935 Large White and 767 Land-race boars performance-tested from 25 kg to 80 kg live weight under individual andad libitumfeeding conditions. Traits analysed were daily weight gain (GR), feed conversion ratio (FCR), daily feed intake (FI) and subcutaneous fat depth (F). Genetic parameters were estimated from sire components obtained by conventional hierarchical analyses of variance and covariance.

Large White pigs grew faster, had higher intakes of feed and utilized it more efficiently than Landrace. There was no breed difference in fat depth. Heritabilities, pooled over both breeds, were 0·36 ± 0·10, 0·27±0·08, 0·62±0·13 and 0·47±0·11 for GR, FCR, FI and F respectively. Noteworthy among the correlation estimates were strong and positive correlations between FI and the other three traits. A positive genetic correlation between GR and FCR arose from a very high coefficient of genetic variation for FI.

Selection indexes emphasising efficiency of lean growth were constructed using some or all of the 4 traits as measured variables. Their use is expected to reduce FI and GR on feeding ad libitum. The results indicate the need to select pigs on the same level of feeding as that used in production.

Carcass measurement and dissection data for 1176 pigs were used to examine the suitability of different measurements for predicting carcass lean percentage in commercial classification and grading. Three carcass weight groups with means of 47, 72 and 93 kg, were analysed separately. Similar results were obtained and those for the 72 kg group are given in this abstract. The s.d. of percentage carcass lean in this group was 3·84. Visual conformation score, carcass length and m. longissimus depth were valueless as predictors (residual s.d. of carcass lean percentages were 3·84, 3·77 and 3·80 respectively). Fat thickness measurements taken on the exposed surface of the split carcass provided a less precise prediction (best residual s.d.=2·89) than fat measurements taken over the m. longissimus using the optical probe (best residual s.d. = 2·20). The probe measurements were taken 6·5 cm from the dorsal mid line at fixed positions along the carcass from 4/5th cervical vertebrae to the 5/6th lumbar vertebrae. Precision of prediction showed a regular pattern with highest levels at the last rib (the P2 position currently used in the Meat and Livestock Commission classification scheme) and at the 13th rib, with lower values anteriorly and posteriorly. These two measurements in multiple regression were more precise than any other pair of measurements for predicting both percentage carcass lean (residual s.d. = 2·16) and the lean percentage in individual primal joints.

Linear measurements and sample joint dissections were examined as predictors of carcass lean percentage using data from 1060 carcasses comprising 108 subgroups (nine genotypes × two sexes × two feeding regimes × three slaughter weights). The potential predictors were examined in terms of precision, cost and the stability of their regression equations.

Probed fat thickness 6·5 cm from the dorsal mid-line at the last rib (P2) was the individual measurement giving the most precise prediction (residual s.d. pooled within subgroups = 2·21 percentage units; r = −0·72). The square root transformation (P2½) gave stable regression slopes over levels of all four factors, though there were important differences in intercepts between genotypes, sexes and slaughter weights.

The precision of prediction from the lean percentage of sample joints was generally improved by including P2½ with each in multiple regression. Used in this way with current Meat and Livestock Commission costings, the ham joint offered the best compromise between cost and precision (pooled residual s.d. = 1·25 percentage units; R = 0·92). Regression slopes on each joint were stable for all factors except for those on the rump back and rib back joints which varied by feeding regime. Intercepts varied by genotypes for all joints except the ham, while intercept differences for feeding regimes were small or non-significant.

The results confirm the value of the ‘sub-sampling with regression’ procedure for predicting carcass composition.

Serum thyroxine levels were observed in 15 Soay ewes and seven rams for 1 year. From June to November the ewes had thyroxine concentrations 40% higher than the rams, and the level in both sexes increased from December to May, demonstrating a seasonal cycle in thyroxine level that could be associated with the annual wool growth cycle. To hasten the onset of wool follicle activity, nine Wiltshire sheep were given thyroxine implants in autumn when the follicles normally become inactive, but only a slight increase in follicle activity occurred. Eighteen Soay sheep were given thiouracil orally for 1 month in spring in an attempt to block a possible seasonal increase in thyroxine secretion that might be associated with the normal regrowth of wool, but no retardation of the spring regrowth was obtained.

The outcome of 37 563 matings of pigs in the Padana Valley of Northern Italy over a 4-year period was studied. The results show a marked reduction in fertility in the hottest months when a fertility rate of less than 50% was found.

Separate studies of sows and gilts show a similar development in the dynamics of the phenomenon, with, however, a time change.

The performance of progeny sired by Piétrain × Hampshire, Hampshire × Large White and Large White boars was compared over the live-weight range 28 to 65 kg. Feeding was twice daily to ‘appetite’ up to a maximum of 2·27 kg meal/day. Breed of boar did not influence rate or efficiency of live-weight gain, mid-line or internal fat depths nor muscle quality. Progeny of Pietrain × Hampshire and Hampshire × Large White sires, compared with those of Large White boars, had higher killing-out percentages (1·4 and 1·2 units) and larger eye muscles in cross-section (19·3 and 11·6%). Significant but small breed differences were noted in joint proportions of the carcass.

Three groups, each of 12 West African Dwarf ewes weighing 17·5 kg at mating, had mean daily digestible organic matter intakes over the final 7 weeks of pregnancy of 249, 435 and 538 g/ewe (Treatments I, II and III respectively). Mean plasma concentrations in Treatments I, II and III were: glucose, 384, 527 and 538mg/l; non-esterified fatty acids (NEFA), 672, 441 and 427 μequiv/1; and ketones, 44·1, 30·5 and 27·3 mg/1. The differences in all metabolites between Treatment I and Treatments II and III were significant (i) <0·001). The mean birth weights of the single lambs from ewes in Treatments I to III were 1·1, 1·7 and 1·8 kg, those from ewes in Treatment I being significantly lighter than those from ewes in Treatments II and III. Lamb birth weights were significantly and positively related to energy intake and to plasma glucose concentrations, and significantly and negatively related to plasma NEFA and ketone concentrations.

The results indicate that the effect of undernourishment during late pregnancy in West African Dwarf sheep may be greater than in European breeds, and that the concentrations of blood metabolites may have a use in monitoring and regulating nutritional state in this genotype.

Crossbred ewes in which the oestrous cycles had been synchronized by the use of progestagen-impregnated vaginal sponges were mated with Suffolk rams either as a single group of 68 ewes (G) or as four subgroups of 17 ewes (S) in 2 years. Ram:ewe ratio was 1:17 in both treatments. G rams performed more mounts and services than S rams. Non-return rate and lambing rate was better for S than G ewes, especially in Year 1, when fewer G ewes were served. Mating activity was greater in Year 2 than Year 1; prolificacy was also markedly improved.

The use of the ‘anti oestrogen’ clomiphene to increase the ovulation rate of the sheep was investigated. Although in most trials clomiphene had a positive effect, this effect was small and inconsistent—especially n i relation to dose. It was concluded that the present approach would not lead to a simple technique to increase ovulation rate and the number of lambs born.

Two groups of 14 intact male weanling beef breed calves were each fed 1·45 kg oat hay daily and concentrates ad libitum, in a 158-day feeding trial. Group ‘a’ received 35 p.p.m. monensin in the concentrate. Group ‘b’ served as a control.

Average daily gain was 1·28 and 1·27 kg, dressing percentage was 58·1 and 58·0, daily intake of concentrate was 8·10 and 8·79 kg and conversion ratio of metabolizable energy into live weight was 74·53 and 81·30 MJ/kg for the monensin and the control group respectively. The percentage of acetic acid in rumen liquor was reduced and that of propionic acid increased very markedly. As a result the acetic acid: propionic acid ratio was reduced from 3·2 in the control group to 1·8 in the monensin group. No peculiarity in the response of intact male cattle to monensin was observed.