The incidence, heritability and effects on lactation performance of twinning in 185 000 calvings in Israeli-Friesian dairy herds are presented. The incidence of twinning in heifer and cow calvings (parities 2 to 6 inclusive) was 0·7 and 5·4% respectively in the large Kibbutz herds and 0·7 and 3·5% in smallholders' herds. The heritability estimate for 144 sires each with over 200 calvings of daughters of parity 2 to 6 was 0·024 ± 0·002. The range of incidence of twinning in these parities among sire progeny groups each with more than 1000 calvings was 2·8 to 7·8 %, apart from one sire, whose daughters had 12·2% twins. His progeny excelled in five characters and as an explanation it is suggested that a rare gene is segregating in the breed with a major effect on the endocrine system and associated pleiotropic effects.

In a population in which generations overlap the improvement in performance in successive years resulting from a single year of selection is not constant, for the genes from a group of selected individuals may take many years to pass through the population. A formal method is developed for predicting responses and discounted returns from improvement in populations with overlapping generations including, if necessary, generations of multiplication of breeding stock.

The method is based on a matrix which specifies the passage of genes between the different age groups and sexes. Simple matrix operations can be used to compute the proportion of genes in animals of both sexes and each age in the population at any time which derive from a group of selected animals at an earlier time. The response produced by these selected animals equals the product of their genetic selection differential and the proportion of genes deriving from them. Comparisons are made between responses predicted using this theory and the classical theory of uniform rates of response, and a method is given for computing the time lag of genes passing through the population.

The results are extended to enable computation of discounted returns from improvement.

Blood groups were determined for eight loci in a total of 742 sheep and for the Tf (transferrin) locus of 512 sheep comprising the Scottish Blackface, the Cheviot and the Welsh Mountain breeds and their crosses. Each breed and cross was represented by a non-inbred F2 generation and by three stages of inbreeding (25, 37½ and 50%). The loci for which all genotypes were distinguished were Hb, M, and Tf, and those for which partial description (including the recessive homozygote) was made were A, B, C, D, R and i.

There were large differences among the three pure breeds only for the frequencies at the Hb and Tf loci. The homozygote frequencies of the Blackface-Welsh and Cheviot-Welsh crosses were approximately half-way between their respective pure breeds. For the Blackface-Cheviot cross the homozygote frequency was considerably lower than either parental breed, suggesting that over all loci there were greater differences in gene frequencies between these two breeds than between the Blackface and Welsh or Cheviot and Welsh.

On average over all loci (except Tf) inbreeding had the effect of decreasing heterozygosity roughly in relation to expectation. Individually this effect was statistically significant only for ii, MbMb, HbAHbA and HbbHbb. For the Tf locus there was no average decrease in heterozygosity as a result of 25% inbreeding but thereafter there was a greater decrease in the purebreds than in the crosses.

There were small (< 0·20) but significant residual correlations between R-O-ii blood types and genotypes at several other loci, and between dd and the M system genotypes.

Sows which had lactated for 2 days were allocated either to a control group or to a group in which oestrus was delayed by the administration of 75 mg methallibure per day for 12 consecutive days, followed by an intramuscular injection of 1000 IU pregnant mare serum gonadotrophin. All sows were mated whenever they were receptive to a boar. The mean time to first mating for the control group of 12 sows was 7·3 days but the mean time to conception was 22·3 days. The corresponding figures for the 9 treated sows were 17·0 and 22·2 days. The treated sows had significantly (P < 0·025) larger litters than the control sows (11·7 v. 8·5). For those sows which conceived at the first post-weaning service, the delayed oestrus treatment increased litter size on average by more than four pigs per litter.

The onset and duration of oestrous and ovulatory activity in 26 Greyface ewes receiving natural daylength changes were determined. Precise ovulation data were obtained by endoscopy and repeated observations of the ovaries of all ewes were made on 10 occasions from 15 July 1971 to 27 June 1972. The natural breeding season for these Greyface ewes was, on average, 22 September 1971 to 18 February 1972. There were significant seasonal changes in the mean ovulation rate of the ewes which appeared to be inversely related to seasonal changes in natural daylength.

An abrupt decrease in artificial daylength of 11 -43 hr applied on 22 June 1971 to 28 similar Greyface ewes advanced their mean onset of oestrous activity by 23 days. The decrease in artificial daylength also advanced the occurrence of ovulation in these ewes. The highest mean ovulation rate of the ewes receiving the decrease in artificial daylength was at least as high as the peak mean ovulation rate measured in the control ewes.

At most stages of the experiment, the plane of nutrition had no significant effect on either oestrous or ovulatory activity in the ewes.

Fleece and skin samples were taken at intervals from birth to 86 days from Scottish Blackface lambs born into one line selected for hairiness, another for fineness and an unselected control line. The birthcoats in the Hairy line were longer than those in the Fine line, and had a predominance of Plateauo and Pi fibre type arrays, whereas P2 and P3 arrays predominated in the Fine line. The Hairy group had significantly more halo hairs and hairy-tip curly-tip fibres, but significantly fewer histerotrichs.

Most secondary follicles had fibres by 56 days, and development appeared to be faster in the Control than in the other two groups. The Hairy and Control groups had consistently high percentages of primary medullation, whereas the Fine group had a peak at 14 days. The Fine group had consistently low secondary medullation, whereas it increased in the Hairy and Control lambs, apart from a set-back at 28 days.

Up to 56 days the Fine group had no fibre shedding in the skin, and the Hairy group had most, although the incidence in any sample was less than 2%. There was more follicle inactivity at 86 days, and the Fine group then had most.

Three experiments were carried out to investigate the anabolic effects of androstenedione administered at various levels to female and castrated male lambs in one experiment and to entire and castrated male calves in two other experiments. Significant increases in growth were not effected in normally fed animals but a sex × dose level interaction was detected in female lambs. Under normal husbandry conditions androstenedione appears to have little application in meat production.

Two forms of supplement, barley or dried grass pellets (G) were given to weaned lambs, set-stocked at pasture, at three stocking rates (43·5, 60 and 87 per hectare). The supplements were given either from weaning or from the time growth rate fell below a target value. The supplements significantly increased the proportion of lambs reaching the target weight (35 kg) and G was significantly better than barley, but slightly more G was given. The substitution value of supplement for herbage was 0·48 ± 0·12 and there was no difference in this value between barley and G. The proportion of lambs reaching the target slaughter weight was not influenced by whether the supplement was offered at weaning or delayed until lamb growth declined below a target level. Increase in stocking rate depressed the herbage intake and growth rate of the unsupplemented control lambs.

In two experiments dried grass cobs or concentrate were offered to calves for 20 weeks after weaning. In Experiment 1 the grasses were harvested in the autumn and in Experiment 2 in the spring. Within each experiment three grasses differing in quality and two levels of concentrate were compared.

Dry-matter intake of the dried grasses highest in digestibility was lower than that of medium digestibility grass, and hardness of package, palatability and mould infestation were implicated as possible causal factors.

Live-weight gains by calves were closely correlated with digestible organic matter (DOM) intake. There was no significant difference between spring harvested grasses and concentrate in efficiency of utilization of DOM for live-weight gain but autumn grasses were used less efficiently than concentrate. At similar rates of gain the replacement value (dry-matter basis) of dried grass for concentrate averaged 142: 1.

Dried poultry waste was substituted for soya and fish meals in an intensive beef ration to ascertain if it was a satisfactory alternative source of nitrogen and in an attempt to reduce the cost of production. The experiment was conducted over 3 years in each of which groups of autumn-born British Friesian male castrates were taken from 150 kg live weight to slaughter at 400 kg on various rations. The treatment rations were made isonitrogenous with the control (14·5% crude protein). When the crude protein of the poultry manure was low (24%) it was necessary to include 27·6% in the diet and poor performances resulted. For manures of 30 % crude protein, or when urea was added to equalize the nitrogen content of diets containing up to 17·5% manure, performance was not markedly different from that shown with control diets. There was a considerable saving in the food cost at the prices ruling when the experiment was conducted and it was concluded that the use of dried poultry manure could increase the profitability of intensively produced beef.

Results are reported of an experiment with 60 pigs (12 pigs/treatment) covering the growing period between 22 and 87·5 kg live weight in which the growth-stimulating properties of sodium salicylate were examined either as a single additive or with copper sulphate. The pigs were individually fed a wet diet ‘to appetite’ and the quality of the carcasses was assessed at slaughter.

No significant improvement in live-weight gain or feed: gain ratio was obtained when either 200 or 700 mg of sodium salicylate/kg was added to the diet. When 700 mg of salicylate was given together with 250 mg Cu/kg diet, the response in growth rate was significantly higher than that obtained from the addition of copper alone.

There was no significant treatment effect on carcass quality as assessed by linear measurements, nor on the dry matter, nitrogen or ash contents of the gluteus medius muscle.