1. Analyses were based mainly upon entries in Volume 101 of the Hereford herdbook (horned section) for 1971, but also used details published between 1900 and 1970.

2. During the calving period July 1969 to June 1970, 2705 herds in Great Britain and Eire registered 17 297 calves. It was estimated that there were some 21 900 cows in pedigree herds; 2664 bulls were used for pure breeding, more than half in one-sire herds. During the period 1964 to 1970 approximately 60% of all bull calves registered by about 6 mo of age were later licensed for pure- or crossbreeding.

3. From a sample of calves, 87% of their sires and 37% of their dams had not been bred in the same herds where the calves were registered. More than half the herds had been established for fewer than 10 yr and the average age was 12·2 yr. The mean generation interval was 5·25 yr.

4. Pedigree analyses revealed a particularly well developed hierarchy within the breed; the effective number of herds at the great-great-grandparental level was only four. The dominant herd (Vern) had made a total genetic contribution to the breed of over 24% judged by the number of independent occurrences of Vern animals in the four-generation pedigrees of sample animals. The average increase per generation in inbreeding over the past 12 generations had been only 0·48%, and there was no evidence of any subdivision of the breed into separate strains.

1. Analyses are presented of 1628 calvings from 397 pedigree Hereford cows kept in three herds whose records were made available by their owners. In addition a further 541 calvings were examined from a sample of 100 cows taken at random from the herdbook entries.

2. Mean age at first paternity for 71 homebred bulls was 25·6 mo. Mean age at first calving varied between the three herds from 29·5to 32·8 mo, with 39·7 mo for the random cows.

3. Random herdbook cows produced an average of 3·3 calves per cow compared with 4·2 from those in the three herds; these latter cows had an average calving interval of 13·1 mo.

4. The pattern of cow and calf disposal varied partly according to the differences in herd age and culling policy. While one herd retained more than 80% of heifers and subsequently sold half of them after two or three calvings for breeding, another retained only one-third but kept these for their full working life. Similarly the percentage of bull calves sold for breeding varied between 43 and 83%.

5. The net reproductive rate, or number of calves weaned per 100 cow-herd-years was 74, 88 and 67 for the three herds, and 66 for the random herdbook cows. The mean generation intervals were 4·1 yr for sires and 5·8 yr for dams whose breeding life in the herds had been completed.

6. The biases in reproductive statistics estimated from herdbook entries were examined, and it was concluded that such data can be of use where better herd records are unavailable.

Data collected on the Meat and Livestock Commission's pedigree recording scheme have been analysed by least-squares procedures to measure the effect of environmental factors on calf performance. In total 5524 birth weights, 16 484 200-day weights and 6248 400-day weights were available for 765 herds and seven breeds. The model adopted included terms representing type of cattle within breed, sex of calf, age of dam, year and month of birth, pre-weaning management, herd average level of performance and herd size as fixed effects with sires as random effects. The model accounted for between 19 and 36% of the variation in birth weights and for 36 to 49% and 39 to 69% of variation in 200- and 400-day weights respectively, depending upon breed. For birth weights and 200-day weights age of dam was a significant effect in all breeds. Sex of calf was the main source of variation at all ages, its relative importance increasing with age. Calves from 2-yr-old dams had 200-day weights 5 to 11% lower and calves out of 3-yr-old dams 2 to 9% lower than calves from 5- to 8-yr-old dams. Month of birth was a significant source of variation in most breeds. Pre-weaning management system was confounded with sex and breed. Correction factors for use in the MLC recording scheme were derived from the data.

Data collected in the Meat and Livestock Commission's pedigree recording scheme have been analysed by least square procedures to assess the relationship between, and heritability of, birth, 200-day and 400-day weights. In total 5524 birth weights, 16 484 200-day weights and 6248 400-day weights were available for 765 herds, in which were represented 2031 sires of seven breeds. Heritability estimates were calculated using the complete data and a set restricted to the larger herds and sire progeny groups. The influence of single effects on the heritability estimates was computed. Linear relationships were established between birth and 200-day weights in the Aberdeen-Angus and Charolais breeds but in the Hereford, Devon and Sussex breeds they were curvilinear. The data from the Hereford breed indicated an optimum range of birth weights. The partial regressions of 400-day on 200-day weights were positive; they were not linear for the Charolais, Devon and Sussex breeds. In these cases there was a more than proportionate increase in 400-day weights for an increase in 200-day weights. The heritability estimates varied widely and were dependent on the method and underlying model used. The most reliable estimates were as follows: birth weight 0·23, 200-day weight 0·38 and 400-day weight 0·27.

The effects of supplementary feeding of licks containing non-protein nitrogen on the reproductive performance of beef breeding cows and on the live-weight gain of growing stock in Botswana is described. The trials involved 1375 breeding cows at five artificial insemination centres, and 360 breeding cows and 269 growing stock under ranch conditions. These licks increased the percentage of pregnancies in cows under lactation stress by up to 20%, but had no effect on the reproductive performance of dry cows or cows with a calf older than 5 mo. In growing animals the provision of rumen stimulatory licks increased live-weight gain by an average of 12%, but was of doubtful economic value.

The three indigenous beef cattle breeds in Botswana, the Tswana, Africander and Tuli, were evaluated for reproductive performance, viability and growth. Over 3 yr the calving percentages of 1389 Africander cows, 305 Tswanas and 357 Tulis were 64·5, 70·6 and 85 respectively. These differences were significant (P<0·05). Mortality of the Africander calves was higher to 2 yr of age, being 11% compared with 7-5% for Tswana calves and 7·4% for Tulis. The 18 mo weight of 219 Tuli cattle was 284 kg, and of 454 Tswanas 279 kg, both superior to the 270 kg for the 786 Africanders. Productivity estimates combining these three traits demonstrate the value of the Tuli breed for Botswana conditions.

Comparisons of the crossbreds produced by the use of Simmental, Brahman, Bonsmara and Tuli sires on Tswana cows showed an advantage in growth to 18 mo through crossbreeding. The 18 mo weights of Simmental and Brahman crosses at 324 and 304 kg respectively were significantly different, and superior to the weight of pure Tswana at 279 kg. Bonsmara and Tuli crosses at 294 and 290 kg also showed a significant increase over the pure Tswana. The use of the Simmental is now recommended for better management conditions, with Brahman crossbreeding on a wider scale.

Least squares analyses were undertaken on 1519 gestation lengths of cows mated to Beef Shorthorn, Hereford, Charolais, German Simmental or British Friesian bulls. Gestation length was influenced largely by breed of sireand byindividual sires within breeds. Approximately 100 calvings are required to give a mean value with a standard error of 0·50 day. Heritability of gestation length was estimated to be 0·24±0·05. It is recommended that Artificial Insemination organizations survey and publish the mean gestation lengths of their bulls and that dairy farmers modify, where necessary, pre-partum feeding in the light of this information.

Thirty-six male piglets weaned at 20 days of age were allocated to an initial slaughter group and to five dietary treatments with proteinlevels of 15·0, 17·2, 19·1, 21·4 and 23·2% crude protein at a common energy level of approximately 3·6 Meal digestible energy/kg. All diets were offered ad libitum and growth response was compared between 5 and 20 kg and 10 and 20 kg live weight. In both periods there were significant improvements in growth and the food conversion ratio from 15·0 to 17·2%, and 17·2 to 19·1% crude protein. Between 10 and 20 kg live weight dietary protein levels above 19·1% were associated with a depression in growth. Carcass protein increased and ether-extractable material decreased with increased dietary protein over the entire range studied. Retention of digestible nitrogen was maximized on the lower protein diets. Conversion of food into carcass lean was maximized on the two highest protein diets.

The length and diameter of the humerus, ulna, femur and tibia were studied at generations 10 and 14 in lines of mice selected for high and low body weight at 10 weeks of age.

Although some deviations from control were not significant, a general pattern of correlated responses was evident. The divergences of high and low line means for bone length and diameter were significant in the four bones, confirming the contribution of these parameters to genetically determined alterations in body weight. Maternal effects did not affect bone length and diameter and heterosis was not important in the genetic control of these parameters. Unlike bone length, bone diameter continued to diverge in response to the additional four generations of selection. There may therefore be differences in the quantitative genetic control of bone length and diameter and consequently in the control of endochondral and intramembranous ossification.

Twenty-one of 23 Finn and 23 of 28 Finn-Merino crossbred springborn males mated at about 6 mo of age during their first breeding season, whereas all of nine Merino males did not do so until the second breeding season at about 18 mo of age. The diameter of the testes of Finn and crossbred males at 6 mo of age was a higher proportion of their diameter at 18 mo than was that of the Merinos. Seasonal studies showed an earlier increase in the testis diameter of Merino males than that of the Finns or crossbred. At all seasons, the intensity of mating behaviour of Merinos was lower than that of the other breed types.

The results support the hypothesis that the reproductive activity of males is correlated with that of related females, and the use of male reproductive activity as a criterion for genetic selection for that of the female is discussed.

A model has been developed in which the voluntary food intake of sheep is related directly to metabolizable energy requirements unless physical or endocrine limitations intervene.

Although no ‘set point’ for body weight or body fat is incorporated in the model, mature sheep offered food ad libitum are predicted to reach eventually a plateau of body weight due to progressive depression of gut capacity by the increasing volume of abdominal fat. The final body weight is proportional to the digestibility of the diet. During pregnancy and lactation metabolic, physical and endocrine factors interact in such a way as to give predicted patterns of feed intake similar to those observed in practice with several qualities of diet.

In some grazing experiments stocking rates are adjusted during the experiment. For these and other experiments in which the initial number of animals per treatment is subsequently reduced a more powerful statistical analysis than the normal analysis of variance may be possible. The methodology of the analysis is presented in this paper and applied to an example.

The generation interval may be defined as the mean age of parents of all progeny born, or as the mean age of parents of only those progeny destined to be selected as replacements. If the mean age of parents of replacements is used, selection differentials must be calculated within parental age groups. If the mean age of parents of all progeny born is used, the selection differential contains a component due to disproportionate selection of replacements from different parental age groups.

Fifty-six Merino d'Aries and 56 Romanov × Merino crossbred lactating ewes were divided into two treatment groups, one (T1) with a ewe-to-ram ratio of 48: 1 (24 Merino and 24 crossbred) and the other (T2) of 64: 1 (32 Merino and 32 crossbred).

Intravaginal sponges with 40 mg FGA (Searle) were inserted in two subgroups at 8-day intervals and withdrawn after 12 and 16 days in situ, in four equal groups at 4-day intervals. A dose of 400 IU pregnant mare's serum gonadotrophin (PMSG) was applied at sponge withdrawal.

Fertility after two oestrous cycles was equally satisfactory in both treatments, though it was lower in treatment group T2. This could be due to the significantly low (P<0·01) number of Merino ewes raddled by the T2 ram (0·33 Merino against 0·92 crossbred) suggesting preferential behaviour by the ram in this treatment.

Comparisons between breeds show a slight superiority in the fertility of crossbred ewes and a highly significant superiority in prolificacy These results suggest that using the described method, 2% of rams seems more than adequate to mate with progestagen-synchronized ewes with a satisfactory fertility level. It is, however, recommended to use homogeneous groups of ewes.

Ruminal ammonia and blood urea levels were compared in fistulated steers eating paddy straw to appetite and given urea alone, urea plus oxalate, and complexes of urea with either formaldehyde (UFC) or oxalate (UOC), introduced into the rumen in a single daily dose. The solubilities of UFC and UOC in rumen liquor were 23·7 and 4·6% respectively of that of urea. Higher levels of ruminal ammonia and blood urea were recorded on urea than on other supplements.