Fifty-six Landrace ♂ x Yorkshire ♀ gilts were divided at 140 days of age into four groups in a 2 x 2 factorial design whereby they were exposed continually (BE) or not exposed (NE) to a mature boar and were or were not blood sampled twice weekly for progesterone determinations.

There was no effect of BE on the age at the first or second ovulation. Since detection of oestrus was only 70·80% accurate, BE apparently delayed the onset of first and second oestrus by 16 days (p = 0·06). Blood sampling advanced first oestrus by 15 days (F = 0·07) and second oestrus by 32 days (P = 0·001).

The results suggest that gilts are unlikely to be responsive to boar exposure at 140 days of age, that observations of oestrus alone may be a misleading end-point of the effect of boar exposure and that the use of blood sampling for the measurement of progesterone as an index of ovarian activity may attenuate the response to stimuli intended to advance puberty in the gilt.

The food intake, growth and development of Ross broiler cockerels were recorded from 1 day old to maturity. At regular intervals, the body composition of these birds was determined and the eviscerated carcasses were chemically analysed. Allometric relationships of component weights and live weight were examined. Historical comparisons with fowl reared during the last 60 years did not reveal any significant changes in the pattern of development, despite faster absolute growth rates and heavier mature weights.

One gilt from each of 24 littermate trios was assigned at birth to one of three treatments which were to adjust litter size to six, nine or 12 piglets. Plasma luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were measured before and after stimulation with 1 ng gonadotrophin-releasing hormone (GnRH) per kg body weight at 55 and 140 days of age. At 150 days, oestrogen was administered and ovarian and uterine responses were measured at slaughter on day 160.

Litter size significantly modified pre-weaning growth rate (P < 0·01) but did not influence gonadotrophin secretion or sexual maturity at slaughter. The family of origin significantly influenced ovarian and uterine development (P < 0·05) and the LH and FSH responses to GnRH at 55 days (P < 0·05). Family size (P < 0·01) and sex ratio of the litter at birth (P < 0·05) also influenced the ovarian response to oestrogen treatment. Significant sire effects on ovarian and uterine development were established (P < 0·05); for the progeny of individual boars significant differences in the response to oestrogen were associated with significant differences in growth rate (P < 0·001).

Variations in family birth and litter weaning weights were negatively correlated with ovarian and uterine development (P < 0·05), whereas all measures of growth from weaning to slaughter were significantly and positively correlated with sexual maturity at slaughter (P < 0·05).

Basal FSH and the responses of both LH and FSH to GnRH at 55 days were correlated with growth rate at day 55 (P < 0·05). Basal LH and FSH at day 140 were correlated with ovarian and uterine development at day 160 (P < 0·05).

Groups of five Yorkshire ♂ x Landrace ♀ gilts were either exposed (BE) or not exposed (NE) to a mature boar commencing at 140, 147, 154 or 161 days of age. Each group of BE gilts was slaughtered 10 days after initial boar exposure, for examination of the reproductive tract, with one of the four NE groups killed at a corresponding age.

NE gilts remained prepubertal throughout. Boar exposure induced ovulation in one gilt at 140 days, two gilts at 147 days, two gilts at 154 days and one gilt at 161 days. Boar exposure also increased uterine weight (P < 0·001). There were no effects of age either on the number of gilts ovulating or on uterine weight. There was a significant positive effect of body weight on uterine weight which was greater in BE gilts than in NE gilts. These results suggest that body weight may be a determinant of the response to boar exposure by gilts between 140 and 170 days of age.

Three portable ultrasonic probes (Scanoprobe, Meritronics and Krautkramer), each used by the one experienced operator, were found to be superior to the visual and tactile assessments of two experienced judges in predicting carcass backfat depth (range 4 to 20 mm) in 132 live steers. Residual standard deviations for the three probe/operator combinations were 2·01, 2·16 and 2·77 mm respectively; those for the judges were 2·96 and 3·09 mm. Combining ultrasonic estimates by multiple regression improved slightly the accuracy of prediction (residual s.d. = 1·87 mm), the level of which was comparable to that previously reported for scanning ultrasonic instruments used on pigs.

A comparison was made of the carcass characteristics of 73 Charolais-sired and 86 Murray Grey-sired cattle slaughtered at about 20 months of age. Six Charolais and 10 Murray Grey sires were used. At a constant slaughter age (597 days) the Charolais-sired calves had significantly heavier pre-slaughter live weight (36 kg) and carcass weight (24 kg) and significantly less fat over the m. longissimus (2·8 mm), but there was no significant difference in killing-out proportion. In the dissected half-carcass, the Charolais-sired calves had significantly more meat (9·3 kg), more bone (3·5 kg) and less fat (1·9 kg). At a constant carcass weight (233 kg), Charolais-sired calves had significantly more meat (1·8 kg) more bone (1·9 kg) and less fat (3·5 kg) in the dissected side. After making allowance for the higher calf mortality from birth to weaning of Charolais-sired calves (0·041), the advantage in carcass weight was reduced to 13 kg. It is concluded that the Charolais breed is superior to the Murray Grey as a terminal sire.

Field data from four breeds (Charolais, Hereford, South Devon and Welsh Black) comprising a total of 145, 68, 262 and 197 records respectively and obtained from 30, 24, 38 and 25 herds were used. Station data from two breeds (Charolais and Hereford) comprising 268 and 687 records respectively, from cattle originating from 27 and 44 herds respectively were also used. The data were analysed to identify environmental factors which significantly affected bull performance traits. In performance traits, herd of origin, in field and station data, accounted on average for 0·50 and 0·18 respectively of the total variance. Year, season and dam's age accounted for 0·02, 0·03 and 0·02 respectively of the total variance, the last being higher in pre-weaning than in post-weaning traits.

Current animal improvement deals largely with selection of stocks for current economic objectives. However, there is uncertainty about how well these objectives will hold in the future, since husbandry and marketing needs and conditions may change unpredictably over time. The scope for developing alternative stocks, selected for different chosen sets of objectives, is considered, so that by substitution or crossing it may be possible to reduce the uncertainty and better meet future needs and conditions. The reduction in uncertainty is modelled by including risk or uncertainty in the discount rate in assessing benefits over time from one cycle of selection. From the national viewpoint, the costs of developing alternative selection stocks are small relative to the possible returns. Thus, if the uncertainties were reduced by having them, a large number of such alternative stocks would be justified. The expected benefits to the community from animal breeding would also be increased thereby.

The induction of cryptorchidism in dairy-bred male calves during the 1st week after birth severely restricted testicular growth and the semen of cryptorchids was practically sterile. Their lifetime performance in a semi-intensive beef system was similar to that of bulls and significantly superior to that of steers. Similar differences were found in carcass characteristics. The grazing behaviour of cryptorchids was similar to that of bulls.

The aim of this study was to estimate the parameters of the lactation curve of dairy cows in Scotland so as to improve the prediction of milk yield. The data used for deriving the parameters were ‘milk test’ yield values recorded by the Scottish Milk Records Association during a 2-year period. The data were grouped, according to lactation number, breed and area of Scotland, into 27 groups. The data pertaining to each group were then averaged so as to produce by regression analysis a curve for each calving week, lactation number and area. Areas and breeds were found to affect the shape of the lactation curves.

Sixteen sheep which had previously eaten wheat only (eaters) were offered wheat, barley, oats and maize separately in a Latin-square design with two sheep per cell and two replicates of the Latin square. Another 16 sheep which had never previously been offered wheat (non-eaters) were offered the same cereal grains in two more Latin squares with two sheep per cell.

The total intake of wheat for the eaters was significantly greater than for the other cereal grains (P < 0·01) and it took 7 to 9 days for other cereals to be eaten. There was virtually no consumption of cereal grains by the non-eaters until days 10 to 12 when some wheat but little other cereal was being eaten.

The results indicate that sheep learn to eat each type of cereal grain separately but if they know how to eat wheat they learn to eat other cereal grains more rapidly.

Normal and dexamethasone-induced parturition were compared in Merino and prolific Booroola Merinos carrying crossbred foetuses. Behavioural observations were recorded for 42 single-, 27 twin- and 10 triplet-bearing ewes. After treatment on day 144 of gestation, ewes injected with dexamethasone lambed 41 h earlier than ewes injected with phosphate buffer. On the phosphate-buffer treatment, Booroola Merino ewes lambed 22 h earlier than Merino ewes. Twin- and triplet-bearing ewes which were induced to lamb had a significantly longer duration of labour than ewes bearing singles which were induced to lamb, and ewes, regardless of litter size, which lambed naturally. Induction of parturition had no significant effect on the lambs' behaviour subsequently. Lamb survival was not influenced by dexamethasone treatment but was significantly influenced by lamb birth weight, which declined as litter size increased.

In a study involving 240 pigs, starter and grower diets containing copper at a high level (180 mg/kg) or olaquindox (110 mg/kg and 50 mg/kg in starter and grower diets respectively) or both were compared with a control diet containing 15 mg Cu per kg. The pigs were weaned at 21 days and given the starter diets until they reached a mean live weight of 10·2 kg, followed by grower diets for 28 days.

In both the starter and grower periods, the effects of the high Cu supplement were not significant. In the starter period, olaquindox significantly improved daily gain (by 25 g) and food conversion efficiency proportionately by 0·07 (P < 0·01). In the grower period, olaquindox did not significantly affect performance.

Lambs from Perendale, Merino × Perendale, Romney, Merino × Romney and Coopworth ewes, and Southdown and Suffolk sires were assigned to three groups. Groups received no treatment, nine weekly doses of 175 mg iron (Fe) per kg body weight or nine weekly doses of 175 mg zinc (Zn) per kg body weight. They were slaughtered 1 week after the last dose. Samples of m. longissimus were analysed for Fe and Zn concentration.

Muscle Zn concentration was greater for Southdown-sired lambs (85·5 mg/kg dried muscle) than for Suffolk-sired lambs (77·6 mg/kg; P < 0·01). Dam genotypes and the interaction of sire breed and dam genotype were not significant for Zn concentration. Heritability of muscle Zn concentration was estimated to be 0·92 (s.e. 0·48). Muscle Fe concentration showed a significant interaction between sire breed and dam genotype (P < 0·05). Heritability of muscle Fe was estimated to be 0·21 (s.e. 0·38). No significant interactions of either sire breed or dam breed × treatment were found.

Formulae are derived to calculate the genetic merit of a commercial population during replacement of one breed by another, based on the rates of genetic change in the old and new breeds, the initial difference between the breeds, the lag between the first use of replacement stock and the appearance of their genes in commercial progeny and the effect of replacement on the old breed improvement scheme. By comparing the merit of the population with and without replacement the value of a change in breed can be assessed. The method is illustrated with an example from pig breeding.

The heat production of fasted and fed littermate pairs of pigs weaned at 10, 17 and 24 days of age was determined between 16 to 34°C. Up to and including 34°C the post-weaning heat production of fasted pigs at 10 and 17 days of age decreased with rising temperature. Rectal temperatures of all 24-day-old fasted pigs were higher at 34°C than at 31°C. For fed pigs weaned at 10 days of age the heat production continued to decrease up to and including 34°C. The decrease between 28°C and 31°C and between 31°C and 34°C was comparatively small and rectal temperatures were slightly elevated at 34°C. The results indicated that for fed pigs weaned at 17 and 24 days the thermoneutral zone was in the region of 31°C and the lower critical temperatures were about 28°C.

Animal Production, Volume 40, Part 3, p. 567.

169. The effect of shearing sheep on intake and heat production