The performance of 38 Friesian cows with either high or low breeding indexes was measured during the whole lactation and in short-term experiments where the cows were individually given cut pasture in stalls, or grazed on pasture.

Cows with high breeding indexes produced more milk fat than did cows with low breeding indexes in the short-term experiments, and during the whole lactation. There was close agreement between the expected differences (predicted from breeding indexes) and the measured differences in milk fat yield. When fresh cut pasture was given ad libitum in stalls the intake per cow was the same between genotypes. However, when intake was expressed per unit of metabolic weight, the mean pasture drymatter intake of the high breeding index cows was 1·05 of that of the low breeding index cows, but the difference was significant in only one of the four indoor feeding experiments.

The difference between genotypes in size was not regarded as significant because the differences in mean live weight at calving (high breeding index – low breeding index) of the animals purchased in the 1st and 2nd years were –28 and 9 kg respectively. During the whole lactation, low breeding index cows gained significantly more live weight and lost significantly less body condition than did the high breeding index cows. No significant differences in live-weight and body-condition changes could be detected during the shorter periods (4 to 5 weeks) of the indoor feeding and grazing experiments.

It can be calculated that the higher milk production of the high breeding index cows could be explained partly because they eat more (per unit metabolic live weight) and more importantly because they divert a higher proportion of the food they eat to milk production and a lower proportion to live-weight gain than do the low breeding index cows.

Six lactating Friesian cows (three high and three low breeding index cows) were subjected to a total of 28 complete energy and nitrogen balances during early and late lactation. In addition, 12 non-lactating Friesian cows (six high and six low breeding index cows) were offered individually pasture indoors for 62 days, beginning at approx. 180 days of pregnancy. Cows within each genotype were randomly allocated to feeding levels which were nominally equivalent to their requirements for maintenance or twice maintenance. Eight of the 12 cows (four high and four low breeding index) were each subjected to two energy balance periods at approx. 210 (period 1) and 230 (period 2) days of pregnancy.

When offered pasture ad libitum during lactation, cows with high breeding indexes consumed significantly more gross energy per unit of metabolic live weight (P < 0·05) than did cows with low breeding indexes, but the two genotypes did not differ in their ability to metabolize the gross energy of the food, or in the individual losses of energy in faeces, urine or methane.

There were no significant differences between genotypes in their heat production at a common energy intake except during restricted feeding in early lactation when high breeding index cows produced less heat than did low breeding index cows (P < 0·05). During lactation high breeding index cows retained a higher proportion of their total energy retention (milk + body tissue energy) as milk in late lactation (P < 0·01) but not in early lactation.

For non-lactating cows, both genotypes required similar amounts of metabolizable energy to maintain zero maternal body energy retention, 0·79 and 0·80 MJ/M0·75 for periods 1 and 2 respectively. The efficiency with which metabolizable energy was converted to net energy (kg) was similar for both genotypes at both stages of pregnancy, the mean value being 0·52. The metabolizable energy required to maintain body condition was estimated to be 0·78 and 0·71 MJ/M0·75 for high and low breeding index cows respectively. The metabolizable energy required in excess of maintenance to promote a gain of one unit of body condition was estimated to be 2290 MJ. This is equivalent to 27 MJ energy retained per kg live-weight gain.

For lactating and non-lactating cows the differences between genotypes in their utilization of nitrogen were small and inconsistent.

An empirically based equation for voluntary food intake, obtained from a multibreed cattle experiment, was used to develop formulae for evaluating the overall efficiency of food utilization of beef production n i a production unit consisting of a dam and her progeny. With these formulae the dependence of overall efficiency on degree of maturity at slaughter was studied, with reproductive rate, dam food costs, terminal sire size and number of calvings per dam allowed to vary.

The formulae were used to study efficiency of food utilization in a traditional beef production system. Maximum efficiency values ranged from 2·3 to 3·5 g of lean tissue per MJ of metabolizable energy. These maximum values were affected to a small extent when progeny were slaughtered much earlier or later than the optimal degree of maturity. In a single-sex, bred-heifer (SSBH) system, the maximum value that could be attained with high technological efficiency and a reproductive rate of unity was 5·2 g, which was 0·5 times more efficient than the highest achievable traditional value. With a reproductive rate of 0·85, the highest efficiency value in an SSBH system was 4·8 g.

Overall efficiency of food utilization was also examined in a sex-controlled system where all offspring for slaughter were male and in a modified traditional system where every surplus female was bred once. Both these systems could give higher efficiency (up to 1·08 times) than in a traditional system, but neither could compete with a well-operated SSBH system.

Formulae to deal with multiple births, partial sex control and other factors are given in the APPENDIX.

An attempt is made to meet the need for a single standardized growth equation to cover the entire period from conception to adulthood in a wide range of mammalian species. It is suggested that an ideal equation is one which adequately describes the overall shape of a growth curve and, in addition, can be extended or modified to give greater flexibility and higher precision as necessary. The structure of such an equation is discussed and results in the derivation of a logistic relationship between adjustable ‘flexibility’ functions of size and time.

Data from eight species are examined and in both its basic and extended forms the equation is found to give acceptable fits to individual species and to provide a useful mean standardized growth curve for mammals.

An abrupt fall in live weight of grazing cattle occurs at the beginning of the growing season in the seasonally dry tropics. In an experiment with grazing crossbred bulls in central Tanzania designed to monitor changes in gut fill, 16 were slaughtered in the dry season and 14 in the early wet season. Bulls were measured and weighed before, and weighed after, a 20-h fast and then slaughtered. Gut fill was measured and empty body weight (EBW) calculated.

During fasting, the ratio gut fill/EBW fell from 0·291 to 0·217 in the dry season and from 0·207 to 0·119 in the wet season. Heart girth/EBW1/3 was 0·2577 and 0·2567 m/kg1/3 in the dry and wet seasons respectively.

It was concluded that live weight before or after fasting is seriously affected by season and can give biased predictions of EBW change. On the other hand, heart girth is little affected by season and can be used to monitor relative increases or decreases of EBW within animals.

Male lambs were selected annually at 18 and 24 weeks of age for high or low concentrations of copper in plasma to produce two closed lines. The difference between the high and low lines increased to approx. 0·5 mg/1 after 5 years when the experiment ended, indicating appreciable additive genetic variation for the concentration of Cu in plasma. The selection lines were derived from a population of Scottish Blackface × Welsh Mountain crosses which had been interbred for many generations. The two parental breeds contributing to the cross normally differ in a number of aspects of Cu status with that of the Welsh higher than that of the Blackface. Both pure breeds were kept as controls. In the final year, Cu concentrations of the high selection line exceeded the values of the Welsh and the low selection line had values below those of the Blackface. Differences between the selection lines persisted at all ages.

There were indications of changes in the frequency of the Hb alleles, associated with the selection for Cu concentrations, but there were no correlated changes in body weight, reproductive performance or lamb mortality during the 5-year selection period when dietary supplies of Cu were not particularly restrictive.

Energy intake was recorded for two batches of 12 individually-penned piglets weaned at 5 kg live weight and fed milk-based diets of 18·7 MJ metabolizable energy (ME) per kg up to 8 kg live weight and 15.4 MJ ME per kg thereafter. The piglets were kept within their thermoneutral zone throughout the experiments. Regression equations describing the relationship between energy intake and days after weaning were developed for each piglet and the means of the regression coefficients and constants were incorporated in an equation for estimating energy intakes for piglets of 5 to 15 kg live weight. The lower and upper critical temperatures were calculated for each piglet on every day of the trial. These values were extrapolated to give temperature recommendations for piglets of 5 to 15 kg live weight kept in groups of 20. The temperatures ranged from 30°C at weaning to 19°C, 23 days after weaning. This represented a reduction in temperature equivalent to 0·5°C per day.

Forty male commercial hybrid pigs, half castrated at 10 days of age, were used in a study to determine whether boars at bacon weight (90 kg live weight) have different fat quality from barrows. Fat quality was defined as the firmness and cohesiveness of subcutaneous fat, both properties being measured using an Instron materials testing instrument. Within each group of boars and barrows, half the pigs were fed a normal commercial diet and half a lower energy diet at a lower level of food intake. The latter diet provided relatively more energy as C18:2 (linoleic acid).

Firmness of subcutaneous fat was related to fat thickness and to concentrations of the chemical components in the fat which are affected by fat thickness. The single best predictor of firmness was C18:0 (stearic acid). Cohesiveness was less closely related to fat thickness and was most associated with the concentration of C18:2.

On the low energy/high C18:2 diet, all the pigs were judged to have soft fat, the difference between boars and barrows being small and explained by the difference in fat thickness. On the normal commercial diet, some barrows had particularly firm fat but they had thicker fat than any of the boars. Most (0·84) of the pigs in the experiment had P2 values (fat thickness over the m. longissimus at the last rib) between 8 and 15 mm. In this range there was no difference in firmness between boars and barrows. Cohesiveness was also similar between boars and barrows.

One hundred entire male pigs growing between 20 and 45 kg live weight were used to investigate the effects on growth performance and carcass composition of eight concentrations of dietary crude protein (CP), ranging from 93 to 238 g/kg and three levels of food intake (giving mean daily digestible energy 15·6, 18·7 and 22·4 MJ).

Growth rate improved with each increase in food intake and with increase in dietary CP up to 175 g/kg (10·5 g lysine per kg) at all food intakes. Food:gain ratio (g food per g growth) improved with increase in dietary CP up to 175 g/kg and with the first increase in food intake.

The rate of protein deposition in the carcass (excluding viscera) increased with each increase in level of feeding, and with increases in dietary CP up to 175 g/kg. The results showed that for pigs given diets deficient in protein, the rate of protein deposition was linearly related to protein intake and independent of energy intake. For pigs given diets adequate in protein, the rate of protein deposition was related to energy intake, and independent of protein intake.

The carcass fat of pigs killed at 45 kg live weight decreased on all feeding treatments with increase in dietary CP up to 175 g/kg, and was higher in pigs given the two higher levels of feeding than in those fed at the lowest level. The effect of feeding level on carcass fat content declined as the protein adequacy of the diet was reduced.

Twenty four entire male and 24 female pigs growing from 48 to 90 kg live weight were used to investigate the effects of five rates of food intake supplying daily between 23 MJ digestible energy (DE) and 41·8 or 39·2 MJ DE for entire males and females respectively, on rate of protein deposition, and on growth and body composition.

Entire males deposited protein faster than females, and for both sexes the rate of protein deposition increased linearly with increase in energy intake up to 33 MJ DE per day (P < 0·001) but was not significantly affected by further increase in energy intake.

Average daily gain increased quadratically (P < 0·001) with increase in energy intake and at all levels was higher for entire males than for females. Food:gain ratio (g food per g growth) which was lower for entire males improved with increase in energy intake up to 33 MJ DE per day and deteriorated with further increase in energy intake.

At 90 kg live weight entire males contained less fat and more protein and water in the empty body than females. For both sexes, body fat content increased at a decreasing rate with increase in energy intake up to 33 MJ DE per day, and at an increasing rate with further increase in energy intake.

Total energy retained was linearly (P < 0·001) related to energy intake and at all rates of energy intake was higher for females than for entire males. The estimates of DE required for maintenance were 15·9 MJ/day (0·664 MJ/kg M0·75 per day) and 12·4 MJ/day (0·51 MJ/kg M0·75 per day) for entire males and females, respectively.

The effects of diet acidification on the growth performance of piglets were assessed in two experiments. In the first, 167 piglets were weaned at 10 days of age and penned in litter-mate groups (seven groups per treatment) which were allocated to one of three diets, with either citric acid (30 g/kg; C), fumaric acid (15 g/kg; F) or no additional organic acid (N); pigs were fed ad libitum for 25 days. The rate of live-weight gain was significantly higher for treatment C than for treatments F or N (216 (s.e. 10), 170 (s.e. 7) and 189 (s.e. 6) g/day respectively; P < 0·05). The voluntary food intake on treatment C also tended to be higher than on the other two diets (220 (s.e. 24), 182 (s.e. 18) and 189 (s.e. 7) g/day for treatments C, F and N respectively).

In experiment 2, 30 crossbred piglets were penned in single-sex groups of three and weaned at 10 days of age. The same diets were used as in experiment 1 but all three diets were available ad libitum to each group of piglets for 25 days. When dietary choice was allowed, piglets consumed significantly greater quantities of the control diet than of the acidified diets (1381, 1422, 2972 (s.e. 132) g/day for diets C, F and N respectively; P < 0·01). When the relative intake of each diet was correlated with live-weight gain, only diet C showed a significant positive correlation (r = +0·74; P < 0·05). Correlations of F and N intakes with live-weight gain were negative. It is concluded that citric acid (30 g/kg) in the diet of early-weaned piglets promotes live-weight gain, although acidification of the diet may decrease palatability.

In muscle and livers of steers given a diet with sodium monensinate a significant decrease in cadmium content was observed. The concentration of manganese, copper, zinc, lead and iron in the analysed organs varied inconsistently. It seems from the results that, in tissues, monensinate influences the metabolism not only of monovalent cations but of polyvalent cations as well.

Melatonin was fed daily at 14.00 h to eight non-lactating and six lactating hinds. Feeding was begun on 27 July 1983 (day 1) and continued until 21 September (day 57). Six non-lactating hinds were used as controls. Blood samples were taken about every 10 days from day 42 until day 73. Plasma progesterone and prolactin were determined by radioimmunoassay. Progesterone values of greater than 0·63 μg/1 were taken to indicate the presence of a corpus luteum and that the animal had ovulated. In the melatonintreated, non-lactating group seven of the eight hinds showed ovarian activity compared with two of the six controls (P < 0·01). None of the lactating hinds treated with melatonin showed evidence of ovarian activity. In both the lactating and non-lactating hinds treated with melatonin, prolactin levels were never greater than the detection limit of the assay (7·4 μg/l), while, in the control hinds mean prolactin levels were elevated on all but the last sampling day. It is, therefore, possible to induce early ovarian activity in non-lactating red deer hinds by feeding melatonin. However, although melatonin treatment similarly depressed plasma prolactin levels in both lactating and non-lactating hinds, it was unable to overcome the inhibitory effect of lactation on reproduction.