In populations where generations overlap and initial genetic differences between age groups differ from steady-state values, genetic responses fluctuate in the early years of a selection programme. The degree of deviation of these selection responses from steady-state responses indicates the usefulness of estimates of economic returns calculated more simply using the steady-state value. Here these deviations and their effect on sums of discounted returns are first defined and the models derived then used in numerical analysis. Some problems of definition of these deviations are discussed.

The results showed that deviations from sums of discounted returns calculated using steady-state responses varied from about −1% when starting conditions were similar to steady-state conditions to more than 20% when initial genetic differences between age groups were taken as zero. Increases in discount rate increased the effect of the deviations from uniform response while the effect of changing age structure was unpredictable. As initial genetic differences between age groups increased, deviations were increasingly positive among selection methods which made better use of those differences.

Where genetic differences between age groups change during a selection programme, proportional contributions of each age group to replacements should also change in order to maximize genetic responses to selection. Sums of discounted returns were estimated for such a ‘dynamic’ selection policy and these were compared with those from ‘static’ selection policies in which genetic contributions of different age groups remained unchanged. Four selection methods, two sets of genetic differences between age groups at the outset, three age structures and two discount rates were used.

The results showed that the superiority of the dynamic policy was greatest where genetic differences at the outset differed most markedly from their steady-state values, where discount rates are high and where there are large numbers of age groups and the selection method allows culling in all age groups. Under most conditions initial genetic differences are probably the most important factor. The proportional advantage in sums of discounted returns is variable and generally small within the framework of this study ranging from zero to roughly 8% unless breeding objectives before and during the programme were negatively correlated. In the latter case the proportional superiority of the dynamic policy was much greater.

1. Adolescent mice were exposed from 33 to 84 days of age to a warm (30°C) or to one of four cold (15°C, 10°C, 5°C or 1°C) environments. Before and after exposure the mice were kept at a normal mouseroom temperature of 23°C. Controls remained at this temperature throughout. Voluntary food intake, live weight and growth efficiency were individually monitored before, during and after exposure, i.e. from 19 to 112 days of age.

2. Food intake during exposure was linearly related to temperature, ranging from 4 g/day at 30°C to 14 g/day at 1 °C. Intakes differed only slightly between males and females. Food intake at 10°C was 70% above controls. Oxygen consumption in adults at 10°C was 74% above controls.

3. Male growth was unaffected by temperature. Superiority over females (56% at 30°C) diminished at lower temperatures (12% at 1 °C) because females grew 41% faster at 1 °C than at 30°C. Females whose growth was depressed at 30°C, showed compensatory growth on returning to 23°C.

4. Growth efficiency was strongly, but not entirely linearly, related to temperature, and increased approximately three-fold between 1°C and 30°C. Efficiency was maximal at 30°C in males and at 23 °C in females. Males grew 76% more efficiently than females at 30°C and only 12% more efficiently at 1°C.

5. Increased food intake in the cold satisfied energy requirements and allowed females to grow faster.

6. Individual differences in appetite persisted throughout life, but rankings in growth and efficiency changed with age. Food intake was strongly correlated with growth rate in weanlings, but growth rate was always the main determinant of growth efficiency.

7. The variance of adolescent growth rate was unaffected by the treatments.

1. Young adult and adult lactating British Friesian female cattle (heifers and cows) were individually fed ad libitum on perennial ryegrass silage (F) made with an additive of formic acid (3·41 of 85% acid/t fresh grass) or on a silage (FF) made from the same crop with an additive of 9·01/t of a mixture of equal volumes of formic acid and formalin (35% w/w solution of formaldehyde). Silage F was supplemented with a concentrate containing 50% dried grass and 50% barley at a low (L) or high (H) level of feeding; silage FF was supplemented with the same concentrate at the lower level only, either with urea at 2·0% of the silage DM (FFLU) or without urea (FFL).

2. Silage F had a lower pH (P< 0·001) and contained more lactic, acetic and total fermentation acids (P><0·001). The DOMD in vivo of the diet was lower (by up to 3 units) for treatment FFL than for the other treatments (P<0·05 in weeks 17 and 18).

3. Over weeks 4 to 18 of lactation the mean dry-matter intakes (DMI) of silage on treatments FL and FFL were similar, but silage DMI was 12 to 13% greater on treatment FFLU than on FL or FFL (P<0·05). Digestible OM intake was greater with FFLU than with FFL (P<0·01) and with FL (P<0·05).

4. Milk yields did not differ significantly between treatments and metabolic blood profiles were normal.

Grass silage with a dry-matter (DM) content of 253 g/kg and containing 142 g crude protein and 670 g digestible organic matter/kg DM was offered ad libitum to 56 first- and second-lactation British Friesian cows during the first 75 days post-calving. In addition the animals received supplementary concentrates containing either 95 (T1), 137 (T2), 174 (T3) or 209 (T4) g crude protein per kg fresh weight, with extracted soya bean meal being used as the protein source. All concentrates were offered at equal levels across treatments, the amounts being 8 kg for first- and 10 kg per day for second-lactation animals. The silage DM intakes recorded over the period were 514, 524, 548 and 545 ± 19·9 kg and milk outputs were 1349, 1451, 1533 and 1628 ±40·2 kg for treatments Tl to T4 respectively. The rate of live-weight change and the milk fat, solids-not-fat, protein, lactose and ash contents of the milk were not affected significantly by any of the treatments. Diet digestibilities were determined using three animals per treatment. The digestibility coefficients were for DM 0·738, 0·746, 0·776 and 0·772 ± 0·0088 and nitrogen 0·597, 0·641, 0·707 and 0·727 ±0·0126 for treatments T1 to T4 respectively.

Milk replacers in which skim milk protein (SMP) was replaced in various proportions by undried or dried fish protein hydrolysate (FPH) were offered twice daily to British Friesian male calves. Various proportions of fish protein and milk protein in the diets were obtained by mixing replacer made from SMP with replacers made from undried and dried FPH respectively. The ratios were: 100 FPH, 0 SMP; 67 FPH, 33 SMP; 33 FPH, 67 SMP; and 0 FPH, 100 SMP. Calves were offered the milk replacers only to a maximum of 5 1 per day until 28 days when concentrates and hay were offered ad libitum. The calves were weaned after 42 days. Feed intake and live-weight gain were recorded for 84 days.

There was a marked decrease in performance to weaning when the milk replacer contained two-thirds or more of the FPH. The FPH-fed calves had lower hay intakes than those fed milk only. Treatment differences in post-weaning live-weight gain reflected mainly differences in concentrate intake. There was no significant difference in live-weight gain nor food efficiency for calves fed either undried or dried FPH in any period to 84 days. The number of treatments for scour was similar for the two groups. However, the calves fed dried FPH had a lower dry-matter intake in the post-weaning period.

The birth weight and growth rate of sucking calves reared by three groups of cows grazing on veld and receiving different amounts of concentrates during winter and the first 8 weeks of lactation were studied over four successive calving seasons. Calves were weaned at either 150 or 240 days. The efficiency of use of concentrates for weaned calf production was calculated.

Calves from cows given most concentrates were heavier (P<0·01) than othei calves at birth, 150 days and 240 days. Milk consumption during the first 98 days of life accounted for most of the variation in pre-weaning growth. Time of weaning had little effect (P<0·05) on birth weight of the next year's calf and although cows in the early- weaning groups produced calves which were the heaviest at 150 days (P>0·05) these calves weighed less than other calves at 240 days (P<0·001). Simple and multiple regression analyses were used to show that birth weight was affected significantly by cow-weight change during winter and these differences were in part independent of treatment and year. Calf birth weight increased as age of dam increased but this was due to increase in weight of cow with age. Within classes, calf birth weight had a significant and positive effect on weight gain after accounting for milk consumption and this was shown to be an effect of sex. Cows given concentrates produced more weight of 240-days-old weaned calf per year than those given none (P<0·01), and late weaning led to the production of more weaned calf per annum than early weaning (P< 0·001) even when allowance was made for differences in concentrate consumption.

The growth and efficiency of live-weight gain of 36 calves confined in pens was studied between 30 and 210 days of age. They were weaned individually at 30, 90 or 150 days old and offered one of three solid foods which contained 200, 300 or 400 g milled roughage/kg both before and after weaning. From 30 to 90 days and from 90 to 150 days weaned calves consumed more (P<0·001) solid food than sucking calves. During both these periods sucking calves consumed more (P< 0·001 and P<0·05 during the first and second periods respectively) metabolizable energy (ME), gained more (P<0·01) body weight and also gained more body weight/MJ ME consumed than weaned calves. Composition of solid food had little effect on calf performance.

The combined efficiency (kg calf weight gain/MJ ME consumed by cow and calf unit) was higher (P<0·001) for suckling cows and calves than for weaned cows and calves between 30 and 90 days. Between 90 and 150 days, weaning had no effect on combined efficiency.

Eight similar calves which were reared by their dams on veld until 210 days of age were heavier at 90 (P< 0·001) and 150 (P<0·05) days than penned calves which had been given concentrates ad libitum since weaning at 30 days. The body weights of these two groups were similar at 210 days.

In two experiments of Latin square design the effect of additives on the ad libitum intake of direct-cut perennial ryegrass silage and its digestibility by steers was determined. The diets were offered to British Friesian castrated male cattle, mean initial live weights 386 and 368 kg for Experiments 1 and 2 respectively. In Experiment 1, untreated silage was compared with silage treated with either formic acid (‘Add-F’) or a formalin and sulphuric acid mixture (‘Sylade’) each applied at the rate of 4·5 1/tonne of fresh herbage. The silages were offered either with or without 5 kg of supplement per day. In Experiment 2, silages treated with 4·5 and 9·0 1/tonne of the formalin and sulphuric acid mixture were compared. Silage treated with 4·5 1/tonne was offered either with or without 5 kg supplement/day. In Experiment 1, the application of an additive had no significant effect on silage digestibility. Although differences were not significant, the intake of silage dry matter was higher from additive-treated silage (91·2 and 80·6 g/kg W0·73 for the formalin and sulphuric acid mixture and the formic acid respectively) than from untreated silage (78·0 g/kg W0·73). Supplementation significantly reduced (P<0·05) silage dry-matter intake and crude fibre digestibility and significantly increased (P<0·05) total dry matter and digestible energy intake. Digestible energy intake from the formalin and sulphuric acid treated silage was significantly higher (P<0·05) than that from untreated silage but differences between the additive-treated silages were not significant.

In Experiment 2, neither rate of application of the formalin and sulphuric acid mixture nor supplementation had any significant effect on intake.

The effects of type of housing (outdoors on earth v. confinement on concrete) and system of rearing (single v. group) on age at puberty and expression of libido in boars were examined. Boars kept on earth lots and reared in groups reached puberty earlier and demonstrated greater sexual aggressiveness compared with contemporary boars maintained under other treatment combinations. Additionally, the effect of rearing boars either in all-male groups or withprepubertal gilts was examined. Age at puberty was not affected by the presence of females, but boars reared without females attained a higher libido score.

An analysis of the records of ewe weight and condition at mating together with subsequent lambing performance for several commercial flocks in the UK revealed no significant relationship between lambing performance and size, measured by weight transformed to a standard condition. The relationship between mating weight and body condition score at mating was linear though there was considerable variation in weight at a fixed body condition, suggesting a substantial spread in skeletal size. Increases in ovulation rate were not discernible from lambing figures over the range of body conditions observed in these flocks.

There was no evidence from three experiments with 60, 600 and 400 Welsh Mountain ewes producing mostly single lambs that housing, tested as an extreme form of shelter, was of benefit to reproductive performance. This was so when the ewes were kept indoors throughout the whole of the winter, or only at mating or during late pregnancy. Also, housing during early lactation did not lead to improved lamb growth rate.

Housing at lambing was associated in two experiments with reductions in perinatal lamb mortality from 5·1% to 3·1% and from 7·2% to 5·9%, though the differences were not significant.

Flushing had no significant effect on conception rate or reproductive performance, but the data were in accord with the view that when applied to ewes in moderately thin condition it leads to reduced barrenness and some increase in twinning.