The effect of chronic administration of bovine growth hormone (GH) on milk production, food intake and live-weight change was evaluated in five sets of monozygotic twin cows on pasture. Purified bovine pituitary GH (specific activity, 0·78 IU/mg) was administered by daily subcutaneous injections (39 IU/day) for a period of 22 weeks (weeks 5 to 26 of lactation). GH treatment resulted in significantly higher yields of milk (23·3 kg/day), fat (0·97 kg/day), protein (0·74 kg/day) and lactose (115 kg/day) compared with the control group (19·8 kg/day, 0·79 kg/day, 0·63 kg/day, 0·99 kg/day). Milk composition did not differ between treatment groups. There was no difference in the intake of cut grass in week 8 but the voluntary intake of the GH group had increased by week 22 (controls, 15·4 kg dry matter (DM) per day and GH group, 17·5 kg DM per day). There were no differences in the rate of live-weight change for the two groups. Serum somatomedin concentrations were significantly elevated in the GH group on weeks 20 to 22 of treatment (0·043 v. 0·135 U/ml). This experiment indicates that cows chronically treated with GH were able to adjust their food intake upwards to sustain a substantial increase in milk production on a diet composed solely of grass.

An analysis was conducted on milk production data collected from 1954 to 1981 at Turrialba, Costa Rica. The local conditions are humid and tropical without a sharply defined dry season. The data included purebred Central American Milking Criollos, Jerseys, their reciprocal F, crosses and backcrosses. Two data sets were created: one with minimum restrictions intended to include production data from all calvings, and another data set intended to include only lactations with normal let-down without the presence of the calf. Data were analysed by maximum likelihood according to a model that included the fixed effects of year of calving, parity of lactation, breed or cross and the regression on year of birth of the sire's first daughter and random effects of sire, cow and error. Breed or cross differences were significant for milk yield, fat-corrected milk (FCM) yield and lactation length but were not significant for fat concentration.

F1 crosses were superior to both parental breeds for milk and FCM yields for both data sets; there were no significant reciprocal effects. The ¾ Criollo backcross had significantly lower milk and FCM yields and shorter lactations than did the F1 or ¾ Jersey. These results suggest that breeding programmes for this type of environment should backcross F1's to Jersey sires and use criss-crossing from then on. The proportion of heterosis for milk, FCM, butterfat concentration and lactation length was 0·26, 0·28 0·02 and 0·15 for minimum-restriction lactations and 0·21, 0·22, 0·01 and 0·09 for normal lactations. Heritabilities for milk, FCM, fat concentration and length of lactation were 0·44, 0·42, 0·28 and 0·41 for minimum restriction lactations and 0·28, 0·27, 0·35 and 0·08 for normal lactations. The relatively higher values for minimum-restriction records suggest a strong genetic effect on ease or difficulty of milk let-down.

A total of 40 individually-penned British Friesian male calves were offered milk substitutes ad libitum from 2 days until 12 weeks of age. Ten milks were reconstitued from either liquid or spray-dried skim milk and a fat-filled (500 g fat per kg) powder and contained 80, 110, 140, 170 or 200 g dry matter (DM) per kg liquid. A further six calves were used in a randomized-block experiment to measure the digestibility, absorption and retention of the diets containing 80, 140 and 200 g DM per kg.

There are no significant differences in the intake, growth rate, digestibility, absorption or retention between calves given milk substitutes reconstituted from liquid or spray-dried skim milk.

At all ages, as the DM concentration of the milks increased, liquid intake decreased but DM intake and live-weight gain increased. The mean live-weight gain was 0·99, 1·13, 1·16, 1·28 and 1·27 kg/day for calves given the milk substitute diet at concentrations of 80, 110, 140, 170 and 200 g DM per kg, respectively. The DM intake to live-weight ratios were constant at all ages when live weight was expressed to the power of 0·61.

No significant adverse effect of the diets was evident from the fortnightly monitoring of jugular blood samples for packed cell volume and concentrations of glucose, urea-N, Na, total protein, albumin, globulin and haemoglobin; nor from faecal consistency, rectal temperature, heart rate and respiration rate.

There was no indication that age affected digestibility, except that digestive efficiency was reduced in calves with diarrhoea, particularly at 2 to 4 weeks of age; nor that dietary concentration influenced digestion or absorption. The true absorption of N, Ca and P from the milk diets was high with mean coefficients of 0·99, 1·01 and 0·95 respectively. The main endogenous losses of N and P occurred through the urine and of Ca through the faeces.

Thirty-two British Friesian calves were individually offered milk substitutes ad libitum twice daily for 12 weeks. The milks were reconstituted to contain 170, 200, 230 or 260 g dry matter (DM) per kg, using spray-dried skim milk powder and homogenized fat-filled powder containing 500 g fat per kg. Drinking water was available ad libitum at all times. The last two blocks of four calves were used to measure the digestibility, absorption and retention of the constituents of the diets at 2, 6 and 11 weeks of age.

The DM intakes of the calves given the four diets were variable but generally increased with increasing DM concentration, mean values being 61·5, 59·5, 66·0 and 73·3 g DM per kg M0.75. As the calves increased in age, their intakes decreased from 67·3 g DM per kg M0.75 at 2 weeks of age to 61·1 g DM per kg M0.75 at 8 weeks of age.

The higher DM concentrations and intakes were associated with the higher rates of weight gain; the mean live weights at 12 weeks of age being 121·8, 123·9, 135·3 and 141·6 kg for the calves offered milks reconstituted at 170, 200, 230 and 260 g DM per kg respectively.

The concentration of the DM had no effect upon the coefficients of digestion or absorption nor on the retention of energy, protein, calcium or phosphorus. The digestibility of protein increased, whilst the absorption and retention of Ca and P declined with age.

Intake of drinking water did not change with age, but was higher for calves given the milk containing 260 g DM per kg than for those given the other three concentrations. The total water intakes were similar for the calves given milks containing 200, 230 and 260 g DM per kg.

The linear relationship of DM intake to DM concentration of the milk substitute suggested a lower level of intake in the present experiment, than was observed in a previous experiment (Ternouth, Stobo, Roy and Beattie, 1985b) when the milk concentrations were lower and drinking water was not available. When the concentrations of the DM were re-calculated to take into account the quantity of drinking water consumed, the relationships between DM intake and adjusted DM concentration for the two experiments were similar.

Data from a large controlled experiment to investigate the effect of level of nutrition on reproductive performance were used to assess the value of production and blood measures as indicators of energy status in lactating dairy heifers. Live-weight change showed the strongest and most consistent relationship to mean energy balance (the difference between metabolizable energy intake and that used for milk production and maintenance) (P < 0·01 to P < 0·001). Body-condition score at a particular time was more closely related to mean energy balance in the preceding 4-week period (P < 0·05) than current energy balance. There was also a lag in the relationship between energy balance and live-weight change and mean body-condition score. Ultrasonic back fat measurements were significantly correlated with both loin and tailhead body-condition score (P < 0·001) but were more strongly related to mean energy balance in the preceding period (P < 0·05 to P < 0·001) than the body-condition scores.

Blood samples were taken from all heifers 2 weeks before calving and 1, 5, 9, 13 and 18 weeks after calving and were analysed for 13 constituents. Concentrations of blood metabolites did not show consistently strong correlations with mean energy balance. The only blood metabolite to be measurably affected by the nutritional treatments applied in lactation was β-hydroxybutyrate.

At best, combinations of production measures and blood metabolites were only able to predict the mean daily energy balance with a 95% confidence interval of ±20 MJ for an individual animal although this confidence interval was reduced to ±3 MJ for 100 animals.

Measurements of growth rate, food intake and food conversion efficiency on 235 Hereford bulls, from the first 4 years of a selection experiment, were used to assess the effects of rearing method, dam age and year/season of birth. Bulls were either reared artificially from birth to 84 days of age, or reared on their own dam to 84 or 168 days of age, to examine whether non-genetic effects on performance could be reduced by early weaning. All bulls were performance tested on ad libitum feeding from 200 to 400 days of age. Artificially-reared bulls had lower live weights at the start of test, which led to higher food conversion efficiency on test. Bulls reared naturally to 84 days of age were least affected by dam age and year/season of birth and performed as well as those weaned at 168 days of age. Earlier weaning of bulls entering central performance tests would reduce these environmental effects on performance, and may increase the accuracy of identifying bulls with genetically-superior growth and efficiency.

Burdens of ticks (Boophilus microplus) on young and adult females (heifers and cows) of six red-and-white Holstein-Friesian (HF) × Guzera (G) grades were assessed by counting the number of semi-engorged tick females on the right side of the animals. The HF grades were: ¼ ½ ⅝ ¾ ⅞ and ≥ Assessments of tick burdens of heifers were made on twelve occasions (357 observations on 193 animals). Cows were assessed six times (380 observations on 83 animals). Cows and heifers were in different pastures. Data were transformed to log10 (2 × count + 1) to normalize their distribution. Direct breed additive (g1, HF-G) and heterosis (hl) effects were estimated within dates of counting. Estimates of h1 on the log scale were not significant (P > 0·05) on 11 out of 12 dates for heifers and on five out of six dates for cows. Estimates of g1 for heifers varied from 0·505 (s.e. 0·492) to 2·376 (s.e. 0·345) (mean g1 = 1·575 (s.e. 0·096)), and estimates for cows varied between 1·009 (s.e. 0·203) and 2·293 (s.e. 0·219) (mean g1 = 1·416 (s.e. 0·080)). These results indicate the presence of important genetic effects on tick burdens. The means of the untransformed number of ticks per animal were, for the six grades in the above order, respectively: 44, 71, 151, 223, 282 and 501, for heifers, and 7, 19, 31, 64, 62 and 97 for cows.

In a multibreed experiment in which 292 heifers from 25 breeds were given a single complete pelleted diet ad libitum, body weight and food intake were recorded every 2 weeks. Relative growth rate, relative food intake and food conversion efficiency were calculated as the ratio of weight gain to body weight, food intake to body weight and weight gain to food intake respectively. The traits were measured over 12-week intervals from 12 to 72 weeks of age, and also over longer intervals of 24, 36, 48 and 60 weeks, each beginning at 12 weeks of age. Relative growth rate and food conversion efficiency declined continuously with increasing age, whereas relative food intake increased to a maximum in the 24- to 36-week period and then declined continuously.

The between-breed genetic coefficient of variation (CVb) for relative food intake was very stable at about 0·04 whether measured over 12-week or longer intervals, whereas the CVVs for relative growth rate and food conversion efficiency were usually higher when measured over 12-week intervals compared with longer intervals. For the entire period from 12 to 72 weeks, the CVb was estimated as 0·029 for relative growth rate, 0·040 for relative food intake and 0·034 for food conversion efficiency. The corresponding intraclass correlations (t2) measuring the between-breed variation as a proportion of the total variation were estimated as 0·08, 0·18 and 0·15. Within-breed variation was thus much greater than between-breed variation for all three traits. Using published estimates of within-breed heritabilities, the parameters g21 measuring the between-breed genetic variation as a proportion of the total genetic variation, and g22, measuring the between-breed genetic variation as a proportion of the immediately selectable genetic variation, were estimated as 0·28 and 0·61 for food conversion efficiency and 0·25 and 0·57 for relative growth rate. Thus, for all three traits, despite their low CVb and t2 values, between-breed selection would be useful prior to within-breed selection.

In an examination of the productivity of goats and sheep on a Masai group ranch in Kenya, estimated least-squares means for pre-weaning mortality were 28·6% for goats and 32·1% for sheep. Three productivity indices based on total live weight of young produced per female per year, per kg female per year and per kg metabolic weight per year gave values of 11·1 kg, 410 g and 0·99 kg for goats and 14·1 kg, 473 g and 1·18 kg for sheep. The effects of environmental variables on these indices, the comparative advantage accruing to the variables, mortality, reproductive success and weight gain were used to construct a proposed set of interventions for the improvement of small ruminant production in south-central Kenya.

In crossbred female lambs given a concentrate diet ad libitum between 8 and 20 weeks of age, daily subcutaneous injections of 0·1 mg bovine pituitary growth hormone (GH) per kg live weight increased daily live-weight gain (347 v. 284 g/day; P < 0·01; no. = 8), food conversion efficiency (3·94 v. 4·49 kg dry matter per kg gain; P < 0·01) and greasy fleece weight (1·49 v. 0·99 kg; P < 0·001). The increase (4·8 kg) in final live weight was due primarily to an increase in the non-carcass components of the body (3·5 kg), with little effect on carcass weight (1·3 kg). However, bovine GH treatment markedly increased the weights of lean tissue (11·4 v. 9·2 kg; P < 0·001) and bone (2·8 v. 2·4 kg; P < 0·001) and moderately reduced the weight of fat (7·0 v. 8·0 kg; P < 0·10) in the carcass. The bovine GH treatment raised plasma concentrations of immunoreactive GH within the physiological range for about 16 h each day and significantly increased mean plasma concentrations of insulin and prolactin. Daily injection of 1 mg bromocriptine had no effect on daily live-weight gain, food conversion efficiency or carcass composition. This treatment markedly reduced plasma concentrations of prolactin but also significantly reduced insulin concentrations. When given in combination with bovine GH, bromocriptine reduced the GH-induced stimulation of insulin concentration and tended to decrease the effects of GH on food conversion efficiency and growth. This interaction was significant only for the effects on greasy fleece and skin weights (P < 0·01).

The extent to which genetic selection to change the concentration of copper (Cu) in plasma also affected the retention of Cu in the liver was assessed in three experiments. In each, the concentration of Cu was measured in samples of liver (taken by biopsy) and of plasma from female sheep of lines selected from a Scottish Blackface × Welsh Mountain population (which had previously been interbred for several generations) for high and low plasma Cu levels (high line and low line) and from purebred Scottish Blackface and Welsh Mountain females.

The concentration of Cu in the liver and plasma was found to be consistently greater in high-line and Welsh Mountain than in low-line and Scottish Blackface females (P < 0·01) in the first experiment in which grazing animals were sampled at both 6 and 10 months of age and in the second, where sampling' was at 6 months only. Mean Cu concentrations in the liver ranged from 13 to 115 mg/kg dry matter (DM) and the correlations with those in plasma varied from 0·28 to 0·53 (all P < 0·05) and was highest when the concentration of Cu in the liver was lowest. The correlation between the concentrations of Cu in liver at different ages was 0·57 (P < 0·05).

In the third experiment, 10-month-old high- and low-line females were given complete diets containing either 5, 10, 22 or 28 mg Cu per kg DM for 12 weeks. Liver samples were obtained at the beginning and end of this period and blood samples were taken every 3 weeks. The concentration of Cu in the liver was greater in high- than in low-line sheep on all diets and increased linearly with the Cu concentration of the diet but at a greater rate in the high line. At the end, the high line had significantly greater concentrations of Cu in plasma on the 5 and 10 mg Cu per kg DM diets.

The experiments showed that high-line females retained more Cu in the liver than low-line females. It is most likely that this was caused by a positive genetic correlation between concentration of Cu in plasma and the efficiency of absorption of Cu.

Forty-one Texel and Texel-Oxford sires were selected over 5 years (1978-82) on high and low adjusted 100-day weight from 174 ram lambs artificially reared from birth and performance tested. The correlated responses to selection were measured in 1103 crossbred progeny, out of an experimental group of crossbred ewes, the progeny being naturally reared in field conditions. Hierarchical analysis of variance was performed, fitting the usual fixed effects and interactions. Co-heritabilities (rAh2h2; rA is the genetic correlation between traits with heritabilities h12 and h22) were estimated from offspring-parent regression and from the realized responses to selection. The selection differentials achieved (1·70 standard deviations in 100-day weight between high- and low-weight sires) were lower than expected, due to mating difficulties, mortality and other requirements for the stock. The co-heritabilities for growth traits were generally positive but small (mean 0·08; s.e. 0·08). Though not statistically significant, lambs slaughtered at a fixed weight from the high-weight rams were slaughtered earlier with lighter and leaner carcasses, as expected. Similarly, lambs slaughtered according to fat cover from high-weight rams were slaughtered later with heavier slaughter and carcass weights. Generally, the co-heritabilities for carcass traits were not significantly different from zero for either slaughter group. On the basis of these results, selection on ram 100-day weight with artificial rearing may not offer any advantage over natural rearing in the improvement of lamb growth and carcass traits.

The variable net energy system for ruminants was developed to allow rations to be formulated in an additive manner. It is demonstrated that the variable net energy system is based on approximations which can cause errors in ration formulation. The extent of these errors is examined for cattle rations, and it is shown that the errors can be significant.

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.

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.

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.

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.

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.

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.