Twenty-four British Friesian dairy cattle in their first lactation were used in a change-over design experiment with four periods, each of 4 weeks duration. The aim of the experiment was to examine the effects of varying both concentrate crude-protein concentration and the proportions of concentrates and grass silage in the diet on milk yield and composition. Twelve treatments were examined in a 3 × 4 factorial arrangement. Concentrates containing four levels of crude protein (CP), ranging from 120 to 206 g/kg fresh weight were offered in diets containing either 400, 500 or 600 g concentrates per kg dry-matter (DM) intake. The grass silage offered as the basal forage had a DM concentration of 214 g/kg and a digestible organic matter concentration of 682 g/kg DM.

Milk yields (kg/day) for cows offered diets containing 400, 500 and 600 g concentrates per kg DM were 17·2, 18·1 and 18·3 respectively; and 17·4, 17·6, 18·0 and 18·4 for cows offered diets with concentrates containing 120, 147, 176 and 206 g CP per kg fresh weight respectively (pooled s.e. 0·13). Increasing either the proportion of concentrates in the diet or the CP concentration of the concentrates did not significantly affect milk fat concentration, but did increase milk protein concentration.

There was a trend towards a greater milk-yield response to protein supplementation with low- rather than high-concentrate diets, although it appears that this trend may only be apparent in situations where total food intake is restricted.

One hundred first lactation Friesian dairy heifers were used to investigate the effect of level of nutrition n i late pregnancy and for a limited period around the time of insemination on reproductive performance. The heifers were housed throughout the period of the trial in cubicle yards and all individuals were given controlled amounts of food.

Fifty animals were offered a ‘high’ (H) level of feeding in late pregnancy (for 10 weeks before calving) and 50 were offered a ‘low’ (L) level. After calving, the heifers were re-randomized with half of each pre-calving group being offered either the H or L level of feeding in early lactation (weeks 6 to 18), thus forming four subgroups of 25 heifers: HH, HL, LH and LL. The level of feeding in late pregnancy for all groups increased towards calving. The high plane heifers consumed 83·6 MJ metabolizable energy (ME) per day on average compared with 64·6 MJ ME per day for the low plane heifers, a proportional mean difference of 0·29 (P < 0·001). After calving, heifers consumed an average of 86 MJ ME per day in week 1 rising to 126 MJ ME per day by week 5. Between weeks 6 to 18 of lactation, the high plane group consumed an average of 146·8 MJ ME per day compared with 119·8 MJ ME per day for the low plane group, a proportional mean difference of 0·23 (P < 0001).

There were no significant effects of feeding treatments on milk yield. Fat yield in weeks 6 to 18 was increased by the high level of feeding in lactation (P < 0001), while protein yields in weeks 1 to 5 and n i weeks 6 to 18 were increased by high plane feeding in pregnancy (P < 0·05) and lactation (P < 0·05), respectively. Heifers receiving the high level of feeding in late pregnancy gained significantly (P < 0·001) more weight than those on the lower level of feeding. All groups lost similar amounts of weight during the first 5 weeks after calving, but there were marked differences in the rate of live-weight gain between weeks 6 to 11 of lactation (P < 0·05 to P < 0·001). In particular, the HL group had the lowest rate of live-weight gain, and this was significantly lower than that of the HH group (P < 0·01).

First ovulation occurred significantly earlier in heifers receiving the high level of feeding in pregnancy P ( < 0·01), but there was no effect of treatment on ovulation detection rate, time to first oestrus, time of first insemination or the proportion of wrongly-timed inseminations. Heifers receiving the high level of feeding in lactation had a successful pregnancy rate to first insemination of 0·42 compared with 0·63 for those on the lower level of feeding (P < 0·05), with the greatest difference observed between the HL group (0·72) compared with the HH group (0·35) (P < 0·01). Embryo loss rate was low in the HL group (008) compared with that in the HH group (0·26; P < 0·l). The net result was that the HH group had a median interval from calving to successful pregnancy of 95 days with an interquartile range of 82 days, whereas the figures for the HL group were 75 and 23 days (P < 0·001). Live-weight change at artificial insemination (AI) and level and change in milk yield at AI had independent effects on reproductive performance and as a consequence net energy output showed the strongest relationship with days to successful pregnancy. Heifers with the highest net energy outputs per day at AI had the longest intervals to successful pregnancy (r = 0·408; P < 0·001).

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.

Two experiments were carried out to compare three methods of harvesting grass silage for beef production. These involved (a) harvesting the grass directly with a flail forage harvester (b) pre-cutting the grass with a rotary mower and harvesting it with a precision-chop forage harvester without wilting and (c) pre-mowing and harvesting with a precision-chop forage harvester after field wilting for 36 to 80 h (experiment 1) or 20 to 24 h (experiment 2). Half of the grass harvested by each method received formic acid at a rate of 2·5 1/t while the other half received no additive. The silages were offered ad libitum and supplemented with 2·6 kg concentrates per head daily to 72 castrated male cattle in each experiment. The unwilted and wilted herbages had mean dry-matter concentrations of 163 and 273 g/kg at ensiling and all silages were well preserved. Harvesting silage with a precision-chop rather than a flail forage harvester produced only a small non-significant increase in intake (0·034) and performance (0·046) and did not affect carcass gain per ha. Field wilting for 36 to 80 h prior to ensiling (experiment 1) reduced animal performance by 0·08 and beef output per ha by 0·11. Wilting for 20 to 24 h in ideal weather (experiment 2) did not affect animal performance or beef output per ha compared with that produced by well-preserved, unwilted silage. There was no significant interaction between method of harvesting and additive treatment.

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.

One hundred first lactation Friesian dairy heifers were used to investigate the effect of level of feeding n i late pregnancy (83·6 or 64·6 MJ metabolizable energy (ME) per head daily and around the time of first artificial insemination (AI) (weeks 6 to 18 of lactation: 146·8 or 119·8 MJ ME per head daily) on reproductive performance.

Heifers receiving the high level of feeding in lactation had a successful pregnancy rate to first AI of 0·42 compared with 0·63 for heifers receiving the lower level of feeding. Heifers receiving the higher level of feeding in late pregnancy followed by the lower level in lactation had a pregnancy rate of 0·72 o t first AI resulting in a median interval from calving to pregnancy of 75 days compared with figures of 0·35 and 95 days respectively, for heifers receiving the higher levels of feeding throughout the experiment (P < 0·001).

Blood samples were taken from all heifers 2 weeks before and on weeks 1, 5, 9, 13 and 18 after calving, and were analysed for 13 components. Of the blood components measured, only p"-hydroxy-butyrate concentrations, particularly at week 9, were affected by the nutritional treatments imposed and consistently related to fertility (P < 0·01 to P < 0·001).

Milk yield and live-weight change, individually and as the components of net energy output, showed the strongest relationships with reproductive performance (P < 0·01 to P < 0·001). The three variables which together accounted for the highest proportion (0·30) of the variation in days to successful pregnancy were net energy output at AI, (3-hydroxy-butyrate at week 9, and mean tail score in weeks 10 o t 13. These enabled the calving to successful pregnancy interval for an individual or 100 first lactation heifers to be predicted with a 95% confidence interval of ±58 days or ±8·6 days, respectively.

Two experiments were carried out to examine the effects on the intake and performance of calves of supplementing a diet, based on high-digestibility, well-preserved grass silage, with fish meal. The effects of fish-meal supplementation were examined at each of two amounts of concentrate intake (1·05 and 2·2 kg/day) in a 2 × 2 factorial design. Fish meal was offered at a rate of 75 g/kg silage dry-matter (DM) intake. Experiments 1 and 2 involved 36 and 40 castrated male calves, respectively, which were initially 7 weeks old and had a live weight of 70 kg. The treatments were imposed for 20 weeks. The silages used in the two experiments had mean concentrations of 22-8 g nitrogen (N) per kg DM; 524 g protein N per kg total N; 59 g ammonia N per kg total N; 0·4 g butyric acid per kg DM; and 712 g digestible organic matter per kg DM. The concentrate consisted of (g/kg) 800 ground barley, 125 soya bean meal, 50 molasses and 25 mineral and vitamin supplement, respectively, and contained 29 g N per kg DM. Silage DM intakes and live-weight gains for the low and high amounts of concentrate supplementation were 1·93 and 1·46 (s.e. 0·055) kg/day and 0·84 and 1·01 (s.e. 0·013) kg/day respectively. The supplement of fish meal did not affect silage DM intake or animal performance for either concentrate intake. Proportionately, only 0·21 of the difference in live weight at the end of the treatment periods between the two amounts of concentrate intake remained at slaughter at 18 months of age. It is concluded that the optimum growth rate for autumn-born calves during their first winter can be sustained by a diet of high-digestibility grass silage supplemented with a maximum of 1·0 kg cereal-soya bean meal concentrate and that no response in performance was obtained to supplementation of this diet with fish meal.

Silages were made from the primary growth of perennial ryegrass (Lolium perenne) or red clover (Trifolium pratense) and were given ad libitum to 52 British Friesian cows, together with 70 kg dry matter per day of a barley/ground maize/soya bean supplement in 2 years. Twenty of the cows were used in both years, providing a total of 72 observations. Three treatments were imposed during weeks 3 t o 20 of lactation: red clover silage, (RC/RC); red clover silage, weeks 3 to 10, grass silage weeks 12 to 20 (RC/PRG); grass silage (PRG/PRG).

Apparent digestibility of the gross energy assessed in lactation weeks 14 and 15 was slightly lower for red clover than for perennial ryegrass (0·683, 0·706 and 0·701 for treatments RC/RC, RC/PRG and PRG/PRG respectively). There was no effect of previous treatment on the digestibility of perennial ryegrass diets (RC/PRG v. PRG/PRG). Cows given red clover silage consumed proportionately 1·22 of the dry matter and 1·09 of the digestible energy ingested by those given perennial ryegrass. The intake of perennial ryegrass in mid lactation was similar, irrespective of early lactation treatment.

During weeks 3 to 20, cows given red clover yielded 1·9 kg more milk than did those given perennial ryegrass (P < 0·01). However, this milk had a significantly lower concentration of fat (P < 0·01; 37·2 and 41·2 g/kg for treatment RC/RC and PRG/PRG respectively). In mid lactation the cows that had received red clover silage in early lactation (RC/PRG) had the lowest yield of milk, milk fat, protein and lactose and the difference between this treatment and PRG/PRG was significant for milk and milk fat yields (P < 0·05).

Red clover silage rather than perennial ryegrass silage of similar digestibility resulted in higher silage intake and higher yields of milk, milk protein and lactose. However, the strategic use of red clover silage to increase yield in early lactation did not induce positive residual effects in mid lactation; rather, there was evidence of a negative effect on milk and milk-fat yield.

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.

Four concentrate mixtures were given with grass silage to 52 cows in early lactation in an 8-week randomized block production trial. A further eight cows were used to measure apparent digestibilities of the total diets in a double 4 × 4 Latin-square trial.

The concentrates contained varied proportions of barley, soya bean meal (soya) and formaldehyde-treated soya bean meal (treated soya) together with fixed proportions of wheatfeed and molasses. Crude protein (CP) concentrations in the concentrate mixtures and sources of supplementary protein were 133 g/kg air-dry concentrate, soya (A); 130 g/kg, treated soya (B); 187 g/kg, soya (C); 169 g/kg, soya plus treated soya (D). In the production trial, 9 kg concentrates were given daily together with silage ad libitum, whilst in the digestibility trial the daily ration was 7·6 kg concentrates and 6·6 kg silage dry matter (DM). The silage contained 462 g DM per kg and 140 g CP per kg DM with an in vitro digestibility of DM of 0·7.

The digestibilities of DM, organic matter and energy showed similar trends. Cows given treatments A and B had significantly lower digestibilities of energy than did those given treatments C and D.

Nitrogen digestibility was highest for cows given treatment C and lowest for those given treatments A and B. Silage DM intake and live-weight change were unaffected by treatment during weeks 1 to 8, all treatment groups being in positive live-weight balance in this period.

Milk yields during weeks 1 to 8 were 18·8 (A), 19·6 (B), 20·3 (C) and 20·7 (D) kg/day. Yields for cows given treatments C and D were significantly higher than those for cows given treatment A. There were no differences between treatments for any constituent of milk, but protein concentration in milk tended to be lower for cows given treatment D. During weeks 9 to 20, there were no significant residual effects of treatment upon milk yield or composition.

Parallel regression analysis of milk yield and yields of milk constituents upon CP concentration in the concentrate indicated significant effects of CP concentration on yield of milk, milk fat, protein and lactose during weeks 1 to 8. When examined during weeks 1 to 20 the responses to increased CP concentration were non-significant. The analysis showed a significant positive milk-yield response to formaldehyde treatment during weeks 1 to 8 and 1 to 20. The response in lactose yield was also significant during weeks 1 to 8 and 1 to 20. There was a trend, though not significant, towards an increased milk-fat yield during weeks 1 to 8 for the formaldehyde treatment, whilst protein yield was unaffected.

Two experiments, each involving 48 British Friesian cattle, were carried out to examine the performance and carcass quality of bulls and steers implanted with one of three oestrogenic growth promoters in a semi-intensive system of beef production in which the animals were slaughtered at 14 to 17 months of age. The treatments used in experiment 1 were: (1) control steers (no implant); (2) steers implanted with 45 mg oestradiol in silastic rubber at 4·5 months of age and with 300 mg trenbolone acetate at 10·5 and 13·5 months; (3) steers implanted with 20 mg oestradiol and 200 mg progesterone at 95-day intervals from 4·5 months and with 300 mg trenbolone acetate at 10·5 and 13·5 months of age; (4) bulls (no implant). For treatments 1 to 4 respectively dry-matter (DM) intakes were 6·74, 7·24, 7·50 and 7·16 (s.e. 0·092) kg/day; carcass weights were 260, 283, 298 and 290 (s.e. 5·4) kg; and fat depths over m. longissimus were 6·5, 5·3, 6·3 and 3·4 (s.e. 0·53) mm. The same treatments were used in experiment 2, except that 36 mg Zeranol® was used in treatment 3 instead of 20 mg oestradiol plus 200 mg progesterone, and the bulls in treatment 4 were implanted with 36 mg Zeranol at 7·5, 11 and 14·5 months. For treatments 1 to 4 respectively DM intakes were 7·37, 7·85, 7·76 and 7·42 (s.e. 0·092) kg/day; carcass weights were 262, 285, 284 and 296 (s.e. 7·1) kg and mean fat depths over m. longissimus were 7·1, 5·9, 7·0 and 5·2 (s.e. 0·54) mm. Implantation of steers increased food intake but also improved food conversion efficiency and did not consistently affect carcass fatness or conformation. Repeated implantation with 20 mg oestradiol plus 200 mg progesterone produced a greater response in performance than a single implantation with 45 mg oestradiol-17β in a controlled-release silastic-rubber implant, while repeated implantation with 36 mg Zeranol produced a similar response to the single silastic-rubber implant. Bulls produced leaner carcasses and converted food to carcass gain more efficiently than implanted steers.

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.

Friesian steer calves born in August were reared from 3 to 7 months of age on either a low (OL) or high (OH) plane of nutrition. A further group born in October (YH) was also reared on a high plane. At grazing, the performance of calves of the same age but different weights (OL v. OH) or the same weight but different ages (OL v. YH) were compared at two levels of herbage allowance, either 30 or 60 kg dry matter per kg live weight. During the winter, the H groups received silage and concentrate and the L group was given silage only. The OL group had a higher concentration of fat and a lower concentration of water in the empty body at the end of the winter than did the OH and YH groups respectively.

Compensatory growth was exhibited by the OL group at both herbage allowances. Differences in the live-weight and empty body-weight gains of OL and OH cattle could be explained almost entirely by differences in the energy content of the gains whereas this was only partially so when the OL and YH groups were compared. The results indicate that YH cattle had lower herbage intakes even though they were of similar weight to OL cattle.

It was concluded that silage nitrogen is used less efficiently by cattle than is generally assumed and that the levels of gut fill and energy content of the live-weight gains in this experiment were not consistent with the general values suggested by the Agricultural Research Council (1980).

In two experiments the effect of giving concentrates to dairy cows from a fully-controlled out-of-parlour dispenser was compared with giving concentrates twice daily in the milking parlour. In experiment 1, 22 cows were used in a cross-over design during the first 19 weeks of lactation. In experiment 2, 34 cows were used in a continuous trial during the first 18 weeks of lactation. Weeks 1 to 3 of each experiment were used as a standardization period during which all cows were given equal amounts of concentrates twice a day during milking. At the end of this period cows were paired and assigned to receive concentrates either in the milking parlour (group P) or to receive 1·0 kg concentrates per day in the milking parlour and the remainder from an out-of-parlour dispenser (group O). Within pairs, between treatments, cows were offered the same amount of concentrates and allowed access to ad libitum silage. Means for milk yield, fat and solids-not-fat concentrations during experiment 1 were 25·1 v. 23·9 kg/day (P < 0·01), 36·5 v. 38·1 g/kg, 89·9 v. 89·1 g/kg (P < 0·001), and for experiment 2 were 27·1 v. 25·4 kg/day (P < 0·01), 39·8 v. 42·6 g/kg, and 90·9 v. 90·9 g/kg, for groups O and P respectively. Mean concentrate intakes during experiment 1 were 8·3 v. 8·4 kg/day (P < 0·01) and during experiment 2 were 8·9 v. 8·6 kg/day (P < 0·01) for groups O and P respectively. In both experiments, group O had more positive changes in weight and body condition score (P < 0·05). It is concluded that the use of the dispenser improved milk yield, weight and condition-score changes, though milk fat concentration was decreased compared to that of parlour-fed cows.

Post-weaning metabolizable energy intake, growth of empty-body chemical components and efficiencies of energy utilization were evaluated for Hereford intact males (17) and females (16) and Simmental intact males (15) and females (16) during a 212-day feeding period. Within each breed × sex subclass, animals were assigned to one of three levels of metabolizable energy (ME) intake: (1) 544 kJ/kg M0·75 per day, (2) 795 kJ/kg M0·75 per day, and (3) ad libitum. Body composition of each animal was estimated at the beginning and end of the feeding period by deuterium oxide dilution.

Protein and water gain of Hereford and Simmental cattle were similar at restricted levels of intake but were greater for Simmental than for Hereford cattle at ad libitum intakes. Similarly, rates of protein and water gain tended to increase more rapidly in response to increased energy intake by males than by females. Hereford males gained fat and energy slightly more rapidly than Hereford females, but Simmental males gained fat and energy at slower rates than Simmental females.

Males had higher maintenance requirements and tended to use ME with less efficiency for maintenance and gain than females. Hereford cattle had lower maintenance requirements and used ME with greater efficiency for both maintenance and gain than Simmental cattle.

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.

Female lambs were given a high energy and high protein diet in varying amounts to achieve either high (H; 220 g/day) or low (L; 110 g/day) rates of gain during two consecutive periods between 4 and 20 and 20 and 36 weeks of age. The effects on body growth and mammary gland development were compared by slaughtering an initial group of four lambs at 4 weeks of age (mean live weight 11 kg), eight L lambs (24 kg) and eight H lambs (33 kg) at 20 weeks of age, and eight LL lambs (36 kg), eight LH lambs (49 kg) and eight HL lambs (48 kg) at about 36 weeks of age.

At slaughter, a single mammary fat pad from each lamb was trimmed and weighed, and the deoxyribonucleic acid (DNA) content of the portion containing parenchyma determined. The mean weight (g) and DNA content (mg) were: 4-week-old lambs, 8·6 g, 1·5 mg; L, 14·7 g, 32·3 mg; H, 30·0 g, 25·9 mg; LL, 46·0 g, 61·2 mg; LH, 86·7 g, 91·0 mg; HL, 70·3 g, 73·0 mg respectively. Relative growth coefficients for mammary parenchyma, estimated from the increase in DNA relative to that for live weight, were 3·7 and 2·4 for L and H lambs respectively, between 4 and 20 weeks, and 1·6, 1·4 and 2·6 for LL, LH and HL lambs respectively, between 20 and 36 weeks of age.

These results are consistent with the hypothesis that a high plane of nutrition can decrease the rate of allometric growth of mammary parenchyma before puberty. However, the cessation of this growth phase was not related to the attainment of puberty, or of a specific live weight or age, but may be related to the availability of mammary fat pad tissue limiting further parenchymal development after 20 weeks of age.

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.

Female crossbred lambs were given a high energy and high protein diet in varying amounts to achieve either high (H; 220 g/day) or low (L; HOg/day) rates of live-weight gain during two consecutive periods between 4 and 20 and 20 and 36 weeks of age. Blood samples were collected via jugular catheters at either 1-h or 2-h intervals over a 28-h period from five lambs on each rearing treatment at 10, 14, 18, 26 and 34 weeks of age for the measurement of prolactin, growth hormone (GH) and insulin. In addition, samples were collected within 2 weeks of slaughter from all eight lambs on each treatment that were slaughtered at either 20 weeks (L and H treatments) or 36 weeks of age (LL, LH and HL treatments) for the assessment of mammary development.

The timing of once-daily feeding influenced the plasma profiles of all three hormones measured, but became less obvious with increasing age. Both mean plasma GH, and the influence of level of nutrition on the mean, declined with increasing age. Mean plasma insulin increased with age but did not differ between rearing treatments until 26 weeks, despite large differences in food intake. Mean plasma prolactin in H lambs increased significantly between 10 and 18 weeks, but a similar response did not occur in L lambs. The greater mammary development of L lambs was associated with increased plasma concentrations of GH and reduced concentrations of prolactin. However, the large variation in total mammary parenchymal deoxyribonucleic acid between individuals within each rearing treatment at 20 or 36 weeks of age was not consistently explained by the variation in any of the hormones measured.