The concepts of metabolic time and size are used, respectively, to compare maturing time and energy exchanges during growth in homeotherms. Maturation rate, expressed per unit of mature size (A1/4) is relatively constant for mammals, irrespective of attempts by man to select for rapid growth. Precocial birds mature more rapidly, however, and altricial birds more rapidly still. These differences are explained in terms of different patterns of energy exchange. It is proposed that maturation time is ultimately determined by size-related constraints on rate of metabolism during growth.

Gestation length and birth weight of 939 calves born at Santa Monica Experimental Station, Valenc, a, State of Rio de Janeiro, were studied. The calves were of six red and white Holstein-Friesian (HF) × Guzera (G) grades: 1/4, 1/2, 5/8, 3/4, 7/8 and ≥ 31/32.

As the grade × sex interaction for birth weight was significant, data for each sex were analysed separately. A model, including the effects of grade, year-season of birth, grade × year season interaction and age of dam as a covariate, resulted in the following least-square means for the six grades in the above order (± s.e.): gestation length for cows carrying male calves, 290·0 (± 0·9), 281·2 (± 0·9), 285·3 (± 0·8), 278·8 (± 0·9), 280·5 (± 0·9) and 279·3 (± 0·9) days; gestation length for female calves, 287·5 (± 0·9), 280·1 (± 1·0), 285·3 (± 0·9), 274·5 (± 1·1), 279·2 (± 0·9) and 276·4 (± 1·0) days; birth weight for males, 34·6 (± 0·7), 28·6 (± 0·8), 34·2 (± 0·7), 32·4 (± 0·7), 35·0 (± 0·7) and 34·7 (± 0·7) kg; birth weight for females, 29·9 (± 0·6), 29·5 (± 0·7), 33·4 (± 0·6), 31·9 (± 0·7), 33·6 (± 0·6) and 32·6 (± 0·6) kg.

Direct (g1) and maternal (gM) breed additive effects (HF - G), and direct (h1) and maternal heterosis (hM) were estimated. The estimates of g7, gM, h1 and hM were respectively, for gestation length for male calves: -21·5 (± 1·9), 10·6 (± 2·1), 1·5 (± 1·8) and -1·6 (± 11) days; for gestation length, for female calves: -21·9 (± 2·1), 14·6 (± 2·5), 5·7 (± 2·1) and -1·5 (± 1·2) days; for birth weight of male calves: -3·5 (± 1·5), 7·1 (± 1·8), -0·6 (± 1·5) and 1·0 (± 0·9) kg; and for birth weight of female calves: 4·7 (± 1·4), 4·1 (± 1·6), 2·8 (± 1·3) and 2·0 (± 0·8) kg. The difference in birth weight between males and females was 4·7 (± 0·9) for G-sired calves and 0·8 (± 0·4) for HF-sired calves.

Age and weight at puberty, age at first conception and number of services per conception of 90 females at Santa Monica Experimental Station, Valencia, State of Rio de Janeiro, were studied. The females were of six red and white Holstein-Friesian (HF) × Guzera (G) grades: 1/4, 1/2, 5/8, 3/4, 7/8 and ≥ 31/32.

A model (1) including the effects of grade and season of birth resulted in the following least-squares means for the six grades in the above order (± s.e.): age at puberty, 770 (± 17), 725 (± 18), 799 (± 26), 788 (± 23), 777 (± 17) and 803 (± 24) days; weight at puberty, 309 (± 8), 334 (± 9), 316 (± 13), 311 (± 11), 303 ((± 9) and 298 (± 12) kg; age at conception, 865 (± 19), 773 (± 19), 867 (± 31), 808 (± 25), 852 (± 22) and 837 (± 25) days; number of services per conception, 1·9 (± 0·2), 1·7 (± 0·2), 1·5 (± 0·3), 11 (± 0·2), 1·6 (+ 0·2) and 1·3 (± 0·2).

A second model (2) was fitted, including the effects of season of birth, direct (g1) breed additive effects (HF - G) and heterosis effects (hl). g1 and h1 were estimated by the partial regression coefficient of the traits studied on, respectively, the expected HF gene frequency and the expected heterozygosity of the females. The estimates of g7 were not significantly different from zero except for age at first conception (g7 = -102 (s.e. 46) days) (P < 0·05). Heterosis effects were significant for age at puberty (h1 = -86 (s.e. 34) days), weight at puberty (h1 = 44 (s.e. 17) kg) and age at first conception (h1 = —119 (s.e. 37) days). These three estimates of h1 amounted, respectively to —0106, 0151 and —0·132 of the estimated mean of the parental breeds.

F-tests on the extra variation due to fitting model 1 after model 2 were not significant (P < 0·05) indicating that epistasis or other genetic effects, not included in model 2, were of little importance for the traits studied.

Two randomized-block experiments have been carried out to examine the effects of increasing the digestibility of grass silage offered to beef cattle by harvesting three crops of grass after shorter growth intervals rather than two crops after longer growth intervals. In experiment 1 early-cut silages were harvested on 1 June, 16 July and 24 August, and late-cut silages on 20 June and 22 August 1979. In experiment 2 the early-cut silages were harvested on 19 May, 2 July and 20 August, and the late-cut silages on 5 June and 12 August 1980. Early- and late-cut silages were made from each of two cultivars of perennial ryegrass (cultivars Cropper and Talbot). The silages were unwilted and had formic acid applied at 2.5 1/t fresh crop.

In experiment 1 the silages were offered ad libitum, both unsupplemented and supplemented with 2·2 kg concentrates per head daily, to 88 cattle of mixed beef breeds and mean initial live weight 337 kg, in a 2 × 2 × 2 factorial design. In experiment 2 the silages were offered ad libitum and supplemented with 2·4 kg of concentrates per head daily to 56 Charolais × (Aberdeen Angus × Friesian) cattle of mean initial live weight 351 kg, in a 2 × 2 factorial design. Grass cultivar did not significantly affect silage intake or animal performance but intake, live-weight gain and carcass gain were significantly higher for the early-cut than for the late-cut silages in both experiments. In experiment 1 silage dry-matter intakes were 6·33, 5·12, 5·80 and 4·67 (s.e. 0·124) kg/day; live-weight gains were 0·72, 0·89, 0·47 and 0·76 (s.e. 0·029) kg/day; and carcass gains were 0·46, 0·60, 0·27 and 0·48 (s.e. 0·016) kg/day, for the unsupplemented and supplemented early-cut silages, and the unsupplemented and supplemented late-cut silages, respectively. In experiment 2 silage dry-matter intakes were 5·49 and 4·95 (s.e. 0·056) kg/day; live-weight gains Were 0·95 and 0·76 (s.e. 0·033) kg/day; and carcass gains were 0·67 and 0·50 (s.e. 0·17) kg/day, for the early- and late-cut silages, respectively.

Automated feeding systems for dairy cows offer the benefits of ‘little and often’ feeding. Published data were collected and analysed to establish the extent of such benefits. Twenty-three publications reporting the results of 35 experiments were examined. Several statistically significant positive responses indicated that milk fat concentration, and to a lesser extent yield of milk, could sometimes be increased by increasing the feeding frequency. However, for ail experiments the average proportional increases (± s.e.) in milk fat concentration and milk yield were fairly small at 7·3 (± 3·3) % and 2·7 (± 1·3) %, respectively. The responses of milk fat concentration and milk yield were apparently correlated (r = 0·43, s.e. = 0·20), and the mean proportional response of milk fat yield was 8·3 (± 3·1) %. There was no evidence that milk protein concentration, lactose concentration or changes in body weight were affected by changes in feeding frequency. Increases in food intake were sufficient to explain some, but not all, instances of increased milk fat production. All statistically significant responses to increased feeding frequency occurred when the milk fat concentration was originally depressed, milk fat depression generally being due to feeding pelleted or highly concentrated diets. Increases in milk fat concentration through increased feeding frequency were generally insufficient to bring the milk fat concentration up to a commercially acceptable level. All statistically significant responses were observed on moving from one or two to three or more meals per day, but the possibility of further responses beyond four meals per day could not be ruled out. It was concluded that cows producing milk of commercially acceptable milk fat concentrations were unlikely to benefit from increased feeding frequency.

East Friesland × Blackface (EFX) ewes with sustained lactation (S) and pure Scottish Blackface (BF) ewes with normal lactation (N) were mated to Suffolk rams. Twenty pairs of twin lambs born to the EFX ewes were used in the experiment. One lamb of each pair was re-allocated as a single lamb to either an EFX or a BF ewe (ER) and the second lamb was artificially reared (AR). At pasture the AR lambs were fed four times daily at levels corresponding to the milk intake of the ER lambs, which was measured weekly. Herbage intake was measured only in wether lambs.

The intake of milk and herbage between weeks 3 and 12 was lower in the AR groups than in the corresponding ER groups but the differences were not significant.

Total metabolizable energy intake as milk in the ER-S group from weeks 3 to 12 was significantly higher than that in the ER-N group (668 v. 450 MJ). Corresponding intakes in the AR-S and AR-N groups were 613 and 416 MJ. Herbage intake over the same period was 187 and 238 MJ for ER-S and ER-N, and 137 and 219 MJ for AR-S and AR-N. Total intake from milk plus herbage was significantly higher in the S group (855 and 750 MJ, respectively, for ER and AR lambs) than in the N group (688 and 634 MJ).

There were negative correlations of herbage intake with milk intake (–0·533) and with live-weight gain (–0·094), and positive relationships between live-weight gain and milk intake (r = +0·832), and between live-weight gain and total intake (r = +0·900). Differences in milk intake resulted in significant differences in live weight at 12 weeks (respectively, 34·6 and 29·7 kg for ER-S and ER-N; 32·3 and 29·1 kg for AR-S and AR-N).

It is concluded that lambs reared artificially at pasture may be used to investigate the relationship between milk and herbage intake. It was suggested that if milk does not meet the requirements of voluntary intake or growth potential, herbage intake may be increased to fill the deficit but this does not compensate fully for a lower milk supply.

In a 2 × 2 factorial experiment, half of each of two groups of ewes in high (20 ewes) or low body condition (20 ewes) were subjected to procedures designed to simulate normal management and climatic stresses, and the effects of these treatments on ovulation rate, embryo survival and endocrine status were investigated.

The mean ovulation rate of ewes in the high condition group was significantly higher than that of ewes in the low condition group (1·8 v. 11) (P < 0·001). Embryo survival rates were unaffected. Neither ovulation rate nor embryo survival were affected by stressful treatments.

Circulating follicle stimulating hormone, luteinizing hormone and prolactin levels were recorded in the peri-ovulatory period. Mean circulating follicle stimulating hormone levels were similar in three of the treatment groups but were generally lower in ewes in the low condition/stressed group. This difference was significant in some of the sampling periods. Neither basal levels of luteinizing hormone nor the size of the pre-ovulatory luteinizing hormone surge were significantly affected by level of body condition or stress but the surge began earlier in ewes in the low condition groups. The difference in timing was not, however, related to ovulation rate. Circulating prolactin levels were consistently and often significantly lower in ewes in poor condition (P < 0·05). Levels were not significantly affected by stress.

While ovulation rate was affected by body condition, the recorded progesterone profiles during the first 2 weeks after mating suggest that luteal function was not affected by any of the treatments applied.

1. Eighteen litters of piglets were weaned at 14 days of age and reared for a further 3 weeks at environmental temperatures of 18, 23 or 28°C. At each environmental temperature three levels of feeding were used. The changes in body composition of the piglets were studied by the comparative slaughter procedure, littermates being analysed at birth, and at 14, 22 and 36 days of age. For comparison, the bodies of suckled piglets of similar weights and ages were also analysed.

2. In the 2 to 3 day period following weaning, body weight decreased, with the greatest reduction being 0·28 (s.e. 002) kg at 18°C. Following this, body weight gain was dependent on both environmental temperature and level of feeding, although at any given level of food intake there was little difference between the values at 23 or 28°C.

3. There were marked changes in the chemical composition of the carcass in relation to the different temperature/feeding level combinations. The most variable component was fat. In the period following weaning there was considerable mobilization of fat, especially at the lower temperatures and feeding levels. The fat content of the early weaned piglets at 36 days of age (68 to 103 g/kg) was significantly less than that of the suckled piglets (169 g/kg) (P < 0·01). The decreases in fat content were accompanied by increases in the dry-matter content of the carcass.

4. The protein content of the carcass remained relatively independent of the different treatments, ranging from 140 to 164 g/kg for the early-weaned piglets, compared with 139 g/kg for those that had suckled their mothers.

5. The differences in the growth and body composition of the early-weaned piglet are discussed in relation to an inadequate food intake following weaning, to changes in nutrient supply and to variations in the climatic environment.

1. Measurements of heat loss, energy and nitrogen balance were made on 18 groups of piglets weaned at 2 weeks, at environmental temperatures of 18, 23 and 28°C, and at three levels of feeding at each temperature.

2. From the experimental results, values of heat loss, energy retention, protein and fat deposition were derived for each temperature, at each of the three levels of metabolizable energy (kJ/kg M0·75 per day) intake: 550 (1·0MEm), 825 (1·5MEm) and 1100 (2·0MEm). The lowest of these levels was the calculated thermoneutral maintenance energy requirement (MEm).

3. From the results the following deductions were made, (a) Heat loss varies with both environmental temperature and metabolizable energy intake, and at an intake of 2·0MEm is minimal between 23 and 28°C. Energy retention varies in an inverse manner to heat-loss, and at 1·0MEm is negative at all environmental temperatures below 28°C. (b) Protein and fat deposition increase significantly with increase in metabolizable energy intake (P < 0·05), with fat deposition being more dependent on temperature than protein deposition. The mean increase in protein deposition per 1°C increase in environmental temperature is 2·05 kJ/kg M0·75 per day. Fat deposition is negative at all temperatures at l·0MEm; at l·5MEm it is zero at 23°C and negative at temperatures below this.

4. Critical temperature was calculated to decrease from 26·9°C at l·0MEm to 23·9°C at 2·0MEm.

5. The efficiency of energy utilization (k) was 0·58 at 18°C, 0·81 at 23°C and 0·74 at 28°C. The corresponding values of the maintenance energy requirements were 739, 615 and 550 kJ/kg M0·75 per day. Estimates of the energetic efficiency of protein deposition (kp) of 0·60 to 0·65, and of fat deposition (k/) of 0·82 to 0·86, were determined at 23 and 28°C.

1. Eight diets of similar energy content, ranging in crude protein concentration from 95 to 256 g/kg, were given at either 2·5 or 3·2 times the energy level for maintenance to entire male pigs growing from 45 to 90 kg live weight.

2. Growth rate improved with increase in feeding level and with increasing dietary crude protein up to 164 g/kg (P < 0·05). The food conversion ratio improved with each increase in dietary CP up to 186 and 164 g/kg on the lower and higher feeding treatments, respectively (P < 0·05). It was also improved with increase in level of feeding of the lower-protein diets but deteriorated with increase in level of intake of the higher-protein diets (P < 005).

3. Rate of protein deposition improved with increasing dietary crude protein up to 186 and 164 g/kg on the lower and higher feeding treatments, respectively (P < 005). The results showed that, for pigs given diets deficient in crude protein, rate of protein deposition was linearly related to protein intake (P < 0·001) but independent of energy intake. For pigs given a diet adequate in crude protein, rate of protein deposition was related to energy intake and independent of crude protein intake.

4. Body fat content at 90 kg decreased with increasing dietary crude protein up to 210 and 164 g/kg on the lower and higher feeding treatments, respectively (P < 0·05), and was increased by raising the level of intake of the higher-crude protein diets (P < 0·05). However, the influence of feeding level on body fat diminished as dietary crude protein was reduced.

A factorial experiment was conducted with 80 first-litter sows and involved two levels of feeding during lactation (ad libitum or 2·0 kg/day), and two levels of feeding between weaning and mating (4·0 or 1·5 kg/day).

Average lactation length was 32·2 days. Sows given 2·0 kg/day during lactation lost more backfat (6·3 v. 0·9 mm; P < 0·05) and more live weight (36·8 v. 9·1 kg; P < 0·05) during lactation than sows fed ad libitum and whose average daily food intake was 4·47 kg. Sows receiving 20 kg/day during lactation took longer to return to oestrus after weaning. Within 8 days of weaning more sows fed ad libitum during lactation ovulated (0·90 v. 0·40; x2 = 20·0; P < 0·001) and exhibited oestrus (0·78 v. 0·38; x2 = 12·8; P < 0·001) than sows whose food intake throughout lactation was restricted. Ovulation rate, subsequent litter size and embryonic mortality were not significantly affected by feeding level during lactation.

Post-weaning feeding level did not affect the interval between weaning and oestrus. However, sows receiving 4·0 kg/day between weaning and mating had higher ovulation rates (14·8 v. 13·0; P < 0·05) and a greater litter size (10·0 v. 8·8; P < 0·1) at the subsequent farrowing.

A factorial experiment involving high (53·1 to 59·5 MJ) and low (25·4 to 27·1 MJ) daily intakes of digestible energy, and high (550 to 745 g) and low (302 to 318 g) daily intakes of crude protein during lactation, was conducted with 68 first-litter sows. Average lactation length was 32 days, and average backfat changes during lactation were +0·3, −1·4, −7·2 and −5·4 mm for the high energy/high protein, high energy/low protein, low energy/high protein and low energy/low protein combinations, respectively. The corresponding mean live-weight losses during lactation were 3·9, 32·5, 29·8 and 35·8 kg. Within 8 days of weaning, more sows receiving high intakes of both energy and protein during lactation exhibited oestrus (0·88 v. 0·53, x2 = 6·7, P < 0·01) than sows whose energy and/or protein intake throughout lactation had been restricted. Nitrogen balances of sows were determined between 18 and 22 days after farrowing. Sows receiving high intakes of both energy and protein during lactation were in positive nitrogen balance, whereas the nitrogen balances of sows on the other three dietary treatments during lactation were similar to each other but negative. Ovulation rate, subsequent litter size and embryo mortality were not significantly affected by energy or protein intake during lactation.

Thirty-two female growing pigs (25 to 55 kg live weight) were subjected to four dietary treatments combining two levels of isoleucine and two levels of leucine to investigate the effects of leucine supply on the requirement for isoleucine.

Results for daily live-weight gain, food conversion efficiency and carcass quality judged by ham dissection indicated that 3·8 g isoleucine per kg diet was marginally adequate for the growing pig in diets containing 13·4 g leucine per kg. An interaction between dietary leucine and isoleucine was demonstrated. Increasing the dietary leucine concentration to 20·4 g/kg clearly resulted in a deficiency of isoleucine in the basal diet, as daily gain, food conversion efficiency and carcass quality were significantly improved by increasing the isoleucine concentration from 3·8 g to 4·5 g/kg diet. Dietary leucine concentration did not influence performance at the higher level of isoleucine supply. Changes in plasma urea and amino acid concentrations confirmed the findings from the growth experiment of the interaction between isoleucine, leucine and valine.

In the first of two experiments of 6 × 6 Latin-square design a diet of barley, soya bean meal and fish meal, known to give an adequate lysine intake (56 g/day) at the level of feeding adopted, was diluted with maize starch to give crude protein intakes ranging between 448 and 923 g/day but with the lysine intake held constant by using synthetic lysine in the diets. The diets were given to sows suckling ten piglets between days 12 and 30 of lactation. Dietary amino acid adequacy was assessed by changes in plasma urea, urinary urea, urinary nitrogen and plasma amino acid concentrations. The dietary amino acid supply became limiting with a protein intake of approximately 730 g/day. In the second experiment, a diet of similar composition was used with varying additions of synthetic threonine, methionine, isoleucine and tryptophan: the control diet contained all the synthetic amino acids added, the basal diet none and the other diets were formed by adding groups of three out of the four synthetic amino acids. On the basis of some of the variables studied, it was concluded that, for this diet, threonine was the limiting amino acid after lysine.

Growing pigs were given diets based on barley (whole or milled), which was either untreated or sprayed with an aqueous solution of sodium hydroxide (270 g sodium hydroxide per kg water) to give 33·8 g sodium hydroxide per kg dry matter. The barley grain was treated with sodium hydroxide while in the whole state. A total of 48 pigs penned individually were given the diets on a restricted scale of feeding from an initial weight of 31·6 kg until slaughter at 81·7 kg live weight. In a metabolism study, eight boars were subjected to the same experimental treatments in two identical 4 × 4 balanced change-over experiments.

The diet containing untreated milled barley gave significantly faster live-weight gain and lower food conversion ratio than the other three diets (P < 0·001). Food conversion ratios were similar for untreated whole barley and sodium hydroxide-treated barley, whether this was whole or milled. Killingout proportion was similar for untreated and treated milled barley but was significantly lower when the diet contained untreated or treated whole barley.

Milling significantly increased the digestibility of energy and nitrogen. Treatment with sodium hydroxide improved the digestibility of energy in the diet with whole barley but not in the diet with milled barley. With both whole and milled barley, treatment with sodium hydroxide increased the digestibility of crude fibre (P < 0·05) but with ground barley the digestibility and retention of nitrogen were reduced considerably by treatment with sodium hydroxide.

It is concluded that treatment of barley with sodium hydroxide did not improve the nutritive value of either milled or whole grain, that it reduced the digestibility of nitrogen and that it severely depressed the utilization of digested energy for growth.

Two groups of six sows and their litters were observed continuously the day before (day 19 post partum) and the 2 days following (days 20 and 21) movement from individual farrowing crates to multiaccommodation pens. Grouping was associated with a decline in the frequency of true nursing events, particularly in the 6-h period immediately following grouping, and with an increase in the number of false nursing events. There was a trend for a reduction in the duration of the milk let-down phase of the suckling sequence in four litters studied in detail and cross suckling was observed in all four litters. Lactational oestrus occurred in nine of the 12 sows. The study yielded little evidence for an association between nursing and suckling behaviour, and the occurrence of lactational oestrus.

Twenty-one pairs of litter sister gilts of similar live weight were selected at 150 days of age and one member of each pair was allocated randomly to the following treatments at approximately 170 days (80 kg live weight): 1. stimulation by mature boar; or 2. an intramuscular injection of oestradiol benzoate. The two groups were fed to appetite and slaughtered at approximately 180 days of age. Examination at slaughter indicated greater ovarian and uterine development in response to oestradiol relative to boar treatment (P < 0·01). The results indicate that oestradiol treatment of gilts followed by post-slaughter examination of the reproductive tract is a sensitive method for evaluating the effects of prior treatments on reproductive development. Significant family and pre-weaning growth effects on reproductive development were also evident on both evaluation methods.

Flemish Giant (FG) and New Zealand White (NZW) purebreds and terminal crossbreds (TX) (Flemish Giant sires × Florida White-New Zealand White dams) were evaluated during a 10-month period for maternal and litter pre-weaning and post-weaning performance. For the doe traits studied, teat number was greatest and litter interval was shortest, while body weight at breeding age (154 days), litter weight at 21 days and day 1 to 21 food intake were lowest for crossbred does. These traits were similar for pure doe groups, with the exception of body weight, for which FG does were heavier. For the preweaning litter traits examined (no. = 101 litters), litter size born was smallest in the TX group (the purebred groups being similar) and was largest at weaning (28 days) for NZW. Proportion of kits born alive was lowest in FG litters, while the NZW and TX groups were comparable. Litter and average birth and weaning weights, litter daily gain, and doe and litter food intake (1 to 28 days) were all highest in FG litters and lowest in TX litters. Repeatability estimates (t) were low (t < 0·05) for litter size and birth weight traits, and were moderate to high (0·18 < t < 0·58) for the remaining maternally influenced characters. For the post-weaning litter traits studied (no. = 86 litters), NZW litters were larger in number and heavier at market age (56 days), and consumed more food from 28 to 56 days than FG and TX litters. Purebred FG litters, however, had heavier average market weights per rabbit, while purebred NZW and TX litters were similar. Mortality due to respiratory disease was higher in FG litters than in NZW and TX litters. Total mortality incidence was lowest in the TX group.

Six sheep were used in a Latin-square experiment to investigate the effect of pre-harvest treatment of pasture with glyphosate (Roundup, a translocated herbicide) on the palatability of hay. Four hays were used: control (C), Roundup (R), quinine (Q, 100 mg/kg) and Roundup with quinine (QR). During each week one of the following choices was given to each animal: (a) C/C; (b) C/R; (c) R/R; (d) Q/C; (e) Q/R; and (f) C/QR. There were no significant effects of treatment on the proportion chosen or on total daily intake, mean meal size or rate of eating. It was concluded that neither pre-harvest treatment with Roundup nor post-harvest treatment with quinine affected the palatability of hay.

Oestrous activity, ovulation rates and embryo survival rates were investigated in Border Leicester × Scottish Blackface (Greyface) ewes with condition scores of approximately 2·75 (moderately fat) or 3·5 (very fat) at mating. Four of a group of 20 very fat ewes failed to show oestrus at the expected time. Very fat ewes had a higher mean ovulation rate (3·36 v. 2·33; P < 0·05) but a slightly lower potential lambing rate (1·10 v. 1·42 lambs born per ewe put to ram) than the moderately fatewes.