Dorset Horn Lambs Research Articles

Vaccination of young Dorset lambs against haemonchosis.

Six Dorset Horn lambs were each vaccinated at age 7 weeks and 9 weeks with 50 micrograms of glycosylated integral membrane proteins, particularly enriched in the protein H11 from the intestinal brush border of adult Haemonchus. At 11 weeks of age the lambs were infected with 10,000 infective third stage Haemonchus larvae. Compared with the average for the control group the vaccinated group of lambs had a 78% reduction in parasite egg output over the patent period, with four of the six better than 93% protected. At autopsy 35 days post-infection the mean total worm burden of the vaccinated lambs was 83% reduced compared with the controls. The serum antibody titres to H11 correlated with the degree of protection.

Influence of dietary protein on the pathophysiology of haemonchosis in lambs given continuous infections

Mixed-sex groups of three-month-old Finn Dorset and Dorset Horn lambs were given complete diets containing either high or low protein. A trickle infection with Haemonchus contortus of 200 larvae, given three times a week, began one month later for a 17 week period. Erythrokinetic, metabolic, digestibility and balance studies were conducted with the male lambs during weeks 12 to 14 of the infection. Clinical signs of haemonchosis became evident only in the low protein diet group. The infection was characterised by severe macrocytic anaemia, hypoproteinaemia, hypoalbuminaemia, reduced survival, high egg counts and a significantly greater worm burden. In the high protein diet group many lambs developed resistance to further infection in contrast to the low protein group in which no resistance developed. Live-weight gain was not affected by infection in the high protein diet group but decreased in the lambs given the low protein diet compared with uninfected controls. However, infection did not appear adversely to affect the digestive efficiency, nitrogen retention or iron absorption in either dietary group although loss of appetite occurred in the low protein group. Examination of a sample joint from the carcases showed infection was associated with a marked reduction in muscle mass in lambs given the low protein diet relative to the control animals.

Border disease: Effect of virus administered intracerebrally to newborn lambs

Six BVD antibody-free Dorset Horn lambs, less than 3-days-old, were infected by intracerebral inoculation of BD virus. Three of these lambs (Group I) were killed 7 days after inoculation (p.i.) and 3 (Group II) 21 days p.i. None of the lambs showed clinical signs of disease. Of the 5 lambs in which infection was established, 3 of Group I and 2 of Group II showed lesions of non-suppurative encephalitis. Although of varying severity and extent in individual animals, the lesions in all lambs appeared to be primarily of the same nature, with the cerebral white matter being the main target. BD virus was isolated from all lambs of Group I, killed 7 days p.i., but from none of Group II, killed 21 days p.i. Thus the induced BD infection appeared to be of a transient nature. The results indicate that the CNS of newborn lambs is capable of supporting BD virus replication with subsequent development of lesions. It is suggested that some of the cases of non-suppurative encephalitis of obscure aetiology occasionally encountered in young lambs in the field may be the result of infection with BD virus.

Pneumonia in sheep associated with dipping in carbolic dips.

Fourteen outbreaks of pneumonia in sheep following dipping in carbolic dips were investigated at seven laboratories. Deaths commonly occurred between one and three days after dipping and fat young sheep of the Suffolk and Border Leicester breeds were most usually involved in outbreaks. The pneumonia was characterised by hyperaemic, oedematous and cyanotic lungs, and the main histological features were hyperaemia, fluid and cellular exudation into alveolar spaces, and widespread epithelialisation of alveolar walls. The microscopic changes differed from pneumonic pasteurellosis and no significant organisms were recovered from any of the lungs. One of 27 Dorset Horn lambs dipped in a batch of dip which had been incriminated in some episodes developed an epithelialising interstitial pneumonia, while two out of three Suffolk lambs dipped in twice the recommended strength of the same batch of dip became ill with severe pneumonia, apparently following skin absorption.

Digestion of a ground and pelleted diet in the stomach and intestines of young sheep from two breeds

Some aspects of digestion were studied in small groups of fistulated wether lambs from two breeds, Dorset Horn and Corriedale. The Iambs were given a ground and pelleted diet of equal parts hay and concentrates once daily at two levels of intake. At the same liveweight (25 kg), the higher intake was associated with reduced digestion of organic matter (OM) and fibre in the whole gastro-intestinal tract; rumen mean retention times were also reduced, as was the proportion of OM digestion which occurred in the rumen. An increase in the amount of protein entering the intestines appeared to be composed of both undegraded dietary protein and microbial protein. Despite a reduction in the digestibility of non-ammonia nitrogen in the intestines, more crude protein was digested there per unit of digestible OM intake (25.9 v..23.6%; P < 0.05). Differences between the breed groups were smaller than those associated with level of feeding. Nevertheless, particularly at lower relative intakes, rumen mean retention times were longer, rumen digesta content was greater, and rumen OM and fibre digestion tended to be greater in the Dorset Horn lambs; crude protein digestion in the intestines per unit of digestible OM intake also tended to be less at the low intakes. More than three-quarters of the cellulose digestion but less than half the hemicellulose digestion occurred in the rumen. The proportion of the urea synthesized that was degraded in the gastrointestinal tract (0.24-0.30) was consistently lower in these lambs than has been reported for adult sheep.

Evidence for a decreased susceptibility to acute radiation lethality in young lambs.

The survival of 2- to 4-day old Romney-lambs was studied following bilateral 60Co irradiation at about 3.5 R/min (the exposure rate in air at the mid-line of the animal). Probit analysis of the data yielded an LD50/60 of 900 R with 95% confidence limits of 700–1150 R. Mature sheep irradiated under similar conditions are known to have an LD50/60 in the region of 250–350 R. These data indicate that very young lambs were less susceptible to radiation-induced hemopoietic failure than adults. Dorset Horn lambs and Romneys born by caesarian section also exhibited low susceptibility when irradiated at 2–4 days of age. There are few data available on LD50 values for very young, large mammals (as opposed to rodents). Consideration must be given to the possibility that large mammals may be less sensitive to radiation-induced lethality shortly after birth than they are at maturity. Further work on the radiation response as a function of age after birth seems warranted and suggestions for some of the parameters which require investigation are made.

THE EFFECTS OF ACID-PRESERVED HIGH-MOISTURE BARLEY AND PELLETING ON THE UTILIZATION OF ALL-CONCENTRATE DIETS BY EARLY-WEANED LAMBS

Sixty-six Dorset Horn lambs weaned at 8 wk of age were fed dry (88% DM) or high-moisture (73% DM), reconstituted, acid-preserved barley-based diets either whole or as 5-mm or 9-mm pellets. Lambs fed high-moisture or dry barley performed similarly in terms of feed intake, average daily gain, feed conversion ratios (DM basis) and digestibility of protein. Digestibility of organic matter was slightly higher (P < 0.05) for dry than moist barley. Whole barley resulted in faster (P < 0.05) growth rates than either 5-mm or 9-mm pellets, 276, 221 and 227 g/day, respectively. The corresponding feed convetsion ratios were 3.94, 4.24 qnd 4.31. Pelleting had no significant (P < 0.05) effect on feed intake or the digestibility of organic matter or protein.

Carcass characteristics of heavyweight crossbred lambs. I. Growth and carcass measurements

An experiment was conducted at Temora Agricultural Research Station to assess the potential for producing heavyweight lamb carcasses from breeds currently used in the prime lamb industry. Growth to slaughter, dressing percentage and carcass measurements of lambs from six breed crosses were compared over a range of carcass weights. The lambs were the progeny of Dorset Horn and Border Leicester rams joined to Merino, Corriedale and Border Leicester x Merino first-cross ewes. Within each breed combination, lambs were allocated to be slaughtered at 34, 44 or 54 kg liveweight. Lambs sired by Dorset Horn rams were slightly larger at birth and weaning, and reached their slaughter weights at a younger age, than lambs sired by Border Leicester rams. Lambs from Border Leicester x Merino ewes grew 10% faster than lambs from Corriedale ewes. The progeny of Merino ewes were always slowest-growing, and this was more marked at higher weights. Carcasses from the slower-growing genotypes tended to be larger in skeletal size and had a lower dressing percentage than carcasses from faster-growing genotypes, when adjusted to the same carcass weight. Fat depth at the 12th-13th rib increased, on average, by 20% for each 10% increase in carcass weight, over the range of carcass weights examined. The rate of increase in carcass fatness was significantly lower in lambs sired by Dorset Horn rams and lambs from Merino ewes, compared with lambs of the other breeds.

The effects of heat treatment and protein quantity on digestibility and utilization of milk substitutes by lambs

SUMMARYThe effects of heat treatment, as measured by undenatured non-casein nitrogen (NCN gN/100 g total N), and of protein: energy ratio on the digestibility coefficients and utilization of milk substitutes by lambs were investigated.Twenty-four Finnish Landrace × Dorset Horn lambs (12 males and 12 females) were given milk substitute diets in which approximately 20 or 30% of the gross energy was supplied by protein (PE20 and PE30). NCN levels were approximately 7, 12, 18 and 5, 11, 16 for diets PE20 and PE30 respectively.All lambs were offered a daily allowance of milk substitutes containing 7·95 MJ of gross energy from 3 to 25 days of age, when they were slaughtered and the chemical composition of the empty body was measured. Gains in the chemical components were measured using the comparative slaughter technique.For each unit increase in NCN value there was an increase in the apparent digestibility coefficients of dry matter (D.M.), crude protein (CP) and ash of 0·13, 0·13 and 0·93 percentage units, respectively. The apparent digestibility coefficients of all the dietary components, except lactose increased by 0·24 (d.m.), 0·24 (CP), 0·63 (fat), 0·75 (ash) and 0·32 (energy) percentage units and the metabolizability by 0·24 units for each 1% increase in PE.Live-weight and empty body-weight gains increased with increasing PE and NCN. Lambs receiving diets PE20 had a daily live-weight gain 42 g less than those on diets PE30; increasing NCN from 5 to 18 gN/100 g total nitrogen, increased live-weight gain by 33 g/day.Increasing PE and NCN increased water gain and nitrogen retention; increasing PE also increased ash retention and decreased fat gain.Lambs on diets PE20 contained more fat and less protein, at the same empty body weight, than those receiving diets PE30.Efficiencies of conversion of dry matter and energy to live-weight gain, and energy and N utilization all increased with increasing NCN, but increasing PE did not affect energy utilization and decreased N utilization.Female lambs had lower growth rates than males (—14·5 g/day) and had a higher total fat gain (128 g).

The effect of temperature and method of feeding on the diestibility of two milk substitutes and on the performance of lambs

SUMMARYThe effects of temperature and method of feeding on the intake characteristics and digestibilities of two milk substitutes were examined.Forty Finnish Landrace x Dorset Horn lambs (20 males and 20 females) were fed one of two milk substitutes from 3 to 25 days of age when they were slaughtered and chemical composition of the empty body was measured.The two milk substitutes contained 30% fat in the dry matter as butter fat (diet B), or tallow and coconut oil (diet TC).The diets were offered at either 34°C (W) or 5 °C (C) and three methods of feeding were used. The diets were given: ad libitumcold (AL1), four feeds to appetite per day either warm or cold (AL2W and AL2C) and four feeds restricted to an intake of 62 g D.M./kg live weight0·5 either warm or cold (RW or RC).Digestibilities of the milk substitutes were not affected by the temperature at which they were offered or the feeding regime used. Diet B had an apparent dry-matter digestibility of 97·6% and diet TC 92·5%. The fat digestibility of diet TC was 13·6 units lower than diet B and this affected the digestibility of all the other dietary components.Lambs tended to consume more of diet B and grow faster.Lambs on treatment AL1 consumed 21% more milk substitute than those on treatment AL2.The composition of the body-weight gain was found to alter with rate of gain.The temperature at which the milk substitutes were offered had only a minor effect on the performance of the lambs. The type of milk substitute and feeding regime used had a greater influence on both lamb intake and growth.

Meat properties of lambs grown to 32 kg at various rates on phalaris or lucerne pastures and an apparent effect of pre-slaughter ambient temperature

SUMMARYOne hundred and fifty-nine Corriedale x Dorset Horn lambs were slaughtered at 32 kg live weight at ages varying from 98 to 303 days. The youngest lambs were from ewes on a pasture with high herbage availability and had not been weaned; the next youngest were from the same group of ewes but had been weaned at 6 or 12 weeks of age. Older lambs had been weaned at 6, 12 or 29 weeks from ewes on a pasture with low herbage availability. The weaned lambs grazed either a phalaris or a lucerne pasture.Lambs that grazed lucerne had a greater dressing percentage and a heavier carcass. Over all lambs, ultimate pH of the m. semimenxbranosus ranged from 5·4 to 6·0 and Warner-Bratzler shear values increased twofold from lowest to highest pH. Minimum ambient temperatures during the nights before the slaughter days ranged from 13 to – 1°C; ultimate pH increased by 0·25 units with decreasing temperature over the range, equivalent to an increase in shear value of 26%. Shear values adjusted for pH differences increased from about 3·4 kg/cm2 at 130 days of age to about 7·5 kg/cm2 at 300 days.Visual attractiveness of 13th rib chops increased with increasing area of the m. longissimus dorsi. Attractiveness was least at intermediate dominant wavelength (redness) of this muscle, but was not related to its reflectance. Optimum fat depth was about 2·4 mm. In general, chops from younger lambs were more attractive than those from older lambs.

Variations in immunoglobulin concentrations in Finnish × Dorset Horn lambs

The serum immunoglobulin concentrations in two-day-old Finnish x Dorset Horn lambs, which were born and nursed indoors, were high and unrelated to littersize, except when young ewes had large litters, or when ewes lambed twice in a year. The results contrast sharply with those from Merino and Blackface lambs, examined previously, whose mean concentrations were much lower, and generally fell substantially with increasing litter-size. The superiority of the Finnish x Dorset Horn sheep in this respect is attributed to the ability of the lambs to suck particularly vigorously, shortly after birth, especially when large litters are compared, and to a higher rate of colostrum production by the ewes. The correlations between the immunoglobulin concentrations in the lambs and their growth and survival were significant but slight.

The effects of quantity and distribution of milk substitute on the performance and carcass measurements of artificially reared lambs

SUMMARYTwenty-seven groups of Finnish Landrace × Dorset Horn lambs each of 2 castrated males and 2 females were offered daily quantities of 180, 248 or 343 g of milk substitute dry matter (DM) for 21, 29 or 40 days. Concentrate pellets were available ad libitum and from 22 days of age 150 g of hay per week was offered. When the mean live weight of a group reached 35 kg two castrates and one female from each group were slaughtered and the carcasses appraised.The growth rate and daily concentrate intake of lambs from 48 days of age until slaughter (reported -in this paper) were not affected by previous treatment, but lambs weaned at 40 days had a higher food conversion efficiency in this period (0·266 kg gain/kg feed DM) than those weaned at 21 days (0·244) or 29 days (0·232).Delaying weaning from 21 to 29 and 40 days reduced overall concentrate DM intake from birth to slaughter (from 88·15 to 81·09 and 73·39 kg) but increased growth rate (261, 276 and 288 g/day) and the conversion of total DM consumed to live weight (0·300, 0·311 and 0·350 kg/kg). Increasing the daily allowance of milk substitute from 180 to 248 and 343 g/day also reduced total concentrate intake (84·18, 80·74 and 77·71 kg). Growth rate and food conversion efficiency tended to increase and concentrate DM intake to decrease when the same total quantity of milk substitute was given over a longer period. Castrated males grew faster than females (290 v. 260 g/day).Weaning age generally had a greater effect than daily allowance of milk substitute on carcass measurements. Fat in the carcass tended to increase with increasing weaning age and significant interactions between daily allowance of milk substitute and weaning age were found for some carcass measurements.

Morphology of Experimental Border Disease of Lambs

Anatomical abnormalities commonly observed in field cases of Border disease (BD) of lambs have been studied in detail in 17 newborn Dorset Horn lambs affected with the experimentally produced disease. Compared with clinically normal control lambs (9), of the same breed and age, the affected lambs were significantly smaller and possessed finer limb bones. Their skulls were shorter, showed excessive prominence of the cranium (doming) and small orbits frequently abnormally positioned in relation to the cranium. The bone of these skulls showed radio-graphically less density, this feature being commonly most evident in the frontal-parietal-temporal area. The study has confirmed dysmorphogenesis as one of the specific features of BD. The greatly variable pattern of morphological lesions in BD and the difficulties which this aspect of the disease creates in its diagnosis in the field are discussed.

The effects of quantity and distribution of milk substitute on the performance of artificially reared lambs to forty-eight days of age

SUMMARYTwenty-seven groups of Finnish Landrace × Dorset Horn lambs, each of two males and two females, were offered daily quantities of 180, 248 or 343 g milk substitute dry matter (DM) for 21, 29 or 40 days. The environmental temperature was 18°C during the first 21 days of rearing, and concentrate pellets and clean water were always available. From 22 to 48 days of age the lambs were offered 150 g of hay per week.For lambs offered 180 g milk substitute DM, increasing the number of days before weaning increased the daily live-weight gain over the 47-day experimental period. For daily allowances of 248 and 343 g of milk substitute, greater gains were effected only by delaying weaning from 21 to 29 days.Increasing the daily allowance of milk substitute DM had little effect on the daily gain of lambs weaned after 21 days but increased gain by lambs weaned at 29 days. For lambs weaned after 40 days growth rate was increased only by the first increment of milk substitute DM. Intake of solid food began when the lambs were 3 weeks old and total intake of concentrates was inversely related to total quantity of milk substitute DM consumed and in general was less when weaning was later.The overall efficiency of conversion of total DM consumed to live weight was positively related to total milk substitute DM consumed and tended to be greatest when weaning was later. A daily allowance of 180 g milk substitute DM for 21 days gave the lowest food cost per unit of live-weight gain.The incidence of navel sucking is discussed.

Experimental Border Disease of Sheep: Effect of Infection on Primary Follicle Differentiation in the Skin of Dorset Horn Lambs

The histological basis of the fleece birthcoat differences observed clinically in pure-bred Dorset Horn lambs as between normal healthy controls and those experimentally infected with Border disease has been studied in the skin. The higher incidence of “hairy” birthcoats in the groups experimentally infected was unequivocally due to the presence of medullated primary fibres, a feature completely absent from the normal healthy controls. No other histological character of significance was affected. Taken with the neurochemical results of Patterson et al . (1971) a clear and definite association now seems established between the medullation of primary fibres in the birthcoat and myelin dysgenesis in the spinal cord suggesting that the critical action of the causal agent is most probably on the ectoderm during the first trimester. At the same time attention is drawn to the effect of the infective agent in producing an excellent phenocopy of certain fleece structures although Border disease as such seems not to have any genetic basis. Les différences histologiques observées cliniquement au niveau des bases des manteaux de naissance des agneaux Dorset Horn de pure race d'après les contrôles sanitaires normaux et de ceux contaminés expérimentalement par la maladie de Border ont été étudiées au niveau de la peau. La grande richesse en poils des manteaux de naissance chez les groupes contaminés expérimentalement fut sans aucun doute le résultat de la présence de fibres médullaires primaires, un élément totalement absent des contrôles sanitaires normaux. Aucun autre caractère histologique significatif fut atteint. Comparé avec les résultats neuro-chimiques de Patterson et al. (1971) une relation claire et définie semble maintenant établie entre la présence de fibres médullaires primaires et le dégénérescence de la myéline au niveau de la moelle épinière laissant supposer que l'action critique de l'agent causai se situe le plus probablement au niveau de l'ectoderme durant le premier trimestre. En même temps l'attention a été attirée par la produetion par l'agent contaminant d'une excellente phénocopie de certaines struetures de la toison bien que la maladie de Border ne semble avoir aucune origine génétique. Bericht über Hautstudien bei reinrassigen “Dorset Horn” Lämmern. Vergleiche zwischen gesunden Kontrolltieren und solchen, die experimentell mit “Border” Krankheit infiziert waren. Klinische Studien der Unterschiede der histologischen Grundlage des Fliesses bei der Geburt. Man fand ein häufigeres Vorkommen einer “haarigen” Behaarung bei der Geburt in der experimentell infizierten Gruppe, verursacht durch das Vorhandensein von markhaltigen primären Fasern. Dies fehlte gänzlich bei der Kontrollgruppe. Keine andere histologische Eigenschaft von signifikantem Charakter war in Mitleidenschaft gezogen. Bezogen auf die neurochemischen Ergebnisse von Patterson et al . (1971) scheint nunmehr ein klarer und definitiver Zusammenhang bewiesen zu sein, nämlich zwischen der Markentwicklung in den primären Fasern des Fliesses zur Zeit der Geburt und der Myelindysgenese im Rückenmark. Hieraus folgt, dass die kritische Aktion der Krankheitsurache höchst vermutlich auf as Ektoderm im ersten Trimester einwirkt. Gleichzeitig wird darauf aufmerksam gcmacht, dass die Ursache der Infektion eine ausgezeichnete Phenokopie gewisser Strukturen des Fliesses produziert, obwohl die “Border” Krankheit ais solche keine genetische Grundlage zu haben scheint. Se ha estudiado en la piei la base histológica de las diferencias de la capa de lana de nacimiento observadas clinicamente en corderos de pura raza Dorset Horn entre animales controles sanos y otros infectados experimentalmente con enfermedad de Border. La mayor incidencia de capas de nacimiento “peludas” en el grupo infectado experimentalmente se debió inequivocamente a la presencia de fibras meduladas primarias, una característica que no se observa por completo en los animales controles sanos. No se encontró ninguna otra característica histológica de significancia. Teniendo en cuenta los resultados neuroquímicos de Patterson et al . (1971) parece que ahora queda establecida una asociación clara y definida entre la medulación de las fibras primarias en la capa de nacimiento y la disgenesia mielínica en la médula espinal, que sugiere que la acción crítica dei agente causai se realiza con la mayor probabilidad en el ectodermo durante el primer trimestre. Al mismo tiempo se llama la atención sobre el efecto dei agente infectivo en la producción de una fenocopia excelente de ciertas estructuras de la lana aunque la enfermedad de Border como tal no parece tener ninguna base genética.

The Acquisition of Trichostrongylid Infection by Grazing Lambs

Summary A grazing experiment was carried out during 1966–1968 in an attempt to identify the source of the larvae which initiate infection in grazing lambs. The post-parturient rise in faecal egg count was abolished in the ewes but was replaced by eggs of T. colubriformis passed by artificaily infected 5-month-old Dorset Horn lambs. The acquisition of infection in 2 years appeared to result principally from the larvae derived from the simulated post-parturient rise but in the third year residual pasture infection was important. Paddocks continuously grazed by lambs over a number of years became dangerously infected with Nematodirus spp.

Border Disease of Sheep: the Diagnostic Significance of Mcdullated Wool Fibre Scores and Counts of Glial Cells in the Spinal Cord

Summary Counting methods have been applied to the assessment of lesions of the fleece and central nervous system of Dorset Horn lambs derived from groups of ewes receiving a variety of exposures to Border disease infection. Lambs fulfilling all the generally accepted diagnostic criteria have been compared with lambs fulfilling some or none of the accepted criteria of diagnosis. Border disease lambs had more medullated wool fibres and glial nuclei in the spinal cord than other lambs, the minimum counts being 37% medullated fibres and 267 glial nuclei per field, values which together were not shown by any of the other lambs. The results suggest that base-line values for these parameters with respect to Border disease might be determined. Limited evidence has been produced which indicates that spinal glia are also increased in natural Border disease in Clun Forest sheep and that these higher values persist for some years after the more characteristic changes of Border disease have resolved. Attention is drawn to the presence of cerebral softenings in lambs following upon each type of exposure to Border disease infection and occurring irrespective of overt evidence of the disease.

Total protein and immunoglobulin concentrations in sera from 2-day-old Finnish Landrace × Dorset Horn lambs

SummaryImmunoglobulin concentrations in 2-day-old Finnish Landrace × Dorset Horn lambs were remarkably high and uniform and were unrelated to litter size. The mean concentrations in singles, twins, triplets and quadruplets were 2.95 ± 0.215, 2.95 ± 0.099, 2.94 ± 0.105 and 3.03 ± 0.142 g/100 ml respectively.

The body composition and herbage utilization of grazing Merino and crossbred lambs during periods of growth and summer undernutrition

A grazing study was undertaken (i) to determine the changes in body energy and body composition of Merino and Dorset Horn x Merino lambs during their first year of life with the object of assessing the nature of body tissue reserves during periods of summer undernutrition, and (ii) to examine the nutritional basis of growth differences between the two genotypes in terms of herbage intake and feed utilization for weight gain. The relations between body weight and (i) total body energy, (ii) body protein, (iii) body fat, and (iv) body water were of linear form and similar for both groups. Each 1 kg of wool-free body weight increment by the unfasted grazing animal was associated with the retention of 3.17 megacalories (mcal) of body energy, whereas on an empty body weight basis 1 kg gain was associated with the retention of 4.18 mcal. Empty body weight was no better a predictor of body energy than the unfasted wool-free body weight of the grazing sheep; by either method more than 97% of the variance in body energy could be accounted for by differences in body weight. Dorset Horn x Merino lambs were significantly heavier at birth (4 5 v. 3.5 kg). They grew more rapidly than the Merinos throughout the 300 day experiment although during the first 200 days relative growth rates were similar. The intake of digestible energy WE) of the two groups was estimated for 112 days from weaning at c. 7 weeks. During this period the crossbreds weighed 20-23 % more than the Merinos; they consumed 23 % more herbage at the beginning and 11 % more at the end (P < 0.01). The efficiency of energy retention was similar for both breeds, 42.5 % of metabolizable energy above the maintenance being stored as wool-free body energy. During the summer drought period the crossbreds used their feed more efficiently than the Merinos; they consumed up to 25 % less feed per kilogram weight and at the same time increased their weight advantage. Differences in body weight underestimated differences in body energy. Crossbreds weighed 30% more than the Merinos at the beginning of the summer drought period, and 36 % more at the end. Corresponding differences for body energy were 40 and 53 %. There was no evidence to suggest that prolonged periods of arrested growth influenced the composition of the body at a given weight.