Lamb Genotypes Research Articles

Differences in composition, muscularity, muscle:bone ratio and cut dimensions between six lamb genotypes

Carcass measurements and composition data were obtained for 198 lambs representing two sexes (104 cryptorchids, 94 ewes). Dorsal images of 166 of the carcasses were obtained on the slaughter chain using a video camera. The lambs were sired by a selection of Poll Dorset (PD), Texel (T), Border Leicester (BL) and Merino (M) rams, born to Border Leicester × Merino (BLM) and Merino (M) ewes giving 6 genotypes (PD × BLM, T × BLM, PD × M, T × M, BL × M, M × M). From the right hindleg and chump, the following muscles were dissected and weighed: semimembranosus, adductor femoris, semitendinosus, biceps femoris, and quadriceps femoris. The femur was weighed, the length measured and a muscularity value calculated as described by Purchas, et al. (1991, Meat Sci., 30, 181). Merino cryptorchid lambs were significantly ( P < 0.05) leaner, as measured by GR tissue depth (for explanation, see MATERIALS AND METHODS) adjusted to a mean carcass weight of 24.2 kg, than BL × M lambs, but there was no difference between genotypes within the ewe group (mean carcass weight of 17.1 kg). Genotype differences were found for conformation using the EUROP scoring system. Merino lambs had a greater proportion ( P < 0.001) of poorer scores, particularly compared to PD × BLM and T × BLM carcasses. The BL × M carcasses had a significantly ( P < 0.05) smaller m. longissimus thoracis et lumborum area than other genotypes with no difference between carcasses of T and PD sires. Muscularity values for the BL × M and M lambs were significantly ( P < 0.05) lower than the other crosses but not different from each other. Carcasses from T sires had significantly ( P < 0.05) higher muscularity values than those from PD sires. A negative association ( r = −0.56) was found between EUROP scores and muscularity when carcass differences were adjusted for, but significant differences between genotypes for muscularity were not necessarily carried through to differences in EUROP conformation score. BL × M carcasses had the lowest muscle:bone ratio and T × M the highest with M carcasses having similar ratios to carcasses from PD-sired lambs. Only small differences were found between genotypes for composition of the hindleg, the exception being BL × M carcasses of both sexes which were significantly ( P < 0.05) fatter than T × BLM, T × M and PD × M carcasses. Carcasses from T sires had significantly ( P < 0.05) more muscle in the hindleg than carcasses from PD sires, with the differences being greater in the heavier cryptorchids (1.8%) than the ewes (1.1%). There was no consistent effect of genotype on the cross-sectional areas of the round and topside cuts within either sex. Differences between genotypes for the proportion of the carcass attributed to the round were small and only for BL × M ewe carcasses was this cut significantly lighter ( P < 0.05) than from all other genotypes, except the PD × BLM carcasses. This trend was similar for the topside and silverside cuts. A moderate correlation ( r = 0.46; P < 0.001) was found between muscularity values determined by dissection and those predicted using the equations of Hopkins et al. (1996, Proc. Aust. Soc. Anim. Prod., 21, 181) based on carcass width measurements taken from video images.

Social relationships between ewes of two breeds

Domestic sheep have a strong social tendency. However, the nature of social relationships varies with season, breed, and size of enclosure. Ewes occupy a smaller home-range, with larger subgroup sizes, in the winter compared with the summer months (Lawrence and Wood-Gush, 1988a). Domestication has apparently led to a reduction in nearest neighbour (NN) distances from feral (Soay) sheep, who do not have strong flocking behaviour, to Merinos with NN distances of less than 2 m (Arnold, 1985). The social choices made by ewes affect their foraging behaviour and may have significant effects on production (Lawrence and Wood-Gush, 1988b). The aim of this study was to examine the social choices of a hill and a lowland breed of ewe when grazing two different environments in spring and summer of 1995 and 1996. The influence of lamb genotype on the behaviour of the ewes was also investigated.

Effects of grazing management and levels of concentrate supplementation on parasite establishment in two genotypes of lambs infected with Haemonchus contortus

The effects of grazing systems, levels of feed supplementation and genotypes of sheep on the establishment ofHaemonchus contortus were studied in 36 St . Croix x Sumatra crosses (HC) and 36 Sumatra (S) male lambs . The lambs were artificially infected with Haemonchus larvae (L3) at a rate of 3,000 L3 per animal and fed concentrate supplement at 0 .5% body weight (BW) or at 1 .6% BW. The animals grazed clean Brachiaria brizantha pastures . Lambs grazing GM1 (grazing system with a 6-week rotational cycle) had egg counts of 3,936 t 1 .525 at week 18, and 29% of the lambs had PCV ranging from 7 .8 to 14 .6% . Lambs in GM3 (grazing system with a 12-week grazing cycle) had egg counts of 6,714 t 2,075 at week 28, and 24%ofthe lambs had PCV ranging from 7 .9 to 13.3% At these periods, all lambs in both GMI and GM3 were dosed with anthelmintics to prevent the death of the animals . Although the overall mean egg counts (4,543 t 318) of lambs on GM2 (grazing system with a 1-week grazing and 6-week resting periods) were higher (P 0 .10) between HC and the S lambs when fed supplement at 0.5% BW, but on 1 .6% supplement BW group, the egg counts were lower (P<0.001) in HC. It is concluded that a one-week grazing and a six- week resting system was superior than grazing systems with a 6- or 12-week rotational cycle . Improving the nutritional status of lambs by increasing the level of supplement offered could depress the establishment of Haemonchus in the Iambs . The HC and S lambs showed similar response to supplements at 0 .5% BW, but at 1 .6% BW the HC responded better than the S. Key words: Grazing management, Haemonchus contortus, feed supplement, sheep

Environmental and maternal effects on early postnatal growth of lambs of different genotypes

Two important North European sheep breeds (the purebred Flemish Milksheep (M) and Suffolk (S) breed) were double reciprocally crossed to develop a highly fertile ewe genotype (SM × MS or F2, called the K.U. Lovenaar), which was finally crossed with Texel sires to improve carcass and slaughter characteristics of the final slaughter lamb genotype: F3 or T × F2. Birth weight, birth date, litter size, number of suckling lambs, age of ewe and sex of lambs had a significant influence on early postnatal growth rate ( P < 0.001). The significantly positive influence of birth weight on earliest postnatal growth rate diminished with increasing age of lambs. Age of ewes affected increases of lamb weights by having an important effect on birth weight. Lambs of 1-year-old ewes were characterized by a significantly lower birth weight and postnatal growth rate compared with lambs of multipareous ewes. A negative effect of litter size was observed on birth weight and on early postnatal growth results until 10 kg. Ram lambs had a higher birth weight and realized better growth until 16 kg, compared with results of ewe lambs. Significant effects of date of birth were also observed. Without correction for significant effects, variation in postnatal growth rate was about 30%. Growth rate of final slaughter lamb genotypes F3, T(MS) and T(F2fix) were significantly different. F3 and T(MS) lambs realized a significantly lower growth rate at 16 kg than T(F2fix) lambs (respectively, 324, 302 and 359 g day −1). However, the number of days needed to reach 16 kg was significantly higher for T(F2fix) lambs due to the low growth performance from 6 to 10 kg. F2 lambs had the highest growth rate of 370 g day −1, while F2fix lambs realized the lowest postnatal growth: 283 g day −1. Variation in postnatal growth rates decreased after correction for the significant effects. Early postnatal growth performance of slaughter lamb genotypes T(MS), F3 and T(F2fix) were comparable (about 320 g day −1 at 16 kg), but were still significantly lower than the excellent growth rate of F2 lambs (404 g day −1). They did not differ in growth data from MS and F2fix lambs. For selection on early postnatal growth one has to take into account maternal and environmental influences.

Environmental and genetic influences on growth performance of lambs in different fattening systems

Purebred Flemish Milksheep (M) and Suffolk (S) breed were double-reciprocally crossed to develop a highly fertile ewe genotype (SM ♂ X MS ♀ or F2, called Lovenaar), which was finally crossed with Texel sires to improve carcass and slaughter characteristics of the final slaughter lamb genotype: F3 or T ♂ X F2♀. Growth of Texel ♂ X (Milksheep ♂ X Suffolk ♀(T(MS))-lambs were studied for efficiency of second reciprocal crossing, while postnatal growth performances during early postnatal life of Texel ♂ X (F2 ♂ X F2 ♀) (T(F2fix)) lambs were tested for preservation of the superior characteristics of crossbreds when fixating the Lovenaar crossing scheme. Influence of birth weight (BW) on growth remained significant after weaning during intensive stable fattening. While lambs of yearling ewes grew slower during the suckle period, during fattening these lambs had a higher growth rate. Differences between lambs born single or as twins, triplets or quadruplets were not significant during fattening. The decrease of superiority of single-suckling lambs during fattening occurred only in non-extra-fed lambs on pasture, not in extra-fed lambs. In all three fattening schemes ram lambs realized a better growth than ewe lambs. Choice of fattening system had an important influence on growth rate. In early-weaned, intensively fattened lambs a post-weaning depression appeared at 18 kg BW. This could not be observed in lambs on pasture, but they showed a reduction in growth from BW of 32 kg onwards. This decrease was observed earlier and more pronounced in non-extra-fed lambs on pasture (28 kg). T(MS)-lambs seemed to be better suited for fattening at pasture, while F3-lambs realized best growth during stable fattening, but with a higher intake of commercial pellet feed. Extra-fed F3- and T(F2fix)-lambs on pasture realized comparable growth to purebred Texel lambs. There were no differences in growth rate between F2- and F2fix-lambs, nor between F3- and T(F2fix)-lambs during fattening on pasture. According to the selected Texel sire, growth performance of F3-lambs was significantly different. F3-lambs born in spring, had a higher growth rate during fattening in comparison to lambs born in autumn. After correction for significant environmental influences, lambs of different genotypes had different genetic growth potentials under different fattening conditions.

Growth hormone pulsatility in ram lambs of genotypes selected for fatness or leanness

AbstractAlthough it is known that growth hormone (GH) influences body composition in ruminants, the precise role of the pattern of GH secretion is not known. We have studied the pulsatile release of GH and insulin-like growth factor 1 (IGF 1) secretion in the male progeny of rams from lines selected either for {fat genotype) or against (lean genotype) fatness. Seventy-two lambs (36 each of the fat and lean genotype) were kept on high-quality pasture and randomly allocated within genotype to treatment at 2, 3, 3·5, 4,5 or 6 months of age. The procedure, which was identical for each sampling period, was to sample each lamb through a jugular cannula every 10 min for 6 h, and then, following an overnight fast, to slaughter and analyse the carcass for fat. All blood samples were analysed for GH and samples taken each hour for total plasma IGF 1. The GH data were further analysed with the pulse detection routine PULSAR. Carcass fatness, adjusted for cold carcass weight, was greater for fat genotype animals than for the lean genotype. GH was pulsatile in all profiles but the pattern differed with time and genotype. Mean GH and pulse amplitude decreased with time but did not differ between genotype, although the lean genotype had higher mean GH at five of the six sampling periods. In contrast, GH pulse frequency and IGF 1 were significantly higher for the fat compared with the lean genotype lambs. GH mean and amplitude correlated negatively with carcass fatness in both genotypes and GH pulse frequency and total IGF 1 correlated positively with fatness for the lean genotype only. When carcass weight and genotype were fitted to these relationships, GH mean and total IGF 1 were found to have independent negative and positive effects, respectively, on carcass fatness. Because GH mean had a separate effect on fatness independent of genotype or cold carcass weight, it is likely that GH secretion influences composition by the same basic mechanism in both genotypes. However, although the slopes of these relationships did not differ significantly between the genotypes, the intercepts were significantly different indicating that over and above the basic mechanism, at any level of GH, the lean genotype lambs were leaner than the fat genotype lambs. This may indicate a measure ofGH resistance in the fat genotype lambs.

Milk yield and milk composition of Flemish milksheep, Suffolk and Texel ewes and their crossbreds

Milk production (MP) and milk composition (MC) were investigated in 72 ewes of West European sheep breeds: Flemish Milksheep, Suffolk and Texel and their crossbreds, during the first 45 days of lactation and related to postnatal growth of their lambs. Without correction for significant influences, mean daily MP was 1.35 ± 0.47 kg and milk fat, protein and lactose contents were 9.00 ± 1.29, 4.85 ± 0.36, and 5.62 ± 0.25%, respectively. Breed, age of ewe and number of lambs suckled influenced MP significantly ( P < 0.05). Flemish Milksheep ewes had the highest average daily MP (3.33 ± 0.29 kg), while Texel ewes produced the lowest amount (1.10 ± 0.15 kg). The other genotypes had 1.5 kg MP. Two-year old ewes produced 200 g/day more milk than one-year olds. Ewes rearing twins had a higher MP of 400 g/day compared to ewes suckling singles. MC was not significantly influenced by genetic and physiological effects. MP and MC did not vary with circadian rhythms, i.e. a.m. vs. p.m. Significant correlations ( P < 0.0001) indicated that ewes with a high MP in early lactation also continued for a good performance at 45 d postpartum. Early postnatal growth of lambs was dependent on genotype of lamb, birth weight and number of lambs suckled ( P < 0.05). Purebred Milksheep lambs realized a high growth rate (306 ± 37 g/d). Crossbred lambs approximated Milk-sheep, while purebred Texel lambs showed the slowest weight gain. These results were associated with milk uptake and conversion by lambs. Twin lambs grew slower than singles. Regression analyses showed that lambs had to have a chance to suck at least five times/d after 4 h of separation before a good estimation of the realized MP was obtained.

REPRODUCTIVE PERFORMANCE OF BOOROOLA × FINNISH LANDRACE AND BOOROOLA × SUFFOLK EWE LAMBS, HETEROZYGOUS FOR THE F GENE, AND GROWTH TRAITS OF THEIR THREE-WAY CROSS LAMBS

Reproductive performance of Booroola × Finnish Landrace (BFL, n = 19) and Booroola × Suffolk (BS, n = 18) ewe lambs heterozygous for the F gene were compared to Finnish Landrace (FL, n = 14) and Suffolk (S, n = 26) purebred controls. FL lambs reached puberty earlier (211.3 d, P &lt; 0.001) than the other genetic groups (237.8, 233.0 and 232.9 d for S, BS and BFL, respectively) whereas weight at puberty was lower (P &lt; 0.001) for BFL, FL and BS (36.8, 36.7 and 47.0 kg, respectively) than for S (61.1 kg). About 95% of BFL and BS ewe lambs had at least one record of three ovulations or more over the first three estruses, including puberty. Mean ovulation rates at breeding (second estrus after puberty) to a Hampshire (H) ram were 3.8, 3.3, 2.2 and 1.7 for BFL, BS, FL and S, respectively (BFL and BS vs. FL and S, P &lt; 0.001). The corresponding litter sizes at birth were 2.5, 2.1, 1.6 and 1.3 (BFL and BS vs. FL and S, P &lt; 0.001) which reflected a higher embryonic loss in the Booroola crosses. Percentage ova loss ranged between 32.8% (BFL) and 12.8% (S) and was related to the level of prolificacy. Lamb mortality at birth was high in BFL (23.7%) compared to BS, FL and S (6.5, 0.0 and 0.0%, respectively, P &lt; 0.001). Litter size at weaning (50 d of age) averaged 1.9, 1.8, 1.5 and 1.3 for BS, BFL, FL and S ewe lambs (BS vs. S, P &lt; 0.02). Growth performance of H-sired progeny from the four genetic groups of ewes showed that H × S lambs had the highest average daily gain in both preweaning and postweaning periods (preweaning ADG: 349.9 g d−1; postweaning ADG: 332.1 g d−1) while the other genotypes of lambs performed equally (preweaning ADG: 267.4, 249.5 and 246.8 g d−1 for H × FL, H × BFL and H × BS, respectively; postweaning ADG: 281.2, 276.8 and 281.8 g d−1 for the same genetic groups). Overall productivity of ewe lambs in terms of kilograms of lamb produced showed a slight, nonsignificant, advantage for Booroola-cross ewe lambs (55.8 and 54.5 kg for BS and BFL) over purebred S (51.6 kg) and FL (44.9 kg). These results indicate that ovulation rate and litter size can be increased by incorporating F gene in both prolific (FL) and nonprolific (S) background genotypes without resulting in any significant difference in total weight of lamb produced per ewe. Key words: Sheep, ewe productivity, Booroola, ovulation rate, crossbreeding

A system for store lamb finishing from roots involving the slimming of overfat lambs and ad libitum concentrate feeding of underfinished lambs

Traditional systems of finishing store lambs from roots are unpredictable and inflexible. Lamb genotype, sex and initial weight significantly affect potential performance and target weights for slaughter at currently fashionable levels of fat cover. Lamb performance can easily be affected by climate. Frequently, producers fail to match crop resources to lamb grazing due to the unpredictable finishing rates. Planning of sales especially to meet marketing contracts is also difficult. Problems are most significant with hill breed lambs and smaller crossbred lambs both which need considerable gains to achieve slaughter condition and weight. With heavy ‘forward’ stores overfat lambs can be a problem.Careful management, appropriate supplementation and regular handling all help to reduce problems but in many situations where labour and handling facilities are limited these inputs also increase costs and difficulties.

The effect of interaction of ewe and lamb genotype on milk production of ewes and on growth of lambs to weaning

ABSTRACTSingle and twin purebred Scottish Blackface lambs born to Blackface ewes, and Suffolk cross lambs born to East Friesland × Blackface ewes, were removed from their dams at birth and cross-fostered to give all combinations of breed of ewe, breed of lamb and number of lambs suckled, with no ewe rearing her own lamb.Milk production was measured on 1 day/week for 14 weeks. The effect of ewe breed was significant in all weeks, with the East Friesland cross ewes having a higher peak and a more sustained level of production than the Blackface ewes. Ewes suckling twins produced more milk than those suckling singles. Breed-of-lamb effect was significant during the first 5 to 6 weeks of lactation, with crossbred lambs inducing a higher milk output than Blackface lambs, irrespective of the breed of ewe suckled. Beyond this stage, lactation potential of the ewe replaced demand by the lamb as the first limiting factor.Food intake by ewes during the first 6 weeks differed slightly with both ewe and lamb breed but there were larger differences associated with number of lambs suckled.The data suggest that lambs on higher milk intake consume a smaller proportion of solid feed but that, in all groups, live weight at 14 weeks of age was closely related to milk intake.

The contribution of maternal environment and lamb genotype to growth of lambs from Merino, Poll Dorset × Merino and Border Leicester × Merino ewes

Suffolk-sired lambs from Merino, Border Leicester x Merino and Poll Dorset x Merino dams were either mothered to ewes of their own breed of dam or to ewes of one of the other breeds of dam, with the exception that lambs born from Border Leicester x Merino, and Poll Dorset x Merino dams were not fostered between Border Leicester x Merino and Poll Dorset x Merino breeds of dam. Liveweight and lucerne meal consumption of the lambs and milk yields of the dams were determined during the first 100 days of lactation. Lambs reared by Border Leicester x Merino dams were significantly heavier than those reared by Merino dams at 16 and 23 days of age. They maintained this advantage throughout lactation, though differences were not statistically significant. There were no significant differences in liveweight of lambs between Poll Dorset x Merino and Merino dams. Suffolk x (Border Leicester x Merino) lambs were significantly heavier than Suffolk x Merino lambs at birth and when 9 days of age. They maintained this advantage throughout lactation, though the differences were not significant. Suffolk x (Border Leicester x Merino) lambs were also significantly heavier at birth than Suffolk x (Poll Dorset x Merino) lambs. However, the difference decreased with age so that the liveweights of the two breeds were similar by 80 days of age. There were no significant differences due either to breed of lamb or to breed of dam in lucerne meal intake by lambs and ewe milk yield. We conclude that the lamb's genetic potential for growth and maternal environment were of approximately equal importance in determining the superior growth of lambs born and reared by first-cross Merino dams compared with those born and raised by Merino dams.

A consideration of the techniques for estimation of milk yield by suckled sheep and a comparison of estimates obtained by two methods in relation to the effect of breed, level of production and stage of lactation

SummaryThree techniques, lamb-suckling and test-weighing, hand- or machine-milking following administration of oxytocin, and measurements of body-water turnover, used for the estimation of milk yield in suckled sheep are discussed in relation to sources of error and comparability of estimates.Results are presented from an experiment in which the lamb-suckling and the oxytocin techniques were compared. The mean daily milk production of Scottish Blackface and East Friesland × Scottish Blackface ewes suckling single and twin lambs was measured by both methods at weekly intervals for 6 weeks. All ewes were well nourished to ensure full expression of milking capacity. Additional information from a similar experiment on Merino ewes is also presented.The oxytocin method gave higher estimates of mean daily milk yield in the first week of lactation, especially in the groups of ewes rearing single lambs and in the Blackface breed. By the second week in most groups and the third week in all groups there were no significant differences between estimates made by the two methods and the relative values of the estimates were not affected by level of milk production, number of lambs suckled or genotype of ewe and lamb.

Corrigendum - Relative efficiency of Merino and Border Leicester × Merino ewes

Thirteen South Australian strong-wool Merino ewes and eight Border Leicester × Merino ewes mated to Poll Dorset rams were fed on lucerne pellets ad libitum during late pregnancy and lactation in pens. Each of their single-born progeny was slaughtered at 33 kg full liveweight. Breeds were compared for efficiency of the ewe (organic matter intake per unit liveweight and clean wool per unit intake), efficiency of the lamb (liveweight and body solids gain per litre of milk), and efficiency of the ewe-lamb unit (liveweight and carcass weight per unit intake). Border Leicester × Merino ewes ate 7% less organic matter per unit liveweight than Merino ewes during lactation; however, there was no significant difference during late pregnancy. In terms of absolute intake 120 Merinos = 100 Border Leicester × Merinos. Border Leicester × Merino ewes were 73% as efficient in converting feed to clean wool as Merino ewes. Merino ewes grew wool of smaller diameter and higher crimp frequency than crossbreds. Lambs from the Merino ewes were as efficient in converting milk into animal tissue as the progeny from Border Leicester x Merino ewes during the first 4 weeks of suckling. Lambs from Merino ewes required on average 25 extra days to grow to the same slaughter weight and ate 110% more organic matter from solid feed than lambs from crossbred ewes. However, the proportion of feed eaten by the lamb was small (10%) compared with that eaten by the ewe. Lambs from crossbred ewes were 23% heavier than those from Merino ewes at the same age. The crossbred ewe-lamb unit, however, consumed 20% more feed and hence was as efficient as the Merino ewe–lamb unit. The Merino ewe–lamb unit was more efficient than the crossbred ewe-lamb unit when the lamb genotypes were slaughtered at the same liveweight (P &lt; 0.05); however, the difference was not significant when expressed as carcass weight per unit intake.

Relative efficiency of Merino and Border Leicester × Merino ewes

Thirteen South Australian strong-wool Merino ewes and eight Border Leicester × Merino ewes mated to Poll Dorset rams were fed on lucerne pellets ad libitum during late pregnancy and lactation in pens. Each of their single-born progeny was slaughtered at 33 kg full liveweight. Breeds were compared for efficiency of the ewe (organic matter intake per unit liveweight and clean wool per unit intake), efficiency of the lamb (liveweight and body solids gain per litre of milk), and efficiency of the ewe-lamb unit (liveweight and carcass weight per unit intake). Border Leicester × Merino ewes ate 7% less organic matter per unit liveweight than Merino ewes during lactation; however, there was no significant difference during late pregnancy. In terms of absolute intake 120 Merinos = 100 Border Leicester × Merinos. Border Leicester × Merino ewes were 73% as efficient in converting feed to clean wool as Merino ewes. Merino ewes grew wool of smaller diameter and higher crimp frequency than crossbreds. Lambs from the Merino ewes were as efficient in converting milk into animal tissue as the progeny from Border Leicester x Merino ewes during the first 4 weeks of suckling. Lambs from Merino ewes required on average 25 extra days to grow to the same slaughter weight and ate 110% more organic matter from solid feed than lambs from crossbred ewes. However, the proportion of feed eaten by the lamb was small (10%) compared with that eaten by the ewe. Lambs from crossbred ewes were 23% heavier than those from Merino ewes at the same age. The crossbred ewe-lamb unit, however, consumed 20% more feed and hence was as efficient as the Merino ewe–lamb unit. The Merino ewe–lamb unit was more efficient than the crossbred ewe-lamb unit when the lamb genotypes were slaughtered at the same liveweight (P &lt; 0.05); however, the difference was not significant when expressed as carcass weight per unit intake.

The influence of lamb genotype on the milk production of Blackface ewes

SUMMARYThe milk intakes of single-born Texel x Blackface and of pure-bred Blackface lambs suckling Blackface ewes was measured using a lamb-suckling technique. The shapes of the lactation curves were similar but the mean milk intake of the Texel x Blackface lambs was substantially greater than that of the pure-bred lambs at all points on the lactation curve. The Texel x Blackface lambs attained a mean maximum daily milk intake of 2418 g and the pure-breds 2075 g; mean total milk intakes during an 84-day period were 157 and 128 kg, respectively. Mean birth weights were 5·08 and 4·98 kg and the subsequent mean daily live-weight gains were 364 and 344 g, respectively, for the Texel x Blackface and pure-bred lambs. It is concluded that lamb genotype had a substantial and significant influence on the milk production of the ewes.

Genetic factors affecting the milk intake of lambs

The actual amount of milk produced by a ewe is influenced both by her own potential to produce milk and by the potential of the lamb to obtain it. Two experiments were carried out to investigate the relative importance of these factors. In each experiment there were two genetically different groups of ewes, of which some reared their own single lambs and some reared single lambs from the other group fostered onto them. In both experiments, the effect of the genotype of the lamb was greater than that of the ewe. In the first experiment, during the period 6–24 days after parturition, ewes suckling strong-wool lambs produced 7.2 g/hr more milk than those suckling Peppin lambs, while the strong-wool ewes themselves produced only 4.6 g/hr more than the Peppin ewes. In the second experiment, during the period 0–28 days after parturition, ewes suckling Corriedale lambs produced 11.6 g/hr more milk than those suckling strong-wool Merino lambs, whereas the Corriedale ewes themselves produced only 4.4 g/hr more than the strong-wool Merino ewes. The lamb genotype effects were statistically significant. In a third experiment, in which the strong-wool and medium-wool Merinos were compared when suckling twins, the strong-wool exceeded the medium by 10.8 g/hr in milk production for the period 6-24 days after parturition. Some implications of the results are discussed.

Selection for weaning weight in Merino sheep. 2. Correlated responses in other production characters

The heritabilities of seven production characters of Merino Sheep and their phenotypic and genetic correlations with corrected weaning weight have been calculated using half-sib and dam-daughter correlations. Realized genetic correlations calculated from responses in Weight Plus and Weight Minus selection flocks, which showed broad agreement with these correlations, were : 17-month body weight, 0.72 ; yield, 0.22 ; clean fleece weight, 0.24 ; and staple length, 0.17. There was zero relationship with greasy fleece weight and crimps per inch, and a large negative relationship between folds and high weaning weight. There were strong phenotypic correlations between lamb growth and milk production through the total weight of milk protein produced. The lamb's genotype was the most important contributor to the difference in weaning weight between the flocks. There were no differences in reproductive performance or survival rates between the flocks but twice as many twin rams and 20 per cent more twin ewes were mated in the Weight Minus flock over the selection period.