Reproductive performance of Border Leicester x Polwarth ewes with and without the Booroola Gene (FECb)

Abstract

Animal Production in Australia 1998 Vol. 22 REPRODUCTIVE PERFORMANCE OF BORDER LEICESTER x POLWARTH EWES WITH AND WITHOUT THE BOOROOLA GENE (FecB ) C.F. KINGAC, D.L. HOPKINSB and P.M. WILLIAMS A B A Department of Primary Industry and Fisheries, PO Box 46, Kings Meadows, Tas 7249 NSW Agriculture, PO Box 129, Cowra, NSW 2794 C 12 Beverly Hills Rd, Newstead, Tas 7250 SUMMARY In February 1988, 190 crossbred ewes were joined to Poll Dorset rams. Of these ewes 145 were a Border Leicester (BL) x Booroola (B) x Polwarth (P) cross a proportion of which carried the Booroola FecB gene and the remainder were BLxP cross. In April another 191 crossbred ewes of similar breeding were joined to Poll Dorset rams. During mating ovulation rate was determined by endoscopy. Mean ovulation rates were not significantly different between joining groups but there was a genotype effect with BLxBxP ewes having a significantly (P < 0.001) higher ovulation rate than the BLxP ewes regardless of joining time. Litter size followed the same pattern as ovulation rate. Early joined ewes produced 1.75 lambs per ewe compared to 1.87 for the late joined ewes. Fewer lambs died between lambing and weaning in the early group (14.4% vs 18.2%). Lambs born early weighed significantly (P < 0.001) more at weaning than those born late such that the early joined ewes produced more kilograms of lamb at weaning (51 vs 44kg). Keywords: lamb, growth, reproduction, Booroola INTRODUCTION Production of prime lambs in Tasmania has been based on crossbreeding systems involving the Polwarth breed as a component of prime lamb dams, with normal marking percentages of around 100% (King 1982). With the knowledge that the Booroola (FecB ) gene offered significant scope to increase the reproductive performance of crossbred ewes (Piper and Bindon 1982) an extensive evaluation of Booroola infused ewes was undertaken in the 1980s in Tasmania. Consideration was given to the importance of joining time in the context of producing larger, leaner carcasses. With a mostly late winter/spring lambing in Tasmania from a March/April joining (Hopkins and King 1990) good quality feed becomes limiting in the summer due to low pasture digestibility (Michell and Fulkerson 1987). This restricts the growth of lambs and the achievement of heavy liveweights (Hopkins et al. 1990). An experiment was conducted with the aim of examining the reproductive performance of crossbred ewes infused with the Booroola gene (FecB ) and those not infused, when joined at two different times, February and April. MATERIALS AND METHODS In February 1988, 190 mixed age crossbred ewes were joined to Poll Dorset rams for six weeks. Of these ewes 144 were a BLxBxP cross, bred by two different pathways as outlined by King (1982) with the gene introduced from either the dam or sire side. These will be referred to as genotypes 1 and 2 and some of each genotype carried a single copy of the FecB gene as established from previous ovulation records. The 46 remaining ewes were a BLxP cross. In April another 191 crossbred ewes of similar breeding and age structure were joined to the same Poll Dorset rams for six weeks. Of these, 145 were a BLxBxP cross and 46 a BLxP cross. Both groups were balanced for genotype, FecB status, age, liveweight and condition score. During mating ovulation rate was determined by endoscopy. All ewes were weighed and condition scored at regular intervals between joining and lambing. At lambing the dam identity and sex, weight and birth status of all lambs was obtained. Rearing status was also recorded. At marking, half the male lambs born as singles or twins were rendered sterile by retention of the testicles in the abdominal cavity (induced cryptorchidism). Male lambs born as triplets or greater were castrated. Analysis of variance was used to assess the significance of the fitted effects of genotype, joining time, age of ewe and all first order interactions on reproduction rate and its components. A bonferroni pairwise test 225 Animal Production in Australia 1998 Vol. 22 was applied to test for differences between means. For lamb growth data, lamb sex, birth type and rearing type were also included as main effects and age of ewe was excluded. The model also included the interaction between genotype and joining time. RESULTS Patterns of liveweight and condition of the ewes joined late or early in the season are shown in Figure 1. At the commencement of the experiment both liveweight and condition scores were similar, these two factors being used to stratify and select the animals. Ewes in the early mating group gained weight during joining and were in significantly (P < 0.001) better condition and heavier at the second weighing (Figure 1). By contrast ewes in the late mating group lost weight and condition during joining. There was no significant difference between joining groups for ovulation rate (Table 1), but there was a significant (P < 0.001) overall genotype effect on ovulation rate. Mean ovulation rate of the BLxBxP 1 ewes was significantly (P < 0.001) greater than the BLxBxP 2 ewes (2.6 + 0.14 vs 2.0 + 0.09), both being greater (P < 0.05) than the BLxP ewes (1.7 + 0.12). The interaction between joining time and genotype was not significant. Fertility was not significantly influenced by joining time or genotype and exceeded 0.9 in all cases. Litter size (fecundity) was not affected by joining time (Table 1), but there was a significant (P < 0.001) overall genotype difference. The mean litter size of the BLxBxP 1 ewes was significantly greater (P < 0.001) than the BLxBxP 2 ewes (2.1 + 0.08 vs 1.8 + 0.06) which in turn had a mean litter size greater (P < 0.001) than the BLxP ewes (1.5 + 0.10). There was no significant interaction between joining time and genotype. Time of joining, genotype and ewe age had no significant effect on reproduction rate or on survival (Table 1). Genotype and joining group differences in birthweight are shown in Table 1. Those lambs born in the late group were significantly (P < 0.05) heavier at birth than those born in the early group. Genotype effects were significant (P < 0.05) with lambs born to the BLxP ewes being heavier (4.13 + 0.09) than those born to the BLxBxP 1 ewes (3.90 + 0.07). Those born to the BLxBxP 2 ewes were not significantly different from the other two groups (4.07 + 0.07). There was a significant interaction between genotype and joining group for birthweight, with lambs from BLxBxP 1 ewes being lighter in the late group compared with those in the early group. Joining time had a significant effect (P < 0.001) on weaning weight (Table 1) with early born lambs being heavier. Between genotypes there was no significant difference in weaning weight of the lambs (Table 1) and no significant interaction between joining time and genotype. Birth type, rearing type and lamb sex all significantly affected (P < 0.001) weaning weight. Since there was no significant difference in the mean weaning age of lambs (120 days) according to joining groups or genotype, lamb growth rate was significantly faster (P < 0.001) for early born lambs, but there was no genotype effect. The only significant interaction (P < 0.05) was between joining group and lamb sex, with early born cryptorchids exhibiting faster growth to weaning than wether and ewe lambs, whereas in the late born group the differences between gender groups were much smaller and non significant. Overall the early joined ewes produced significantly (P < 0.001) more kilograms of lamb, per ewe joined at weaning, with no significant genotype effect or interaction between these factors. Lamb sex, birth type and rearing type effects were all significant (P < 0.05). 58 3.4 56 Liveweight (kgs) 3.2 54 52 50 48 46 40 77 105 133 166 192 236 Day Fat Score 3 2.8 2.6 2.4 2.2 40 77 105 133 166 192 236 Day Figure 1. Liveweight and fat score change over time for early x and late q joined ewes 226 Animal Production in Australia 1998 Vol. 22 Table 1. Least squares means with standard error of dif ferences (s.e.d.) for reproduction rate and its components according to joining time and genotype BLxBxP Early group Number of ewes joined (Ej) Ovulation rate* Number of lambs born (Lb) Fertility - ewes lambed/joined (El/Ej) Litter size (fecundity) - lambs born/ewes lambed (Lb/El) Survival - lambs weaned/born (Lw/Lb) Number of lambs weaned (Lw) Reproduction rate - lambs weaned per ewe joined (Lw/Ej) Birthweight (kg) Weaning weight (kg) Weight of lamb/ewe joined (kg) Late group Number of ewes joined (Ej) Ovulation rate* Number of lambs born (Lb) Fertility - ewes lambed/joined (El/Ej) Litter size (fecundity) - lambs born/ewes lambed (Lb/El) Survival - lambs weaned/born (Lw/Lb) Number of lambs weaned (Lw) Reproduction rate - lambs weaned per ewe joined (Lw/Ej) Birthweight (kg) Weaning weight (kg) Weight of lamb/ewe joined (kg) * BLxBxP2 82 1.96 118 0.95 1.69 0.85 95 1.22 3.83 31.0 51.1 82 1.93 122 0.93 1.84 0.87 105 1.40 4.32 27.0 45.0 BLxP 46 1.62 60 0.95 1.42 0.93 57 1.32 3.96 31.3 50.1 46 1.82 67 0.87 1.63 0.81 59 1.32 4.29 27.3 44.6 Overall 190 2.06 290 0.96 1.75 0.87 236 1.32 3.90 31.2 51.2 191 2.09 296 0.91 1.87 0.85 254 1.45 4.16 26.8 44.4 Mean s.e.d. 62 a 2.58 112 0.97 2.13 0.82 84 1.43 3.92 31.5 52.3 63 a 2.51 107 0.93 2.13 0.86 90 1.63 3.87 26.0 43.5 a a b b 0.21 0.06 0.14 0.08 0.16 0.15 0.93 2.06 b b b b 0.21 0.06 0.14 0.08 0.16 0.15 0.93 2.06 b b Determined for 188 ewes per group. Superscripts indicate differences between means within rows at P = 0.05 DISCUSSION Early joining did not suppress the ovulation rate of the ewes when considered as two groups which may have been expected and consequently there was no significant difference in litter size. This outcome is likely a reflection of the fact that the early joined ewes gained weight and were in better condition during mating (Knight and Hockey 1982). There was however a difference between genotypes, the BLxBxP 1 ewes being superior to the BLxBxP 2 and BLxP ewes in ovulation rate. This superiority was not exhibited in the weight of lamb weaned per ewe. The ewes showing the superior reproductive performance had the FecB gene introduced by the crossbred B x P ewe with a greater number of ewes carrying a copy of the gene than when it was infused from the ram line as for the BLxBxP 2 ewes. Endoscopy data showed 34% of BLxBxP 1 ewes carrying the gene, compared with 23% for the BLxBxP 2 genotype. Since not all ewes carried the gene a direct comparison cannot be made to the work of Bindon et al. (1984) or, King (1987) because in these cases only ewes carrying the FecB gene were used. Their results highlighted considerable potential for the Booroola gene in crossbred flocks. Ewes carrying the FecB gene have been shown to mark 1.96 lambs/ewe joined compared to similar ewes not carrying the gene which marked 1.50 lambs/ewe joined (King 1987). Bindon et al. (1984) found that ewes carrying the FecB gene when joined to either Suffolk or SIROMT rams weaned a greater weight of lamb per ewe joined (39.7 vs 32.8) than ewes from a non-Booroola background. Although more lambs were weaned from the late joined group, the early joined ewes produced 4.4 kilograms more lamb at weaning and the lambs had the potential to produce a large lean carcass as the later born lambs faced declining feed quality. Nutrition becomes a very important factor if increases in profitability from improved lambing percentages are to be realised (Earl and Dunstan 1988). Producers who target their production at the heavy weight market also face the problem of growing out lambs particularly if they lamb 227 Animal Production in Australia 1998 Vol. 22 late in the season. Non-castration of male lambs is one strategy that can be used to assist the production of heavier lambs, but satisfactory results will only be achieved if adequate nutrition is provided (Hopkins et al. 1990). ACKNOWLEDGMENTS This project was part of a larger study partially funded by the Australian Meat Research Committee now the MRC. The assistance of the former staff at the Elliott Research Station particularly Mr K.L. Gilbert is gratefully acknowledged. REFERENCES BINDON, B.M., PIPER, L.R. and CHANG, T.S. (1984). In Reproduction in Sheep. (Eds D.R. Lindsay and D.J. Pearce) p. 243. (Australian Academy of Science: Canberra). 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