Abstract:
THE EFFECT OF GENETIC IMPROVEMENT WITHIN PURE BREEDS ON THE PERFORMANCE OF THEIR CROSSBRED LAMBS W. J. YATES* and W. A. PATTIE Summary In 1966 and 1967, two flocks of Merino ewes, one selected for high weaning weight and the other selected for low weaning weight, were mated to Border Leicester rams. In 1967, half of the lambs born in each flock were crossmothered at birth to ewes in the other flock. Crossbred lambs from the high weaning weight flock were significantly heavier at birth and weaning, and had heavier carcass weights than did those from the low weaning weight flock. These differences were approximately half of the differences that existed between the two purebred Merino flocks. Comparisons of the weights of crossmothered and normally mothered crossbred lambs from each flock indicated that the lambs' genotype accounted for approximately half of these differences, the remainder being attributed to the milk production of the dams. I. INTRODUCTION In discussing alternative methods of improving the efficiency of lamb production, Pattie and Smith (1964) suggested that intensive selection within parent breeds used for crossbreeding would be more useful than establishing new breeds or improving existing dual purpose types. Such an approach would allow continued utilization of hybrid vigour for reproduction which has been shown to be extremely important in the Border Leicester x Merino ewe (B. J. McGuirk, personal communication). Body weight and wool production were shown to be only slightly affected by hybrid vigour in the same study. Thus selection within the parent breeds may lead to improvement of the crossbred lambs. To test this hypothesis, crossbreeding and crossmothering experiments were carried out using Border Leicester rams and ewes from two Merino flocks that have been selected for five generations for high and low body weight at weaning. Pattie (1965 a) showed that this selection had produced a 30 per cent difference in weaning weight between the flocks. In addition there were differences of approximately 20 per cent in the carcass weight of weaners, mature body weight and milk production (Pattie and Trimmer 1964; Pattie 1965 b; and Pattie and Williams 1966). The experiments reported here were designed to examine the effects of these differences on crossbred lamb production. *Agricultural Research Station, Trangie, N.S.W. ?N.S.W. Department of Agriculture, Sydney. 154 IL MATERIALS AND METHODS The two Merino flocks used in these experiments were described in detail by Pattie (1965 a). They consist of 100 ewe -flocks which were established at Trangie Agricultural Research Station in 1950. One has been selected for high weaning weight (Weight Plus), the other for low weaning weight. (Weight Minus). Details of the base flock, selection procedures and flock management have been given by Pattie (1965 a). During the last week of February 1966, the two flocks were joined as one mob with five Border Leicester rams. This joining continued for five weeks and lambing commenced in late July. All ewes and lambs were run together as one flock, individual lambs being identified and weighed at birth, marked when seven days old, then weighed again at weaning at an average age of 80 days. Wether lambs were slaughtered immediately after weaning and their carcass weights recorded. Slaughter data are not available for ewes as they have been retained for further crossbreeding experiments. In March 1967, the two flocks were again joined with Border Leicester rams. However, at this joining, oestrus was synchronised by the use of progestagen impregnated intravaginal sponges. Lambing commenced in the first week of August with all ewes that had mated within the first ten days of joining being confined to a small observation paddock. The lambs from approximately half the ewes in each flock were crossmothered at birth to ewes of the other flock. As the ewes lambed, they were confined in a small portable pen. Their lambs were removed and exchanged with lambs from a ewe that had already lambed or with the next ewe to lamb in the other flock. Each ewe and her foster lamb were kept in the pen for approxiniately 24 h. Birth weights and weaning weights (at an average age of 93 days) were recorded for all lambs. Ewe lambs were again retained for further crossbreeding experiments while the wether lambs were slaughtered 109 days after weaning. Body weight at slaughter and carcass weights were recorded. AI1 body weights and carcass weights were individually corrected for age using actual weight gains from birth to the time of weighing. These data were then examined by analysis of variance of unweighed subclass means. The main effects considered were genotype and sex for 1966 drop lambs, and genotype-rearing classification and sex for the 1967 drop lambs. Multiple range tests (Snedecor 1956) were used to assess differences between the four genotype-rearing classifications in the latter drop. Single and twin born lambs were treated separately as in some cases there were interactions between type of birth and other main effects. Only data from surviving twin pairs were included in these analyses. The relative contributions of genotype and rearing to the differences between crossbred lambs from the Weight Plus and Weight Minus flocks were calculated from the results of the crossmothering experiment. The effects of rearing were estimated as the average difference between lambs of the same genotype mothered either normally or crossmothered, expressed as a percentage of the difference between normally mothered lambs from each flock. Conversely the effects of genotype were estimated as the average difference between lambs suckling the same dams, which were born in either the Weight Plus or Weight Minus flocks, expressed as a percentage of the difference between normally mothered lambs from each flock. 155 III. RESULTS Mean corrected body weight data for the 1966 drop crossbred lambs are presented in Table 1 together with the number of lambs and mean percentage differences between Weight Plus and Weight Minus lambs. Both genotype and sex differences were significant (P < 0.05) for all characters except birth weight of twin born lambs. Corrected body weight data of the 1967 drop crossbred lambs, half of which had been crossmothered, are given in Table 2. The birth weights of Weight Plus lambs were significantly heavier than those of Weight Minus lambs, but within each genotype there were only small differences in birth weight between lambs mothered normally and crossmothered lambs. In only one case (Weight Minus, single born lambs), was this difference significant. Weight Plus lambs that were normally mothered had significantly heavier weaning and slaughter weights than normally mothered Weight Minus lambs. The effect of crossmothering Weight Plus lambs onto Weight Minus ewes was to significantly reduce weaning and slaughter weights of both singles and twins. Conversely crossmothering Weight Minus lambs onto Weight Plus ewes significantly increased weaning and slaughter weights except for the slaughter weight of twins. The differences between genotype-rearing classifications in carcass weight were not significant although the effects of crossmothering were in the same direction as with weaning and slaughter weights. The estimated differences in each of the body weight measurements between Weight Plus and Weight Minus lambs that are due to either genotype or maternal environment (rearing) are shown in Table 3. Here the differences in weaning weight have been calculated for males only as well as for both sexes as subsequent slaughter and carcass weights are only available for male lambs. TABLE 1 156 TABLE 2 IV. DISCUSSION The results of both the 1966 and 1967 observations demonstrate that differences between flocks selected for high or low weaning weight are reflected in the growth of their crossbred progeny. The differences in weaning weight between first cross lambs from Weight Plus and Weight Minus ewes is of the order of 12 to 14 per cent. This is approximately half of the difference previously observed between the purebred flocks. As the ewes were mated to the same Border Leicester rams, this indicates that the genotypes combine additively or that the hybrid vigout effect was approximately the same for each cross. In either case, the producer would receive the benefit of improvement but its extent may depend on the relative TABLE 3 157 importance of the genetic ability of the lambs to grow and the milk production of the dams. The lack of any large differences in birth weight between normally mothered and crossmothered Weight Plus single and twin lambs and Weight Minus singles indicates that effective randomisation had been achieved within these groups at lambing. However, the significant difference in birth weights among the Weight Minus twins indicates that heavier than average lambs were crossmothered. Although interpretation is confounded by the cell with -only one lamb, this difference would cause a slight over-estimate of the importance of milk production in the determination of later body weights. The average effect of crossmothering indicates that approximately half the difference in body weight between crossbred lambs from Merino flocks selected for different weaning weights is due to a difference in the lamb's genotype for growth. The remaining half is due to the differences in milk production of the ewe. This conclusion is slightly different to that reached by Pattie (1965 b) from the results of a crossmating and milk production experiment with the two pure Merino flocks. That work indicated that lamb genotype was the main determinant of differences in weaning weight and that the higher milk production of Weight Plus ewes was necessary for their lambs to fully express their superior genotype. In this case, crossbred lambs from Weight Minus ewes had sufficient genetic growth potential to respond to increased milk supply. Therefore, an enterprise that aims to improve production from crossbreeding by selection for weaning weight in the ewe flock should ensure a sufficient level of ewe nutrition to allow the expression of potential in milk production. V. ACKNOWLEDGMENTS We would like to thank the staff of the Agricultural Research Station, Trangie, for assistance with data collection in this experiment. VI. REFERENCES PATTIE, W. PATTIE, W. PATTIE, W. PATTIE, W. PATTIE, W. S NEDECOR, A. A. A., A., A., G. (1965 a). Aust. J. Exp. Agric. Anim. Hub. 5: 353. (1965 b). Aust. J. Exp. Agric. Anim. Hub. 5: 361. and S MITH , M. D. (1964). Aust. J. Exp. Agric. Anim. Hub. 4; 80. and T RIMMER , B. (1964). Proc. Aust. Soc. Anim. Prod. 5: 156. and W ILLIAMS, A. J. (1966). Proc. Aust. Soc. Anim. Prod. 6: 305. W. (1956). 'Statistical Methods'. (Iowa State University Press: Ames).