Selecting Romneys For Fertility

Press, Volume CIII, Issue 30478, 27 June 1964, Page 8

Selecting Romneys For Fertility

"1 consider that the evidence which has been presented in this paper shows quite conclusively that very worthwhile gains can be made by selective breeding,” said Dr. L. R. Wallace, director of agricultural research for the Department of Agriculture and director of the Ruakura agricultural research centre, speaking to the Ruakura farmers’ conference last week about a 16-year-old experiment in which selection has been practised to raise fertility in a Romney flock.

Sixteen years ago three flocks each of 100 ewes were established. They were known as the “high,” “control” and “low” fertility flocks. The sheep for these flocks came from a mixed-aged flock of about 1000 Romney ewes for which individual lambing records had been kept for several years. The ewes that went into the high fertility flock were either animals that had previously produced twins or, in the case of two-tooths, were animals whose dams had a history of twinning. Ewes for the low fertility flock were, on the other hand, animals which had never previously produced twins or, in the case of two-tooths ewes whose dams had not produced twins. The control flock was a random sample. Since 1948, each of the flocks has been completely closed and self contained. Since the experiment began, the size of the flocks has been increased to about 133 with about 33 two-tooths being brought into each flock each year. Once a ewe enters a flock she remains in it for at least three lambing seasons. In both the high and low fertility flocks no attention has been paid in selecting replacements to size, conformation or wool. For the high fertility flock the animals chosen have been those whose dams had the best twinning records. For the low fertility flocks they have been those whose dams have regularly produced singles. In selecting replacements for the control flock, no attention had been paid to the fertility performance of the dams —instead the best looking two-tooths have been chosen.

Lambing percentage, expressed as the number of lambs alive at 28 days per 100 ewes mated, was 80 per cent for the high fertility flock over the first four years of the experiment and it has risen steadily to average 111 per cent in the 1960-63 period. “Whereas during the first four-year period the average lambing percentage of the high fertility flock was only 1 per cent higher than that of

the total flock, over the last four years the average difference has been 26 per cent,” said Dr. Wallace in: giving the results of the exercise.

“The lambing percentage figures may appear rather low,” he added, “but it must be remembered that they relate only to the two-tooth, four-tooth and six-tooth ewes, and that more than a third of the animals were twotooths.”

In summarising the results, he said that the improvement in lambing percentage had resulted both from a reduction in the percentage of dry ewes and from an increase in twinning rate. Although the ewes bred for high fertility had in general been rather heavier than those of the control and low fertility groups, their improved lambing percentage had not been the result of any increase in the size of the ewes—it had been shown that over the years ■ as the lambing percentage increased, the average live-weight of the ewes actually declined. This did not mean that liveweight was not an important factor in fertility, but it did show that it was possible to breed small, highly prolific animals.

In discussing the trial generally, Dr. Wallace said that in all three flocks the dry ewes had tended to decrease with age, but this decrease appeared to be much more pronounced in. the high fertility than in either of the other two flocks. The high fertility flock had rather more dry ewes at the two-tooth stage, but considerably fewer at all later stages. Of the reason for the relatively high percentage of barren two-tooths in the high fertility flock, Dr. Wallace said that one factor that could have been of importance was that a large proportion of the two-tooths which entered this flock were born as twins, whereas most of those entering the other two flocks were born as singles.

At both the two-tooth and four-tooth stage in the high fertility flock the proportion

of dry ewes had been higher among those born as twins than among the singles, and it was Interesting to note that the animals born as singles actually produced more twins than did animals bom as twins, the difference being particularly marked at the two-tooth stage.

In each flock ewes that were bom as singles had given a higher lambing percentage than ewes bom as twins.

The twinning rate in the control and low fertility flocks had been fairly similar both in the first and second elght-year periods. It appeared that no progress had been made In breeding against twinning. Substantial progress had, however, been made in breeding for twinning, for in both periods, and particularly during the last eight years, the twinning rate had been much higher in the high fertility than in either of the other two flocks. During the last eight years the twinning rateof the high fertility two.tooths had almost been as high as that of the fourtooths in the first eight years, and the twinning fate of the four-tooths had in the later years been almost as high as that of the six-tooths in the earlier years. Mortality among twin lambs tended to be higher than among singles, and as there were more twins born in the high fertility group than in either of the other two groups this was the main factor responsible for the higher average death rate in the high fertility group. In all three flocks fleece Weight had declined and this was greater in the high fertility flock than in either of the others. It was difficult to decide whether , the genera] fall in fleece weights during the experiment was caused by the sort of breeding practised or something else. It had certainly been paralleled by a general reduction in the average live-weight of the ewes. However, it was his personal feeling that a major factor in the fall in both .fleece weight and live-weight had been the increased stocking rate which had occurred during the experiment. Not only had the size of flocks been increased but the area on which they had been run had been reduced. This did not, however, account for the greater reduction in fleece weight of the high fertility group, particularly over the last four years. In all three flocks ewes born as singles had produced somewhat heavier fleeces than those born as twins. Most of the ewes entering the high fertility flock had been twins and this would account jto some extent for their

lighter fleece weights. The I fact that more of the high fertility group ewes had themselves reared twins had also probably been of some importance. In all three flocks and in the case. of two-tooths, fourtooths and six-tooths there was a fairly consistent difference of about 101 b In favour of single lambs at weaning. Again, at weaning, lambs from four-tooths tended to be about 51b to 61b heavier than those from two-tooths, and lambs from six-tooths about 11b heavier than those from four-tooths. This applied to both single and twin lambs.

Lambs from the high fertility group—both singles and twins —had also been somewhat heavier at weaning than those from either of the other two groups. It appeared that high fertility animals were either inherently faster growing than those of the other two groups, or that the ewes of the high fertility group were somewhat superior from point of view of milk production. As a • result of the higher lambing percentage and the greater growth' rate of the lambs, the high fertility group ewes, showed up very favourably when a comparison was made on the basis of the weight of lamb weaned per ewe.

In all three flocks quite a high percentage of the sheep that failed to lamb as twotooths failed again at either the four-tooth or six-tooth stage, and quite a proportion were dry at both of these two stages. On the other hand comparatively few of the ewes which lambed as two-tooths were dry as fourtooths or as six-tooths and very few were dry at both these ages. This provided a fairly strong argument for immediately culling twotooths which failed to lamb—particularly where the total number of these was not very large—say not more than 10 to 15 per cent. A further point in this respect was that two-tooths which failed to produce a lamb did not, on average, have a very high lambing percentage even if they did lamb at later stages. Dr. Wallace said that no claim was made that the programme followed in breeding for an improved lambing percentage in this investigation was the best that could be employed in practice. He believed that the programme could be modified in several ways to speed improvement.