Selection for aseasonal lambing and fecundity

Brian H. Magee, Farm Manager
Michael L. Thonney, Professor

The purpose of this project is to increase aseasonality of lambing and increase fecundity, where fecundity includes fertility and mothering ability to produce large numbers of healthy lambs at weaning time. Efficiency and speed of growth, and yield of lean meat are traits primarily controlled by stage of growth (weight in relation to mature weight). Therefore, because selection for increases in growth traits will also lead to increases in mature size, selection for growth rate in replacement rams is not considered important in this project.

Dorset (150 ewes). The Dorset flock is divided into the A, B, and C subflocks. Most non-defective ewe lambs are kept for replacements and followed for a few lambings. The most prolific and aseasonal are kept long-term. STAR ewes lamb five times at 7.2-month intervals at some time in their lives. All-STAR ewes lamb five times at 7.2-month intervals at some time in their lives and produce twins or better at each lambing. The fastest growing ram lambs out of All-STAR ewes are kept for breeding if they have sound feet and legs and no other defects. Some ram lambs might be kept from ewes that are nearly All-STARS. Such ewes might be those that have had a fast-growing set of twins at each lambing but have had one long (9.7-month) interval or are young ewes having not yet lambed five times but on schedule to become All-STARS. Inbreeding is minimized by breeding rams out of flock A to ewes in flock B, rams out of flock B to ewes in flock C, and rams out of flock C to ewes in flock A.

Finnsheep (100 ewes). All non-defective ewe lambs are kept for replacements and followed for a few lambings. The most prolific and aseasonal are kept long-term. STAR ewes lamb five times at 7.2-month intervals at some time in their lives. All-STAR ewes lamb five times at 7.2-month intervals at some time in their lives and produce triplets or better at each lambing. The fastest growing ram lambs out of All-STAR ewes are kept for replacement if they have sound feet and legs and no other defects. Some ram lambs might be kept from ewes that are nearly All-STARS. Such ewes might be those that have had a fast-growing set of triplets at each lambing but have had one long (9.7-month) interval or are young ewes having not yet lambed five times but on schedule to become All-STARS. Finnsheep rams from a private flock are used to minimize inbreeding.

Finnsheep x Dorset (300 ewes). Replacements for these commercial ewes are selected from the March-April lambs because 1) their mothers lambed out of season (or they wouldn't be lambing in the March-April season); 2) prices usually are lowest when the March-April lambs are ready for market; 3) they can be bred to lamb at 12 months of age in the March-April season or at 14 months of age in the June season, then at 22 months in the January or March-April seasons, and then at 30 months in the fall. Ewe lambs are less likely to breed for fall lambing and this strategy ensures productivity from replacement ewes. Replacement rams are from the Finnsheep and Dorset flocks. Large mature size terminal sires (Suffolk rams) are used to produce fast growing and efficient lambs that are lean when ready for market.

Selection progress. The aseasonality and fertility of the Dorset ewes from 1982 through 1987 was documented in the early part of the last decade (Lewis et al., 1996). Fertility ranged from 21% in the unfavorable mating seasons of March and June to 55% during favorable mating seasons of January, August, and October (69% of ewes in October breeding groups lambed). There was no change in aseasonality across the years evaluated. Prolificacy (lambs born per ewe lambing) ranged from a low of 1.39 for ewes mated in March to 1.58 for ewes mated in October and, again, little phenotypic progress was made from 1982 through 1987. Records since 1987 need to be added to the data base to assess progress made over the last decade.

Literature Cited

Lewis, R.M., D.R. Notter, D.E. Hogue, and B.H. Magee. 1996. Ewe fertility in the STAR accelerated lambing system. J. Anim. Sci 74:1511-1522.