Breeding Issues (Conservation Genetics, Scrapie, et)
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Howling Oak Ranch_______________________________________________ |
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Flock Improvement using Selection. Part 1: Calculations without weights by Karen Gerhart, Howling Oak Ranch St Croix Sheep The usefulness of the St Croix breed is partly due to the moderately high prolificacy, early age at first breeding, and ability to breed year round shown by the St Croix ewe. Several methods for maintaining these important traits while also improving productivity of the flock as a whole are outlined below. All were derived from the Sheep Production Handbook (SPH, 1997) and have been used in our flock. Flock productivity is influenced by many traits, only some of which respond well to selection. Low ewe fertility (number of ewes lambing) and poor lamb survival are generally indications of a problem in flock management. Ewe fertility has shown low heritability (0-10%), as has lamb survival, meaning that they are more linked to environmental causes than to genetic makeup. Both fertility and lamb survival respond quickly to improvements in nutrition and husbandry practices; adjusting nutrition and breeding management practices can quickly improve the number of pregnant ewes in a flock, as well as the number of lambs that survive the first few days after birth. (An exception would be a flock with a high degree of inbreeding, which might see a substantial increase in heterosis and corresponding reproductive traits by bringing in less related stock.) Productivity traits that respond well to selection include prolificacy (number of lambs born per ewe lambing, or litter size), fertility of ewe lambs (age at first breeding), degree of seasonal breeding, pounds of lamb produced per ewe, and various measures of lamb growth (pre-weaning weight, post-weaning weight, growth rate). Keeping the ewes that perform well for these traits will improve a flock’s production. How can a shepherd determine which ewes are poor performers, in a way that is even-handed and might convince him or her to part with a favorite, for the good of the flock? We can use our breeding records – as purebred breeders, we have a record of the number of lambs registered per ewe. A more detailed analysis of our ewes is possible if we have more information, such as the date the ram was added, lamb birth dates, number of lambs born and raised per ewe, lamb birth weights, and lamb weights before and after weaning. These are the types of information collected for the NSIP (National Sheep Improvement Program). First we’ll look at some productivity traits that can be compared using flock records alone, without any lamb weights. These are ewe prolificacy, age at first breeding and rebreeding interval. Estimating Prolificacy (litter size at birth) Say that I have 3 ewes with different lamb production histories. All three ewes have given birth to 5 lambs over 3-4 years. However, a young ewe that has not yet reached full body size is investing more (and thus has greater potential prolificacy) if she produces the same number of lambs as a mature ewe. Similarly, a prime-age ewe is expected to raise more lambs than a young or old ewe. A correction factor can allow more accurate measurement of each ewe’s genetic potential. To make the correction, multiply the number of lambs produced by the age factor from Table 1, to effectively adjust all the ewes to the same age (4 years). Then add all the adjusted numbers for that ewe together, and divide by the number of times she has lambed. Adjusted prolificacy (number of lambs born per lambing per ewe) using Table 1 according to the directions in the Sheep Production Handbook: Table 1. Multiplicative adjustment factors for adjusting number of lambs born (litter size or prolificacy) for age of ewe
According to these calculations, Ewe B is the superior producer, and Ewe C is lagging in her lamb production. But Ewes A and B have failed to lamb during one breeding cycle, while ewe C has had lambs every year (or every time she was exposed to a ram). While the SPH formula is technically correct, in that prolificacy can only be measured when a ewe has lambs, I don’t like their formula because it does not penalize ewes A and B for failing to lamb in one year (or season). If their formula is changed so that the adjusted number of lambs is divided by the number of potential lambing events (instead of the number of actual lambings), we get my preferred estimate of a ewe’s “effective” prolificacy: Adjusted “effective” prolificacy (number of lambs born per breeding season per ewe): When calculated this way, Ewes B and C both have a similar effective prolificacy, while Ewe A has produced somewhat fewer lambs per breeding season. I feel that this is a more accurate view of the relative production value each of these ewes. In general, the SPH recommends a target average prolificacy of 2 (2 lambs born per ewe per lambing) for a commercial flock. If St Croix sheep are to be used as a maternal breed – that is, to improve lamb production and vigor in commercial operations – the breed would likely benefit from selection for a higher prolificacy (> 2 lambs born per ewe). That being said, it is important to evaluate the nutritional status of your flock before trying to increase the number of lambs born per ewe – raising your flock’s prolificacy is appropriate only if there is sufficient feed for ewes with multiple lambs. Because not all flocks have sufficient resources for a ewe to successfully raise triplet lambs, not every flock owner should try to increase prolificacy. Age at First breeding The age when a ewe first breeds is strongly correlated to her lifetime lamb production. That is, ewes that lamb at a young age tend to produce more lambs over their lifetime than do ewes that begin producing lambs when older. The St Croix ewe will breed early and often: St Croix ewes have given birth for the first time when as young as 10 months of age. However, lambs born to such a young ewe are less likely to survive; if they do survive they place great demands on the mother’s metabolism. While there is no ideal age for a ewe to have her first lamb – it depends in large part on the resources available to support the young ewe and her lamb(s) – most shepherds keep ewe lambs away from rams until they are 7-8 months of age or more. Ewe lambs that are exposed to a ram at 7-8 months will lamb at 12-13 months. If the flock has good nutrition, such ewes can successfully raise lambs while continuing to grow themselves. Alternatively, ewe lambs can be kept from the ram until 12 months of age, when a substantial proportion of their growth is complete. Few shepherds delay breeding ewe lambs past 12 months of age; ewes that do not lamb before 24 months of age tend to have problems with conception for the rest of their lives. In our flock management system, the ewe lambs are first exposed to a ram at around 7 months of age; winter-born ewe lambs tend to breed within 1-2 months but fall-born ewe lambs are more variable. We use birth dates to keep track of the age at which a ewe produces her first lamb, and do not penalize ewe lambs as long as they breed for the first time prior to 12 months of age. We sort our pregnant ewe lambs into their own pasture where they get more supplemental feed than the mature ewes up to the time that they lamb. We also supply creep-feed for the lambs to reduce their demands upon the ewe. This high-input system makes sense in our situation, because we have to supply supplemental feed for most of the year regardless of the ewe’s breeding status; it may not be appropriate for flocks in a low-input system where there is a more reliable source of pasture but ewe lambs grow to mature size more slowly. Selection for year-round breeding Another valuable trait of the St Croix breed that can be preserved and improved via selection is the ability to breed at any time of year. Assuming that you allow your ewes to breed in more than one season, selection can be based on the interval (in days) between lambs, and / or the interval between when the ram was added and when the ewe was bred. Candidates for culling are ewes that have longer than average intervals between lambs, suggesting that they are more seasonal in their breeding or need more time for recovery. Longer rebreeding intervals are especially noteworthy if the ewe is also producing relatively few lambs at each breeding (low to average prolificacy). We use lamb birth dates to estimate the date when a ewe was bred (average gestation length in sheep is 147 days). Any ewe that did not breed within the first two cycles (34 days) after the addition of the ram gets a ‘black mark’ – meaning that she has a strike against her that may result in her being culled from the flock, unless she is excelling in other areas. Productivity traits such as a ewe’s prolificacy, age at first reproduction, and ability to lamb year-round can be calculated using flock records and used to determine which ewes are the better producers. By eliminating the poor producers in your flock, you can increase lamb production and help maintain the strengths of the St Croix sheep breed. Flock productivity can be improved more rapidly by collecting additional information on each ewe’s productivity, such as lamb weights, as I’ll outline in part 2. Flock Improvement using Selection. Part 2: Calculations involving weights
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Last modified 27 May 2008
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