Sheep Industry News August 2021

Improving the Genetics of American Sheep Flocks

RON LEWIS, University of Nebraska-Lincoln & LUIZ BRITO, Purdue University G husbandry or feeding – genetic improvement garners huge ben efit by being permanent, cumulative, and in most cases, highly cost effective. We want to justify those claims. We will begin with some of the in's and out's of selection programs, including the factors af fecting progress and some limits to overcome. Those constraints can be addressed through a systematic genetic evaluation system, which combines information collected across flocks to estimate genetic merit more reliably. That is our second topic. Lastly, we venture into the opportunities that new molecular (genomic) technologies offer sheep breeding. Importantly, they complement rather than replace our traditional tools by allowing us to acceler ate genetic gains for a broader more inclusive set of traits. The outcome is even more sustainable and profitable flocks. THE IN'S AND OUT'S OF SELECTION Genetic selection programs are built on some guiding prin ciples. Foremost is defining their direction, which is based on pinpointing those attributes or traits that we consider most im portant economically to improve in our flocks. Next, we measure our animals’ performance for those traits. Lastly, we combine that information to identify and then select those individuals with the highest genetic merit for the traits we want to improve. The outcome is bettering our flock in our desired direction. The rate of genetic gain we can achieve – our selection re sponse – is affected by four factors. The first is selection accuracy. The more we know about an animal and its relatives, the more accurate is its evaluation. The reliability of our decision making improves, thereby increasing selection response. The second is selection intensity. By being more choosey when selecting the rams and ewes we breed, their genetic merit becomes higher. This also leads to faster selection response. Third is the genetic variation intrinsic to each trait we wish to improve. At least within a breed, this tends to be static for extended periods. Its context might be easiest to understand in terms of the heritability. The heritability is the ratio or proportion of genetic to total variation present for a trait. It therefore ranges from zero to one. Values near zero indicate the environment defines nearly all differences in the way animals perform. Values enetic selection is one strategy for changing the perfor mance of farm animals. Although it is relatively slow compared to some other methods – such as improved

near one indicate that animals’ genetics essentially define their performance. The higher the heritability, the greater the selection response. The fourth and final factor is generation interval, which is the average age of parents when their progeny are born. When the generation interval is shorter, selection response accelerates. In short, we want to increase selection accuracy and intensity while reducing generation interval. In practice, this can be challenging since there are antagonisms among these factors. An increase in selection accuracy and in tensity typically coincides with an increase in generation interval, and vice versa. This necessitates forethought in our design of breeding programs and is where our new molecular (genomic) technologies can help. We will return to genomics a bit later. Beyond addressing the antagonism affecting selection re sponse, we must consider two further constraints in the layout of our breeding program. One is biological and the other is struc tural. Compared to other farm species – with poultry and swine as good examples – sheep are older in age at sexual maturity and produce litters of only a couple offspring. That slows turning over generations and reduces the number of candidates available for selection. Furthermore, structur ally the size of many seedstock operations is small. Introducing breeding stock from outside flocks becomes an attractive option. Yet that too can be challenging. Differences in management and geography (environment) among flocks can mask differences that are genetic. This is where a systematic genetic evaluation pro gram fits in. breeds have been provided through the National Sheep Improve ment Program. Participation in NSIP is dominated by four breed-types: hair, maternal, fine-wool and terminal sire. The primary role of NSIP is to provide Estimated Breeding Values for a range of production traits. An EBV estimates an animals' genetic merit for a particular trait and can be used to predict how an animal's progeny will perform. When EBVs are estimated more accurately – particularly in younger animals – selection decisions are more reliable and can be made earlier. Selection response thereby increases. Many eco nomically important traits are evaluated through NSIP, includ ing: live weights from birth through adult age, reproductive rate (number of lambs born and reared), ultrasonic measures of fat QUANTIFYING FLOCK GENETICS Since 1987, genetic evaluation services in American sheep

14 • Sheep Industry News • sheepusa.org