ESTIMATION OF GENETIC PARAMETERS ANALYSIS OF SHEEP REPRODUCTIVE TRAITS USING FIELD DATA AND MODELING
According to Dickerson (1970), he stated that the cost of animal products in the market depends primarily on the efficiency of three basic functions: (1) reproduction (2) growth of the young (3) female production. The high mortality observed in sheep and lambs according to the National Agricultural Statistics Service (NASS), 2017 cost the US government about 63% loss in targeted profit. In 2010, sheep and lamb losses from animal predators and non-predator causes in the United States totaled 634,500. Losses of sheep totaled 234,500 head or 37.0 percent of total losses. Lambs losses were 400,000 head or 63.0 percent of the total. (NASS, 2010). Most of female productive traits are normally and continuously distributed. increase in the number or total weight of lambs weaned per ewe can be achieved by increasing the number and the weight of lambs produced per ewe within a year (Duguma et al., 2002). Genetic analyses of these traits are well understood. During the history of sheep breeding, these productive traits have been the focus and tremendous genetic improvement has been made in these traits. Number of offspring and growth of lamb, indicated by body weights and rate of gain at different phases of growth are among the most economically important and easily-measured traits. (Petrovic et al., 2012).
Knowledge of the particular trait and phase of the animal’s growth upon which to base selection is therefore of utmost importance. In recent years, more studies have addressed how to efficiently utilize maternal genetic effects in genetic improvement of these traits. In sheep production, reproductive traits such as fertility, prolificacy, fecundity and lamb survival have been recognized as major factors influencing profitability and they are a major discourse in the animal science field. Studies involving genetic relationships and reproductive traits recently became a matter of scientific concern as they were seldomly studied in the past; this is due to these traits having low-medium heritabilities and they don’t exhibit a noticeable response to phenotypic selection. Among these reproductive traits, litter size is the most important, and thus most studied as an increase in the numbers of lambs weaned per ewe per year offers the greatest single opportunity for increasing the efficiency of any kinds of sheep production systems. Litter weight weaned per breeding ewe is a convenient biological measure of overall ewe reproductive ability (Martin and Smith, 1980; Ercanbrack and Knight, 1998; Snowder, 2002, 2007). In Norwegian Sheep Breeding Scheme, litter size has been considered as one of the major traits with high economic value (about 12% of the National Sheep Breeding goal) (NGS, 2013), In the U.S., the National Sheep Improvement Program (NSIP) was established in 1986 to provide within-flock genetic evaluations for U. S. sheep producers. The program utilized single-trait prediction methods with variances derived from literature values. A system for across-?ock genetic evaluation of Targhee sheep was implemented in1995 (Notter,1998) and currently involves approximately 15 ?ocks and 1,600 breeding ewes. Expected progeny differences are estimated for 7 traits by the US National Sheep Improvement Program (NSIP; Bradford, 2003).
Generally there are two methods of estimating and evaluating sheep reproductive traits: (1) Linear method. (2) Non-linear method. Linear method consider only the direct genetic variance as an important factor, but others (maternal, environmental) as unimportant ones. Analyses of discrete traits assuming the threshold model in sheep breeding are scare but not scarce in large ruminant. Pattersson and Danell (1985) analyzed litter size and lamb survival in four Swedish sheep breeds, and Bodin and Elsen (1989) studied variability of litter size in different French sheep breeds using a threshold model. Other studies in sheep where the discrete nature of the data was taken into account are those of Gilmour (1983), who analyzed data on foot-shape in lambs, Thompson et al., (1985) who estimated heritability of fleece rot incidence and score in Australian Merino, and of Loaiza-Echeverr et al., (2013) who used nonlinear model to describe scrotal circumference growth in bulls. Because iterative procedures are required to find solutions to nonlinear systems of estimating equations, however, computations are more difficult than with linear model techniques. This aspect has prompted the following question: How advantageous are nonlinear over linear procedures when compared under the same circumstances? Most studies carried out opposed the use of non-linear model ((Meijing and Gianola, 1985; Hofer, 1990; King, 1991; Matos, 1993; Perez-Enciso, 1995). Although, they were cases where both gave similar results.
For linear methods, for the sole purpose of genetic evaluation, Henderson’s BLUP method is recommended while for the purpose of estimating genetic reproductive traits in sheep, recently developed restricted maximum likelihood (REML) has been the method of choice in animal breeding.
The merit of REML has in estimating genetic reproductive traits over BLUP is that REML estimators maximize only the portion of the likelihood that does not depend on the fixed effect and can eliminate the bias of Maximum Likelihood estimators and all information available is utilized in an optimal way while the Henderson’s best linear unbiased predictors (BLUP) won’t function properly because it considers fixed factors which the REML doesn’t.
2.0 GENERAL BREED CHARACTERISTICS
The Rambouillet is a finewool breed that is well adapted to rangeland conditions and typically does well in arid and semi-arid environments. They have traditionally been touted for their longevity, hardiness, and mothering ability. The Rambouillet sheep were first imported to the United States from France, with the name ‘French Merino’ in the mid-1800s. The name, French Merino eventually changed over to Rambouillet around the late 1800s. This name was derived from the town and area where the sheep were raised in France, from a flock produced from some of the most elite Merino sheep that originated from Spain. Today the breed is the base of most range flocks in the western United States. The Rambouillet is a white-faced, mid-to-large size breed whose mature ewes generally weigh between 64-86 kg and produce high quality grease fleeces weighing 4-6 kg, and measuring 19-24 microns in fiber diameter (Bradford, 2003). The Rambouillet has historically offered two economically important products, wool and lamb. However, in the last 15 years, wool has been on a downward trend in terms of both demand and price. With no obvious reversal of this trend in sight, selection within the Rambouillet breed has increasingly leaned towards lamb production.
2.0.2 TARGHEE BREED
The Targhee is one of America’s youngest breeds of sheep, having been developed in the 1920’s. Like Corriedales and Columbias, Targhees are a cross-bred species, trying to maximize high quality wool with increased musculature. To meet this demand the U.S. Sheep Experiment Station, Dubois, Idaho, began selective breeding in the fall of 1926. They took a group of cross-bred ewes of Rambouillet, Lincoln and Corriedale blood lines and bred them to their best Rambouillet rams. Through many years, variations within and between generations declined. The new breed was named “Targhee” after the U.S. National Forest where the animals grazed during the summer. The forest was named for a chief of the Bannock Indians who had lived in the area in the 1860’s. The Targhee is a dual-purpose sheep with good meat type and a heavy fleece of high quality wool. They are especially popular in Montana, Wyoming and South Dakota, where their ¾ fine wool and ¼ long wool breeding is favored by western ranchers. The Targhee sheep has an average body weight of 200-300 lbs for the rams and 125-220 lbs for the ewes, has a fleece quality of 25-21 microns, 64-58 spinning cotton weight, has a fleece weight of 10-14 lbs for the ewe, has a staple length of 3-5 inches, and a lambing percentage per ewe of 150-200%.
2.0.3 POLYPAY BREED
The Polypay breed came as a result of developing a breed that can combine into a composite breed with the potential for greatly increased reproductive capacity along with desirable growth rate and carcass quality. (Hulet et., 1984). Development of the Polypay breed (the name was coined to suggest more than two pay- ing crops per year, i.e., one wool and two lamb crops) was begun with matings made at the U.S. Sheep Experiment Station in 1968 (Hulet et al., 1984). Fertility in Polypay ewes is high as 85%, 9% lower than in Dorset ewes and 12% lower than in the Polypay × Dorset crossbred ewes (Fahmy and lavallee, 1990).
2.1 EWE PRODUCTIVITY, HETEROSIS AND BREED COMPLEMENTARITY
Ewe productivity is defined as total weight of lambs weaned per ewe exposed and it is dependent upon the component traits of fertility, litter size, lamb survival and growth (Snowder and Fogarty, 2009), this is a major source of concern for longetivity of the sheep industry. There is much greater potential for increasing both biological and economic efficiency of lamb production through genetic improvement in reproductive rate than through improvement in growth rate or body composition (Dickerson, 1978). Total production costs accounted for by replacement and maintenance of breeding females are proportionally much higher for sheep and beef cattle than for the other meat-producing species, mainly because of their relatively low reproductive rate. Improving reproductive performance is likely to increase both the biological and economic efficiency of animal production enterprises (Dickerson, 1970).
Heterosis, or hybrid vigor, refers to the increased vitality or “doing ability” of the crossbreds as compared to the average of the parental breeds. The only way to get heterosis is through crossbreeding. Nitter (1978) defined heterosis as the average performance of crossbred sheep relative to the average performance of purebred breeds that produced the cross. When breeds are crossed, new combinations of gene forms are created in the crossbred sheep. Traits that are lowly heritable (reproductive traits, traits influencing resistance to stress, survivability and longevity) are the ones that respond the most to crossbreeding (that is, have the greatest amounts of heterosis). These are traits that are difficult or slow to improve through selection. In general, as heritability increases, the amount of heterosis decreases. Fogarty (2006) observed that the levels of heterosis are low for growth traits (3-10%), while lamb survival (10%) and reproduction traits (10-40%). The effect of heterosis may seem minor when only one trait is considered.
However, when total productivity (for example, lamb survivability and growth rate) is considered, heterotic effects accumulate to provide a rather substantial improvement over straightbred sheep. Also, mating F1 ewes to a ram of a third breed to produce crossbred offspring can maximize heterosis.
Complementarity refers to the combining of desirable traits from two or more breeds into one animal.
Nitter (1978) defined complementarity as the improved production efficiency that results from crossbreeding systems that let strengths of the sire breed offset weaknesses of the dam breed and strengths of the dam breed counter weaknesses of the sire breed. For example, the Hampshire is known as a heavy mature-weight breed that produces fast-growing lambs with desirable carcass characteristics. However, Hampshires are not especially noted for prolificacy or mothering ability. Because of this lack, the Hampshire is referred to as a specialized “sire” or “ram” breed. The Polypay is known primarily as a specialized “dam” or “ewe” breed because of its prolificacy, milk production and overall mothering ability. They are not as well known for rapid growth rate, muscling or carcass quality. So, if a Hampshire ram is mated to Polypay ewes, the offspring benefit from the maternal environment provided by the ewe breed and can be expected to grow faster and have more desirable carcasses as a result of the ram breed’s contribution. Because the Polypay is a more prolific breed than the Hampshire, the producer can also expect to wean more lambs than would be weaned from straightbred Hampshire sheep. The efficiency of mating Hampshire rams to Polypay ewes would be much greater than the reciprocal mating of Polypay rams to Hampshire ewes. Although the resulting offspring are genetically the same (half Hampshire and half Polypay), fewer lambs would be expected and production costs would likely be increased due to higher feed requirements of the heavier Hampshire ewes as compared with Polypay ewes. Complementarity greatly improves the efficiency of meat production by mating ewes of specialised dam breeds to rams of specialised sire breeds. Breed diversity allows producers to benefit from complementarity. To maximize the benefits of breed complementarity, breeds must be chosen wisely.