Evaluation of three-ram cohort serving capacity tests as a substitute for individual serving capacity tests

¹ U.S.D.A.–A.R.S., U.S. Sheep Experiment Station, Dubois, ID 83423

^* To whom correspondence should be addressed. E-mail: John.Stellflug{at}ars.usda.gov.

	Abstract

Alternatives to time consuming, laborious individual serving capacity tests (ISCT) are needed to classify ram sexual behavior. The objective for Study 1 was to evaluate the relationship between the first, 3-ram cohort test (COSCT) scores and the average of 5 ISCT scores. Objective for Study 2 was to determine whether 1 or 2 additional COSCT improved the ability to predict ISCT scores. For Study 1, rams (n = 69) were assigned to either a COSCT given before or after 9 ISCT. For Study 2, rams (n = 127) were given 3 COSCT before or after 6 ISCT. For repeated COSCT, rams were initially grouped at random and subsequently rerandomized so each ram was grouped with at least 1 different ram for each test. For both studies, number of ejaculations from COSCT was compared with average number of ejaculations across the second through sixth ISCT. A threshold between high and low performing rams was defined in each analysis as mean ISCT scores of sexually active rams. Rams with average number of ejaculations in ISCT greater than threshold were classified with high sexual activity, whereas rams below threshold were classified with low sexual activity. Rams with no ejaculations in ISCT were classified as sexually inactive. Data from Studies 1 and 2 were used to evaluate the relationship between first COSCT and average of 5 ISCT scores. Data from multiple COSCT were fit to various models to determine whether ability to predict ISCT scores was improved with 1 or 2 additional COSCT. The best model for ISCT and COSCT was a piecewise linear regression model. The first COSCT correctly identified all sexually inactive rams in both studies. The first COSCT, however, also classified 56% of low sexually active rams and 18% of high sexually active rams as inactive. Rams had a 71% probability of high sexual activity in ISCT if they were classified as sexually active in the first COSCT. We conclude that a single COSCT is a reliable, albeit more conservative, and efficient alternative to a series of ISCT for characterizing sexual activity of rams. Multiple COSCT can provide some protection against culling rams with high sexual activity (i.e., approximately 50% less with 2 additional COSCT) and still retain most of the efficiency compared with ISCT. It is important to use high performance rams for breeding because they will approximately double the number of ewes bred and lambs sired compared with low performance rams if a high number of ewes need to be serviced daily.

Key Words: rams, sexual behavior, serving capacity tests