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A Fertility Index for Rams

U. S. Department of Agriculture¹

Abstract

Semen observations on 39 Rambouillet, Targhee and Columbia rams representing a wide range of semen quality were used to derive indexes of fertility. At least two consecutive, naturally ejaculated semen samples were obtained from each ram prior to breeding. Each ram was pen mated to five ewes to obtain fertility data. All possible simple correlations were calculated among 13 semen traits, inbreeding coefficient, type of birth and rearing, age of ram's dam, face-cover score and fertility. Using multiple regression methods and including ultimately only those traits making worthwhile contributions to the multiple correlation, the following fertility indexes were developed: Index A=174—17.19(pH)+0.29 (% live normal sperm) –.08 (% abnormal –.20 (% abnormal necks). Index B=217-4.38 (motility score) –18.89(pH) –.19 (% abnormal sperm) –.14(%abnormal necks). The standard partial regressions for traits used in Index A were pH=–.255, percent live normal=0.293, percent abnormal=–.082, and percent abnormal necks=–.210. In Index B the regressions were motility score =–.210, pH=–.240, percent abnormal =–.202, and percent abnormal necks=–.151. The multiple correlations and standard errors of estimate for the two indexes were 0.74 and 0.73, and 21.3% and 21.3%, respectively. The correlations between indexes A and B and the actual fertility of all semen-tested rams (136 head) used in 1959 were 0.76 and 0.73, respectively. These are higher than the correlation of –.49 between the semen score previously used and fertility.

¹ From the U. S. Sheep Experiment Station and Western Sheep Breeding Laboratory, Sheep and Fur Animal Research Branch, Animal Husbandry Research Division, A.R.S., Dubois, Idaho in cooperation with the University of Idaho. Published with the approval of the Director of the Idaho Agricultural Experiment Station, Moscow, as Research Paper No. 543.