Reproductive characteristics of Ethiopian highland sheep II. Genetic parameters of semen characteristics and their relationships with testicular measurements in ram lambs

A study was conducted to investigate the effect of season on, and to estima te heritabilities of, and genetic correlations among, semen and spermatozoa characteristics and the relationship of these characteristics with measure s of testicular or scrotal size in 6-, 9- and 12-month old ram lambs of Men z and Horro sheep breeds indigenous to Ethiopian highlands. A total of 278 ram lambs with substantial pedigree information were involved in the study. There was improvement in all semen and spermatozoa traits with age, the me ans at 12 months being consistently superior to values at 6 and 9 months of age. There were no significant breed differences in any of the traits stud ied, except semen volume at 9 months (0.67, S.E. 0.07 for Horro versus 0.39 , S.E. 0.05 ml for Menz) and proportion of dead spermatozoa at 12 months (0 .18, S.E. 0.03 versus 0.23, S.E. 0.02, respectively). Season was significan t (p<0.05) for most of the traits studied, and differences were attributed primarily to nutrition. Semen collected in the wet season had higher sperma tozoa concentration while samples collected in the dry season had higher pr oportion of abnormalities. After correcting for differences in total sperma tozoa abnormalities, the wet season (of collection) had the highest spermat ozoa output (volume x concentration) in 12-month old ram lambs. Heritabilit y estimates varied substantially in magnitude, ranging from zero to over 0. 4. Traits with non-trivial heritability estimates were mass motility at 9 m onths (0.32, S.E. 0.11), individual motility at 9 months (0.32, S.E. 0.12) and at 12 months (0.16, S.E. 0.12) and proportion of abnormal spermatozoa a t 9 months (0.35, S.E. 0.13). Genetic correlations among semen and spermato zoa characteristics, as well as correlations with testicular measurements, were medium to high and generally favourable, but were associated with larg e standard errors. The genetic correlations of scrotal circumference with s emen volume (0.55, S.E. 0.11), mass motility (0.62, S.E. 0.20), individual motility (0.54, S.E. 0.12), concentration (0.25, S.E. 0.04) and proportion of abnormal spermatozoa (-0.75, S.E. 0.24) in 12-month old rams indicated t hat selection based on this trait, which is highly heritable and easy to me asure and can be measured early in lift, should have appreciable favourable correlated response in semen quality and spermatozoa production. (C) 2000 Elsevier Science B.V. All rights reserved.