Reproductive characteristics of Ethiopian highland sheep I. Genetic parameters of testicular measurements in ram lambs and relationship with age at puberty in ewe lambs

A study was conducted to estimate heritabilities of, and genetic correlatio ns among, body weight (WT) and testicular measurements - scrotal circumfere nce (SC), testicular diameter (TD), testicular length (TL) and epididymal d iameter (ED) - in ram lambs between 6, 9 and 12 months old, and relationshi p of the testicular traits with age at puberty in ewe lambs (AP). Two fat-t ailed sheep breeds, the Horro and Menz, indigenous to the Ethiopian highlan ds were studied. Experimental lambs were produced by mating 250 oestrus-syn chronized ewes of each breed to 10 sires in a single-sire mating system ove r three mating periods which produced, for this study, a total of 361 ram l ambs and 148 ewe lambs, with substantial pedigree information, in two dry s easons (October/November 1992 and 1993) and one wet season (June/July 1993) . Mean AP was 322.7 days. Breed was not significant (P > 0.05) for all test icular measurements, except ED at 9 months for which the Horro had a higher (P < 0.01) value (20.6, s.e. 0.38 versus 19.3 s.e. 0.30). Horro ewe lambs had their first oestrus 28 days earlier (P < 0.05) than the Menz. Heritabil ity estimates were obtained from a sire model (M1) and from an individual a nimal model with additive direct and maternal effects (M2). The maternal ef fect was not fitted in R 12 for AP. Repeatability and heritability estimate s were also obtained by analysing records at the three ages in a repeated m easures model (M3). M1 and M2 heritability estimates were generally low to moderate but were associated with large standard errors. Heritability estim ates for testicular measurements were generally similar to those for WT. Am ong the testicular measurements, SC had the highest heritability estimates at 6 (0.45, s.e. 0.24 for M1 and at 12 months (0.41, s.e. 0.21 and 0.41, s. e. 0.20 for M1 and M2, respectively). Model 3 heritability estimates were 0 .29 (s.e. 0.14), 0.24 (s.e. 0.12), 0.09 (s.e. 0.07), 0.11 (s.e. 0.08) and 0 .12 (s.e. 0.08) for WT, SC, TD, TL and ED, respectively. Only repeatability estimates for WT (0.34, s.e. 0.02) and SC (0.25, s.e. 0.03) were significa nt. Heritability of AP was estimated at 0.16 (s.e. 0.21) by M1 and 0.14 (s. e. 0.19) by M2. Genetic correlations of AP with testicular measurements, es pecially with SC (-0.57, s.e. 0.38 and -0.78, s.e. 0.33, at 9 and 12 months , respectively) and TD (-0.72, s.e. 0.33 and -0.83, s.e. 0.48), were desira ble and generally high. Adjustment of testicular measurements for liveweigh t consistently resulted in a reduction in estimates of heritability and gen etic correlations. SC was suggested as the most appropriate selection candi date for the genetic improvement of both male and female reproductive perfo rmance in these breeds. (C) 2000 Elsevier Science B.V. All rights reserved.