Seasonal variation in testes size in Polish Merino rams and its relationship to reproductive performance in spring

Observations relating testes size to body weight in growing and adult Polish Merino rams were performed during 3 consecutive years in a flock of 1700 pedigree ewes. October-born ram lambs were measured for testis diameter (TD), scrotal circumference (SC), and body weight (BW) at 100 (January), 150 (March), 180 (April), 240 (June), 365 (October), and 480 d (February) of age. In adult rams (n = 63 to 73 per year) BW and SC were measured in March, May (prior to breeding), August and October. Semen quality was evaluated before each breeding season, and the results of controlled reproductive performance (2 natural services to designated rams) were analyzed. Indices were applied to relate testis size to body weight: TD BW , SC BW and TW BW . Heritability estimates of testes indices in ram lambs at 100 to 240 d and at 480 d of age ranged from 0.21 to 0.36 (SE = 0.067 to 0.090). In adult rams SC measurements within months in consecutive years were repeatable (r = 0.55 to 0.68; P< or =0.01). Single measurements of adult ram gonads were not related to other reproductive traits, but significant relations were found when dynamic changes in testes size were examined as the percentage of testes weight change from March to May. Consistent correlations (positive or negative r = 0.2 to 0.5; P < or = 0.01) between seasonal changes in adult ram testes, semen quality, reproductive performance and in testes size in growing ram lambs were observed. The data suggest that the best time for selecting Polish Merino rams for a seasonality in reproduction is in March at 150 d of age.