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This study examined the effects of live mass on ovarian follicular dynamics in heifers before and after the onset of nutritional anoestrus. The effects of decreased live mass on secretion of progesterone, LH and FSH were also investigated. Ovarian characteristics were monitored in 16 postpubertal Brahman heifers using daily transrectal ultrasonography. After two normal oestrous cycles, 11 heifers (treatment group) were allocated to a restricted diet, until the cessation of their oestrous cycles, that resulted in a decrease in live mass of 0.8 kg day−1. Heifers were then fed an ad libitumdiet, resulting in increasing live mass until resumption of oestrous cycles. Five heifers (control group) were fed an unrestricted diet throughout the experimental period. The concentrations of progesterone in plasma were monitored twice a week and the concentrations of LH and FSH were determined on day 6 after ovulation, before initiation of dietary restriction, after a 17% decrease in live mass and after the onset of anoestrus in the treatment group. Onset of anoestrus was preceded by linear decreases in the size of ovarian follicles and corpora lutea, and in the persistence of the first dominant follicles of oestrous cycles. These changes were proportional to the decrease in live mass (P< 0.001). The frequency of pulses of LH or mean concentration of FSH on day 6 of the oestrous cycle were not changed after a 17% decrease in live mass, and did not increase following the onset of anoestrus, although concentrations of progesterone were < 0.2 ng ml−1, suggesting that release of LH was suppressed at this time. Increasing live mass following anoestrus resulted in a linear increase in dominant follicle size and persistence, which were correlated with changing mass (P< 0.001); ovulation resumed when dominant follicles were 105 ± 4% of the size of initial ovulatory follicles.

Summary.Scrotal insulation for 48 h raised subcutaneous scrotal temperature by 4°C in hybrid Bos indicusx Bos taurusbulls. The incidence of decapitated spermatozoa in the ejaculate increased significantly between 6 and 14 days and that of protoplasmic droplets and tail abnormalities between 20 and 23 days after insulation, respectively. Simultaneously, the percentages of spermatozoa with lost and damaged acrosomes increased significantly 12–17 days after insulation. At slaughter 23 days after scrotal insulation sperm production rates and gonadal reserves had not been affected by insulation, but epididymal reserves were markedly reduced, particularly in the cauda. Elevated testicular temperature therefore had an effect on immature spermatozoa in the caput epididymidis and on spermatids, but it is suggested that selective sperm resorption in the rete testis and excurrent ducts may prevent some of these changes being expressed in the ejaculate.

Summary.The hybrids were carrying 50% (N = 4) or 75% (N = 4) zebu (Brahman) blood. The spermatogenic cells were labelled in vivowith [3H]thymidine. The relative frequencies of the seminiferous epithelial cycle stages, determined from examination of 6350 tubule cross-sections, did not differ between the 2 hybrid types examined, between regions of the testis or between testes within animals. Frequencies were generally similar to those for Bos taurusbulls except that the frequency of stage 4 tubules was lower and that of stage 8 tubules higher. The mean duration of the seminiferous epithelial cycle was 13·41 days and was similar for both types of hybrid. The durations of meiosis and spermiogenesis were calculated to be 20·7 and 17·4 days respectively, estimates which are similar to those for Bos taurusbulls. From the relative frequency of cell stages and estimates of the duration of the cycle, a time divisor of 5·11 days is suggested as being appropriate to determine daily sperm production from counts of spermatid reserves in testicular homogenates of these hybrid bulls.