The influence of nutritional levels and shade structure on testicular growth and hourly variations of plasma LH and testosterone levels in young Creole bulls in a tropical environment.

Twelve Creole bulls 13.3 months of age were divided into three groups according to live weight. The first group received a low nutritional level and was housed in a shaded stable; the second group received a high nutritional level and was housed in the same stable as the first group; the third group received the same ration as the second group but was exposed to solar radiation. Live weight and testicular diameter were recorded fortnightly. After 65 days, the blood of the bulls was sampled every 20 min for 12 h. Plasma LH and testosterone were quantified by radioimmunoassay. There was no significant influence of solar radiation on the testicular growth or the hormonal levels. Underfeeding decreased testicular growth and the average plasma LH and testosterone levels. However, this action was effective only in decreasing the number of hormonal pulses per day (0.63 vs 2.55 LH pulses per day and 0.75 vs 2.50 testosterone pulses per day); it did not change the basal hormone levels. It is suggested that this decrease in LH pulsatility might be responsible for the slow testicular growth in the underfed bulls.