Effect of pinealectomy and plane of nutrition on wool growth in Merino sheep.

The effects of surgical pinealectomy and plane of nutrition on wool growth and plasma prolactin concentrations in young Merino wethers were investigated. In young pinealectomized wethers maintained at a low live weight under conditions of minimal stress, the decline of conditioned clean wool production on midside patches was slowed when compared to pineal-intact controls. This difference appeared to be due in part to the observed greater secondary wool follicle density in the pinealectomized wethers; mean fiber diameter was affected to a smaller extent, while staple length growth rate was not significantly altered. Circulating prolactin profiles showed a seasonal variation (high in summer, low in winter) in both pinealectomized and control wethers. There was no difference in wool production between pinealectomized and control wethers when the diet of the same wethers was subsequently supplemented with formaldehyde-treated cottonseed meal. While the role of melatonin in the regulation of wool growth remains to be determined, it is suggested that the hormone may have a transient effect on wool production in young wethers under conditions of limited nutrition, but not at higher nutritional levels, and that melatonin may be involved in partitioning of nutrients to the wool follicle.