Photoperiodic responses in sheep with special reference to wool growth

Abstract

Progress has been substantial in the field of endocrinology in recent years. Knowledge has been accumulated to the point that it is now regarded as being fairly comprehensive and very advanced in certain sections. The relationship between the endocrine glands and the pituitary body has been established to the extent that it is almost axiomatic to look upon the pituitary as the master gland of the body. However, knowledge of the factors infuencing the pituitary is as yet by no means complete, and only comparatively recently has it been possible to demonstrate some of the photoperiodic responses of the pituitary to the light-dark environment in which an animal is placed. Hence the contribution which an understanding of photoperiodicity can make towards a knowledge of the response of the animal body to its environment is of considerable scientific importance. For the purpose of this treatise it is important that the term photoperiodicity be defined as clearly as possible. In brief, therefore, photoperiodicity in animals may be defined as "The response of animals to the number and length of daily exposures to light dark sequences." 54 Corriedale ewes in New Zealand on a constant ration throughout the year were subjected to various light treatments and their monthly growth of wool compared with controls having normal daylight hours. The experiments extended over a period of 5 years and in addition to wool growth, observations were made on ewe lactation, lamb production and growth, and the thyroid glands.