Veterinary Physiology-III

LIGHT

• The period of light during the day is called photoperiod; it is the time between sunrise to sunset. Photoperiod varies with latitude and season and it is related to the path of sun. Length of photoperiod near the equator varies only by few minutes; by ±2 hours at latitude 30° and by ± 19 hours at latitude 60°.
• The eyes in mammals and eyes and skull in poultry perceive photoperiod. The information is relayed to the hypothalamus and pineal via superior cervical ganglion; the pineal encodes this information by producing more melatonin in dark than in light (circadian rhythm).

Effects of light on production

• Providing supplementary light of 4-8 hours with natural day light to cows increases milk yield by 7-16%; in ewes, birth weight of lambs is increased; in pigs, the litter weight is increased.
• In heifers and lambs, growth rate increases by extending the photoperiod; in growing pigs, feed intake or weight are not affected. In poultry, providing 14L, 10D enhances growth rate up to 12 weeks of age; after 12 weeks, very little effect is produced on growth. Low light intensity or blue light, which is perceived as less bright by birds than red or white is preferable to stimulate growth up to 18 weeks.
• In ewes, timing of puberty is determined by photoperiod and nutrition. With adequate nutrition, GnRH and photoperiod regulates puberty. During short day with longer duration of melatonin GnRH pulse frequency increases with subsequent LH release and ovulation. Long day length with short melatonin duration causes refractoriness and ovaries become inactive.
• Sheep is the most affected animal by changes in photoperiod. Seasons of high temperature and long photoperiod cause anestrous and decreased fertility and sperm production in ram. These effects are related to GnRH and LH production.
• In cattle, photoperiod does not affect reproductive activity. Growth and shedding of hair coat in cattle is influenced by length of photoperiod.
• In long day breeders like horse and birds require more light hours for growth, reproduction and production.
• In short breeders like sheep and goat require less light hours for growth, reproduction and production.