The cow eye : function and effect of light on milk yield

Sammanfattning: The purpose of this review was to study the cow’s eye physiology, focusing on how light affects the cow’s milk yield. It has been shown in studies that more hours of light per day (16 hours of light a day, as opposed to about 9-12 hours of light a day), increases milk yield for dairy cows. This review contains suggestions on how this is connected with the physiology of the eye. Vision is an important sense for most mammals and is used to check out the animals’ surroundings. Sensory neurons in the retina of the globe help in turning photons from the light into signals to the brain. The brain can then translate the information and start a range of responses in the animals. For instance, ganglion cells in the eye´s retina send signals to the pineal gland in the brain, to stimulate or inhibit production of melatonin. Increased light hours decrease the secretion of melatonin, important for sleep patterns and regulation of seasonal reproduction. Artificial light seems to work as well as natural light, in this aspect. Studies have shown relations between longer photoperiods (16 hours of light per day) and elevated prolactin levels, which leads to increased milk production. However, in dry cows, short photoperiods (8 hours of light per day) show an increase in milk yield in the subsequent lactation. Studies also show connections between increased levels of the growth hormone bovine somatotropoin (BST) and longer photoperiods.