Effects of environmental and artificial light and the relationship of the pineal gland upon the reproductive biology of the Brahman cow (Bos indicus)

Abstract

A series of three experiments were undertaken characterizing the Brahman cow as a seasonal breeder. The addition of light to create a daylength of 14 hours appeared to increase the reproductive capabilities of Brahman cattle during the fall artificial insemination breeding season. Although the proportions of animals that were assessed as having normal reproductive cycles (via rectal palpation of ovarian structures) were similar between the control and light treated groups, fewer (P <.05) animals in the light treatment had silent heats than in the control treatment. Further, conception and pregnancy rates had a tendency to be higher in the light treated groups than the control group. In a second trial, the effect of melatonin and decreasing daylength upon exhibition of estrus, estrous cycle length and corpus luteum function was examined in Brahman cows. Twice daily administration of melatonin had no effect on exhibition of estrus, estrous cycle length or corpus luteum function. However, luteal phase serum total progestogens decreased (P <.05) from September to December, presumably, as a result of decreasing length of photoperiod. In a third study, corpora lutea from Brahman and Hereford x Holstein heifers were removed within 20 days of both the summer and winter solstices. Brahman cattle generally had corpora lutea that were smaller (P <.001), contained lower (P <.10) progesterone per gram of luteal tissue during the summer and contained lower (P <.01) progesterone per corpus luteum than the Hereford x Holstein heifers. The decreasing daylength from summer to winter caused an increase (P <.01) in total corpus luteum progesterone content in both breed types. As a result of this difference, there was a tendency for the corpus luteum weight and progesterone concentration (μg progesterone/g of luteal tissue) to increase from summer to winter in both breed types. Overall, these data indicate that progesterone release may be inhibited during short photoperiod days. The mechanism of action is not known. These data further add to the concept of the seasonality of reproductive function (relative within the genus Bos) exhibited by Brahman cattle.