Introduction: Goat milk production is very important in Iran. Artificially extending day length affected milk production in several domesticated species. In the dairy cow industry, the use of photoperiod management is usual because it is a safe, non-expensive, and effective method to increase milk production. Dairy cows with late gestation exposed to short days produce more milk than cows exposed to long days. But, long-day photoperiod during early lactation increases daily milk yield. In contrast to cows, small ruminants such as goats have seasonal reproductive cycles and therefore their response to photoperiod may differ. To date, no study was conducted to evaluate the magnitude of the response of native goats of Iran to photoperiod. Therefore, the objective of this study was to compare the magnitude of the effect of artificially shortening and extending day length on milk yield in black native goats of Chaharmahal and Bakhtiari province in Iran.
Materials and methods: Six weeks before the start of giving birth in the flock, 80 does whose parturition dates were close to each other were selected from a local black goat flock and each one was randomly assigned to one of the following four treatments (n=20 for each treatment). Treatment 1: Does exposed to natural day length during pregnancy and lactation (control); Treatment 2: Does exposed to natural day during the pregnancy period and exposed to long day length at the beginning of the lactation period; Treatment 3: Does exposed to short day length at the end of the pregnancy period and exposed to natural day length during the lactation period; Treatment 4: Does exposed to short day length at the end of the pregnancy period and exposed to long days at the beginning of the lactation period. Milk production and levels of prolactin, melatonin, and IGF-1 hormones were measured. The does were milked manually once a week at 07:00 starting one week after parturition. Before milking the does, their kids were separated for 12 hours. Blood samples (five mL) were collected at the start of the experiment, 20 days before parturition, parturition, 15 and 30 days after parturition from each doe via jugular venipuncture into a sterile evacuated tube and immediately placed on ice. Plasma was harvested by centrifugation at 2000 rpm for 20 min and stored at -20 °C until use.
Results and discussion: In treatments 2, 3, and 4, the amount of milk obtained in each milking time increased until the third week of the lactation period, and then its increasing process slowed down. In the control treatment, the amount of milk obtained in each milking time increased until the third week, but the amount of milk obtained in the fourth week was less than that in the third week, although this difference was not statistically significant. The average milk obtained in four milking times for treatments 1, 2, 3, and 4 was equal to 868.5, 959.25, 1005, and 1085 mL, respectively. Over the monitored lactation period, the averages of milk yield in the second, third, and fourth treatments were 10%, 15%, and 24% more than that in treatment 1 (control). According to the results of this research, it was reported that milk production in goats that were placed in short-day length conditions in the last third of their pregnancy period was 26% higher than in goats that were in long-day length conditions at the same time. In another study, a 20% increase in milk production was reported in Saanen goats that were exposed to a long light period after giving birth. The amount of prolactin hormone in treatments 1 and 2 in 20 days before delivery was higher than in treatments 3 and 4 (P<0.05), but no significant difference was observed between treatments 1 and 2 as well as between treatments 3 and 4. At the time of parturition, the amount of prolactin hormone in all treatments was similar. At 15 and 30 days after delivery, the amount of prolactin hormone was higher in treatments 2 and 4 than in treatments 1 and 3 (P<0.01). Before giving birth, the amount of melatonin in female goats of treatments 3 and 4 was higher than in treatments 1 and 2 (P<0.01). Fifteen days after birth, the amount of melatonin hormone in female goats of treatment 2 was lower than that of female goats of treatment 1 (control treatment) (P<0.05), and the difference between other treatments was not significant. The amount of melatonin in different treatments was similar 30 days after birth. In the 20 days before parturition, the difference between the female goats of treatments 3 and 4 with those treated with natural day length (treatments 1 and 2) was significant (P<0.01) in terms of IGF-1 hormone levels. At 30 days after birth, the difference between female goats of treatment 4 and other treatments was significant in terms of the amount of IGF-1 hormone (P<0.05). According to the results of the present research, it has been reported that the long lighting period increases the concentration of prolactin and IGF-1 hormones. Decreased secretion of melatonin hormone with the increasing length of photoperiod has been reported in cattle and other animals. Increasing the duration of the light period reduces the time when melatonin secretion is at its peak.
Conclusions: In general, the application of a short light period before parturition and a long light period after parturition increased the amount of milk produced by female goats.