Effects of melatonin exposure duration and estrous cycle phase at the initiation of estrus synchronization on reproductive performance in Awassi ewes during the breeding season

Sheep, like many animals, are sensitive to the length of the day. Their bodies use the changing seasons as a calendar, turning their reproductive systems on and off to ensure that lambs are born when the weather is mild and food is plentiful. This seasonal rhythm is controlled by a tiny gland in the brain that releases a hormone called melatonin when it gets dark. In the wild, this system works perfectly, but for farmers who want to raise sheep year-round or ensure a steady supply of lambs, this natural timing can be a hurdle. To manage this, scientists and farmers often use a technique called estrus synchronization. This involves placing a small sponge inside the sheep that releases hormones to pause the reproductive cycle, and then removing it to restart the cycle so that many ewes come into heat at the same time. The goal is to make breeding predictable and efficient.

For years, researchers have wondered if adding extra melatonin to this process could make it even better. Since melatonin is the body's signal for night and rest, giving it to sheep might trick their bodies into thinking it is the perfect time of year for breeding, potentially leading to more pregnancies or stronger lambs. However, the results of past experiments have been mixed. Some studies suggested it helped, while others found no benefit at all. A key question remained: does it matter how long the sheep are exposed to this extra melatonin before the breeding cycle starts? Does a long period of exposure work better than a short one? To answer this, a team of researchers in Turkey decided to test different timing strategies on a specific breed of sheep known as Awassi, which is common in the Middle East and known for its hardiness.

The researchers gathered 120 healthy adult female sheep and divided them into four equal groups. They wanted to see if the length of time the sheep received melatonin implants before the main breeding treatment made a difference. One group received a melatonin implant twenty days before the breeding treatment began. A second group got the implant ten days before. A third group received the implant on the very same day the breeding treatment started. The fourth group, serving as a control, received no melatonin at all. All four groups then received the standard breeding treatment: a hormone-releasing sponge was placed inside them for twelve days. When the sponges were removed, the sheep were given a shot of hormones to trigger ovulation, and they were placed with fertile rams to mate. The researchers also paid close attention to where each sheep was in her natural cycle when the sponge was inserted, checking if being in a specific phase of her cycle changed the outcome.

Over the following weeks, the team tracked every detail of the sheep's reproductive lives. They watched to see when the sheep started showing signs of being ready to mate, recorded how many successfully mated, and measured hormone levels in their blood to check for pregnancy. Later, they used ultrasound machines to confirm which sheep were pregnant and counted how many lambs were born. They also weighed the newborn lambs to see if the treatment affected their size. The results were clear and consistent across all groups. The length of time the sheep spent with the melatonin implant did not change the outcome. Whether the sheep had the implant for twenty days, ten days, or just one day, they performed exactly the same as the sheep that received no melatonin at all.

The timing of the sheep's natural cycle when the sponge was inserted did cause a small difference in when they showed signs of being ready to mate, but this did not translate into any real advantage. The sheep that started the treatment at different points in their cycle still ended up with the same number of pregnancies and the same number of lambs. The researchers found that the melatonin treatment did not increase the rate at which sheep became pregnant, nor did it reduce the number of embryos that were lost early in development. It did not make the sheep have more twins or triplets, and it did not make the lambs heavier at birth. In fact, the only thing that seemed to matter for the final results was the natural health and condition of the sheep, not the extra hormone treatment.

This study suggests that for these sheep, during the time of year when they are naturally ready to breed, adding extra melatonin does not provide a boost. The idea that a longer exposure to melatonin would prime the body for better results was not supported by the data. The researchers concluded that the reproductive system of the Awassi ewe during the breeding season is already working at its peak, and adding more of the hormone that signals "night" does not improve the process. While melatonin is a powerful regulator of the body's internal clock, its ability to act as a performance enhancer for breeding seems to depend heavily on the specific conditions, such as the time of year and the breed of the animal. For farmers working with these sheep during their natural breeding season, the study indicates that the standard synchronization method is sufficient, and the extra step of administering melatonin for varying lengths of time offers no additional benefit.