The main findings of the experiments were the effects of the melatonin treatment of dams at lambing on lamb weight and growth rate, in parallel with an increase in the fat content of the milk, especially at the end of lactation. Furthermore, the milk fat levels of treatment ewes persisted from the beginning to the end of lactation. To our knowledge, this is the first evidence of this effect in a small ruminant, particularly if exogenous melatonin is implanted at lambing. Previous studies on the effects of melatonin implants on milk production and/or offspring growth focused on implantation in the second half of pregnancy or in the middle of the milking period. It has been reported  that melatonin implants at day 35 of lactation did not have a significant effect on milk yield and composition in Manchega and Lacaune ewes, and a melatonin implant at mid-milking did not have a significant effect on milk yield throughout the milking period in Lacaune and Assaf dairy ewes . Cashmere goats that had received an implant at day 50 of lactation presented a higher fat content in the milk of than was in the milk of control goats; however, daily yields of milk, milk protein, and milk lactose were lower in the implanted goats than they were in the controls goats . Melatonin administration to does in the dry period, seven weeks before kidding, produced a galactopoietic response in the subsequent lactation and an improved daily weight gain of their suckling kids, especially, males .
In our experiments, lambs that were reared by implanted ewes had a higher growth rate than did the lambs from control ewes, which might have been due to a higher volume of milk from their implanted dams (not measured), and or the higher fat content of milk, especially at the end of lactation. A high milk fat concentration, milk energy content, and milk energy concentration contributes to faster lamb growth. The administration of exogenous melatonin coupled with the simulation of a short-day photoperiod in summer had significant effects on the milk levels of solids, protein, fat, and lactose, and on the fatty acid content of sheep milk . Furthermore, the higher content of total solids in the milk of M Rasa Aragonesa ewes might have contributed to the higher growth rates of the lambs because total solids are a combination of fat, protein, lactose, and minerals. In one study, a 30% reduction in overall milk solids was correlated with a 20% reduction in total milk energy production .
Although milk yield was not measured in our experiment, the significant increase in protein and lactose in the control group of ewes throughout lactation, which did not occur in the M group, might indicate lower milk production in the C group because of a dilution effect . This dilution factor was cited  as an explanation for the correlation between low levels of protein and lactose in milk and high milk production in sheep.
The three experiments did not indicate any direct effect of implanting melatonin in the lambs whether they were reared by their dams or artificially. Evidence of treatment with melatonin in suckling lambs is limited. Implanting lambs with a single melatonin sachet subcutaneously on the back at 3–4 weeks of age  did not affect growth rate, and a pinealectomy of prepubertal sheep did not affect growth rate . The administration of melatonin by intravenous infusion or through transdermal patches to newborn lambs that had been subject to induced asphyxia, reduced the pathologies caused by asphyxia . In newborn lambs, treatment with melatonin reduced the in vivo pulmonary pressor response to changes in oxygenation  or presented an antiproliferative effect against pathologies such as pulmonary arterial hypertension in neonates . Melatonin given to human newborns with sepsis reduced the number that died because of its highly effective antioxidant and free-radical scavenging properties . Although melatonin treatment has positive effects on the newborn’s health, no evidence of an effect on growth rates has been presented. Melatonin treatment of ewes in mid- or late pregnancy has positive effects on lambs. It has been reported  that maternal melatonin supplementation in the second half of pregnancy improved the survival of second-born twin lambs, so that melatonin implants have potential as a simple and cost-effective strategy to reduce neonatal losses of twin lambs on farm. Our group has demonstrated an improvement in colostrum quality if ewes are implanted at the fourth month of pregnancy , and higher survival and growth rates from birth to weaning of Merino lambs . Probably, the increased survival is mediated by the high colostrum quality, and by an increase in BAT and birth weight if maternal melatonin implants were inserted from day 100 of gestation .
Our experiments indicated no significant differences in the growth rates of male and female lambs, and even among artificially reared lambs, females had higher growth rates than did males. It has been observed that ewe lambs at the end of milk rearing had a higher average daily gain than did ram lambs, probably, because of differences in the feeding behaviour of males and females fed ad libitum ; specifically, females tend to consume less milk per meal than do males, but consume more meals per day than do males .
The higher growth rate of lambs reared by implanted dams might have been mediated by a higher melatonin concentration in their mother’s milk, which in turn might have affected the lamb’s health. In cows and goats, milk melatonin levels reflect blood concentrations of melatonin, with a short delay , and it was speculated  that melatonin that is supplied to the infant via breast milk plays a role in improving sleep and reducing colic in breast-fed human offspring. The lamb can readily produce an appropriately timed melatonin rhythm by 1–6 weeks of age  and, although maternal melatonin is predominant before birth, significant but low-amplitude increases in nighttime melatonin can occur within the first week . In any case, the absence of differences between c and m lambs from the M group of ewes in Experiment 2, and the results from Experiment 3, in which lambs did not receive milk from their mothers, did not receive extra melatonin from milk, lead us to conclude that neither exogenous or endogenous melatonin of the lambs, or melatonin from their mothers’ milk have a direct effect on growth rate.