Shorten the productive cycle is one of the main mechanisms to increase productivity in any livestock system and in sheep herds this is not different. Since the gestation period is constant, the way to alter the duration of the cycle is to work on the interval between parturition and the following conception. For that, the return to oestrus after lambing is a key point to be improved. The postpartum period is a delicate time in the production cycle and it has been thoroughly studied, not only in sheep, but in all major livestock species in order to allow the recovery of the females in a quicker and more satisfying way. In this context, suckling management is a very interesting tool to promote a faster return to cycling, besides also having the potential to be beneficial to lamb development.
Suckling management consists on altering the pattern of lamb suckling. The more traditional suckling pattern, which will be referred from now on as “normal suckling”, consists on letting the lamb with the ewe 24 hours a day until weaning. In this type of management, Munro (1956) reported, based on 16 hours a day of observation, that twin lambs feed, in average, 22 times per day during the first 6 weeks of life, while single lambs suckles 14 times per day, considering 6 daylight hours, in the same period. The amount of times a lamb suckles diminish significantly after the first to third weeks of lactation, probably due to the improvement on their ability to suckle, but also because the ewe becomes less receptive to the lamb with time (Hinch, 1989).
It has also been shown that the stimulus provoked by sucking is detrimental to the return of normal reproductive activity (Scharir, 1990; Ascari, 2012). In order to ovulate again after giving birth, the levels of FSH (Follicle-Stimulating Hormone) and LH (Luteinizing Hormone) have to return to their normal patterns through the recovery of the hypothalamic-pituitary axis, which controls the production and release of the main reproductive hormones. This recovery happens in two phases: the first one occurs between the second and fifth week after parturition and it is marked by irregular releases of GnRH (Gonadotropin-Releasing Hormone), sufficient to stimulate LH production, but not its release in great quantities (Nett, 1987 in Ascari, 2012). So, the early period after parturition in sheep can be characterized by a gradual recuperation of ovarian activity; high concentration of prolactin (due to suckle stimuli), that starts to decrease after the first week; and low concentration of LH, that increases slowly (Restall & Starr, 1977). It is in this period, as well, that the uterus goes through involution, in order to be prepared for the next gestation (Hafez, 2004). The second phase of recuperation of the hypothalamic-pituitary axis occurs when the stocks of LH return to their normal concentration and the LH pulses released into the circulation have sufficient amplitude to stimulate ovarian follicular development and estradiol secretion. Estradiol promotes the expression of its own receptors in hypothalamus and anterior pituitary, which increases their sensibility to its positive feedback and enhances GnRH and LH pulses, promoting the terminal follicular development and ovulation. The first phase is relatively independent of suckle stimulus, but the second phase, on the other hand, seems to be highly related to that (Nett, 1987 in Ascari, 2012).
That can be explained by the detrimental effect suckling has on GnRh output (Schirar, 1990), due to the influence of prolactin, mainly.
Considering different mechanism to fight the detrimental effects of lactation to the reproductive cycle, perhaps the more common one and that is currently widely implemented, is to anticipate the age of weaning. Schirar (1989) reduced the lactation period in ewes by having the lambs weaned 24 hours after birth, which accelerated significantly the return to oestrus when compared to ewes that were kept feeding their lambs continuously. In the first case, oestrus was observed, in average, 22 days after they gave birth, while in the second scenario, oestrus occurred around 35 days after parturition. This results differ from those obtained by Ascari (2012), though. The author weaned the lambs at 10 days of age, but that did not result in any significant difference on return to oestrus in the ewes, besides being detrimental to lamb development. For that last part, the results found by Schirar (1989) have to be considered as an interesting information to relate suckling with regular reproductive activity, but not to be implemented as a valid tool in productive systems. Less drastic shortenings, however, can be used to try and explore the beneficial aspect of weaning on the return to cycle and that has been seen in commercial herds with frequency.
Suckling management comes around, then, as an alternative, but mostly as a complimentary action to precocious weaning, helping to accelerate the production cycle. Suckling managements consist on separating the lambs from the ewes in some periods throughout the day, diminishing, then, the suckling stimuli and, therefore, reducing the postpartum anoestrus.
A study developed by Assis (2009) tested two variations of suckling management and its effect on return to cycle and lamb development. The first variation on suckling consisted of separating lambs and ewes throughout the day, but reuniting them during the night (forming Group 1). The second variation, in turn, restricted suckling to two times a day, half hour in the morning and another half hour in late afternoon (Group 2). In both cases, the ewes were kept in the pasture during the day, without any contact (including sound, sight and smell) with the lambs. Also, in both groups, suckling management started to being applied when lambs were 15 days old. Before that, normal suckling happened. A control group was also formed, in which normal suckling was implemented during the whole period of lactation, that lasted in average 60 days for all groups. In the control group, ewes remained confined with their lambs during the hole experiment, therefore, without contact with pasture.
Other factors were considered in the experiment, as breed, number of lambs born (one or two per ewe) and if it was the ewes first, second or third parturition. However, data showed that these factors did not influence the interval between parturition and the first oestrus.
The different suckling managements, on the other hand, proved to have a participation on the return to cycle in the ewes analysed. The ones from the control group presented their first oestrus at 49,1 days after giving birth, in average, and while the ewes of Group 1 did not show a significant difference from this value, the ewes from Group 2, which had the most restricted time of suckling, diverged significantly from the control group, with an average return to cycle at 36,7 days, which supports the hypothesis of suckling interfering with the hypothalamic-pituitary axis and presenting suckling management as an effective mechanism to overcome this.
Other parameters to be considered to associate suckling management and the return to cycling, aside from the prolactin influence on GnRH release is the weight variation and body condition score of the ewes in the postpartum period. In Assis (2009) study there was no evidence of suckling management influencing in both of these parameters, but another study, from Leite (2010), differs from that. In a similar experiment, he found differences in ewes that fed their lambs only twice a day when compared to those that stayed with their offspring during the night and that can be accounted as another benefit to help a more precocious oestrus after parturition.
Assis (2009) and Leite (2010) also analysed in their experiments the influence of suckling management on lamb performance, which can represent still another plus in deciding to use suckling management as an alternative to normal suckling in commercial herds where the main product is the meat.
As the lamb grows, its nutritional requirements increase and milk (sole base of its feeding in the first days of life) becomes insufficient to meet them. For that, is imperative to adapt lambs to consume solid feeds as soon as possible, and restricting suckling is a mechanism that can be used to speed up this process. The results of Leite (2010) corroborate this idea, as it showed that lambs subject to suckling management had a better weight gain and final weight than lambs of normal suckling regimes. Assis (2009), on the other, did not find any significant differences between the lambs of the three groups considered in her experiment, but that can still be seen as a good result, since the lambs of control group did not show any advantage from those who had their contact with their mother restricted, as it would be expected.
In conclusion, suckling management presents itself as a cheap and simple tool that can be applied in virtually any commercial farm and that delivers significant improvements to the system. The effects on shortening the lactation anoestrus are the more evident and the main reason to try to implement this type of management in the herd, but the probable gains, both to the lambs’ performance and to the ewes’ body condition score recovery after parturition, must also be taken into account to influence the decision to adhere to it.