Agriculture Reference
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The degree of stimulation of prolactin secretion may also be an important consideration.
King et al. (1996) administered high levels of porcine prolactin to first-litter gilts from
day 102 of pregnancy through lactation. While concentrations of RNA and DNA in
mammary tissue biopsies were not affected by prolactin administration, milk yield was
reduced in sows given prolactin. Other evidence in that study suggested that the sows
may have undergone premature lactogenesis with the elevated peripartum prolactin
concentrations. In another study of administration of recombinant porcine prolactin
from days 2 to 23 of lactation in third-parity sows, no significant effects on either milk
yield or mammary composition were observed (Farmer et al. , 1999). This absence of effect
could be due to the fact that mammary receptors for prolactin were already saturated in
control animals, thereby preventing the exogenous prolactin from having any biological
action (Farmer et al. , 1999).
It would be of interest to find ways to increase circulating prolactin concentrations
using feed additives that are not pharmaceutical agents so that they could be used in
commercial swine operations. One possibility might be the plant extract silymarin (from
milk-thistle) which was found to have hyperprolactinemic properties in rats (Capasso
et al. , 2009) and hypergalactenemic actions in women (Di Pierro et al. , 2008) and cows
(Tedesco et al. , 2004). In a recent study it was demonstrated that silymarin can increase
prolactin concentrations in gestating sows, yet, the increase was not significant enough
to have beneficial effects in terms of mammary development (Farmer et al. , 2014). More
specifically, 4 g of silymarin was fed twice daily from 90 days until 110 days of gestation,
leading to a 51.8% increase in circulating prolactin concentrations 4 days after the onset
of treatment. However, this effect was no longer apparent 15 days later. The absence
of beneficial effects on mammary development may be due to the fact that prolactin
concentrations were not increased enough or for a long enough period of time. Indeed,
in the study by VanKlompenberg et al. (2013), where a positive effect of increased
prolactin concentrations was seen on secretory activity and epithelial cell differentiation
of mammary tissue, prolactin concentrations were increased almost four-fold within 24
h of treatment and remained greater for 6 days. In the project using silymarin (Farmer
et al. , 2014), no blood was obtained 24 h post-treatment so it is not known if prolactin
concentrations peaked earlier. In any event, it is possible that a larger dose of silymarin
could have had a greater effect. Yet, depending on the required duration of treatment, this
would most likely not be economically feasible for producers to use on a regular basis.
4.4
Role of milk removal
Milk removal is also critical for mammary development and function during lactation.
An accumulation of an autocrine feedback inhibitory factor(s) occurs in the mammary
alveoli as part of the normal process of cellular secretion of milk components, inhibiting
further secretion (Wilde et al. , 1995). If milk is not fully removed from a gland by the
piglet, then the gland will reduce further milk secretion and eventually initiate the process
of involution (discussed below). Suckling also stimulates secretion of prolactin and other
hormones (Algers et al. , 1991; Spinka et al. , 1999), and weaning results in a rapid decline
in plasma prolactin concentrations (Bevers et al. , 1978). The removal of the feedback
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