Agriculture Reference
In-Depth Information
some piglets are able to suckle two teats (Illmann
et al.
, 2007) but it is unknown whether
this increases milk production.
13.4.2
Milk production and nursing frequency
The amount of suckling allowed by a sow can directly affect instantaneous milk production.
Two mechanisms have been proposed as to how piglets can stimulate a greater milk
output. The first is teat massaging during the post-ejection phase of nursing. Empirical
evidence indicates that the duration of the post-ejection massage in the last few nursings
has a short-term positive effect on milk output during following nursings (Jensen
et al.
,
1998; Špinka and Algers, 1995). Yet, performing longer post-ejection massages over days
or weeks of lactation does not increase milk output (Špinka and Algers, 1995; Torrey and
Widowski, 2007). The second, and stronger effect, is through more frequent initiations of
nursings by the piglets, whereby the frequency of milk ejection is increased and the more
frequent emptying of mammary glands enhances milk production. From a functional
point of view, the feedback loop from hungrier piglets through more frequent nursing
initiation to enhanced milk output resembles the provisioning-begging-provisioning
cycles between bird parents and their nestlings and can be understood as a case of honest
signaling of need. Non-nutritive nursings may serve as a cost that the sow imposes on
piglet nursing initiation if it becomes too frequent (Špinka
et al.
, 2011). Thus, the need of
the litter for milk is expressed in the frequency of nursing initiation and in the propensity
to engage in long teat massage. The sow may consent to it by allowing nutritive nursings
and permitting long post-ejection massaging phases; or the sow can withstand the
demand by failing to release milk and/or by curtailing the post-ejection massage. This
feedback loop may contribute to the stability of milk production at a level that is adjusted
to the needs of the litter while being affordable by the sow.
13.4.3
Milk allocation to individual glands/piglets
Through their behavior at the udder, piglets can individually influence the portion of
milk output produced by their mother that they will acquire. Stronger piglets are able to
displace weaker siblings from teats, leading to the danger of very unequal milk distribution.
However, neither very unequal milk allocation to piglets (Milligan
et al.
, 2002) nor
wastage of energy through intense sibling competition is beneficial for reproduction of
the sow. It seems that the nursing behavior and physiology of the sow were shaped by
natural selection to curtail competition among siblings. First, the pre-massage phase with
loud grunting gives all piglets the opportunity to assemble at the udder and find their
teat, and the increase in grunting frequency announces to all piglets the incoming milk
ejection. Second, the brief and synchronous window of milk availability in all teats does
not give the stronger piglets a chance to usurp milk production of more than one teat.
Thus, the brief milk ejection in combination with acoustic signaling by the sow ensures
a largely undisturbed milk intake for all littermates (Drake
et al.
, 2008; Fraser, 1980).
Three phases of piglet competition can be distinguished as lactation progresses. During
the teat sampling phase (approximately the first 12 h), piglets compete over the number
of teats suckled and the amount of colostrum ingested. The short and sharply delimited
