Nursing behavior - The Gestating and Lactating Sow - page 294

types of housing systems from very intensive (Fraser, 1977; Illmann and Madlafousek,
1995; Illmann et al. , 1999; Puppe and Tuchscherer, 2000; Valros et al. , 2002; Whatson and
Bertram, 1980) to semi-natural (Castrén et al. , 1989a; Newberry and Wood-Gush, 1985),
and also in the wild boar (Horrell, 1997). Non-nutritive nursings start appearing within
the first 24 h postpartum, as soon as the cyclic pattern of milk release gets established
(Castrén et al. , 1989a; Illmann and Madlafousek, 1995), and occur over the whole course
of lactation, possibly with some peak of occurrence around week 2 (Valros et al. , 2002).

Non-nutritive nursings prolong the intervals between nursings with milk ejections
(Castrén et al. , 1989a; Fraser, 1977; Špinka et al. , 2011). Because the increase in milk
available in the mammary glands with prolonged intervals is only slight (Špinka et al. ,
1997), the net effect of non-nutritive nursings is an overall decline in milk consumption
by piglets. Therefore, it could be that the absence of milk ejection in some nursing
episodes is a strategy by the sow to down-regulate the milk output when piglets are
initiating nursings too frequently (Špinka et al. , 2011). This assumption is supported
by the fact that the probability of a nursing to be non-nutritive strongly increases if the
nursing occurs soon after the last milk ejection, and also if the nursing was initiated
by the piglets rather than by the sow (Castrén et al. , 1989a). This strategy seems to be
physiologically based, i.e. related to the central refractory period described above rather
than cognitively guided. Experimental evidence confirms that in the domestic sow,
motivation to nurse can be separated from the instantaneous (in)ability to release milk.
Indeed, Fraser (1975) showed that when a sow is separated from her litter, stimulated to
exhibit the milk ejection oxytocin reflex (without milk removal) by hand massage and
shortly thereafter reunited with her litter, the sow starts a nursing willingly. However, the
nursing is always without milk ejection, in spite of the fact that her litter is hungry and
her glands are full of milk.

While non-nutritive nursings do not provide any milk to piglets, they may still play a
role in the hormonal state of the sow as prolactin and somatotropin are increased after
non-nutritive nursings, although the concentrations are still lower than after a nutritive
nursing (Rushen et al. , 1993).

13.3.5 Nursing synchronization and allo-suckling

When lactating sows are housed in groups, their nursings become highly synchronized.
For instance, in groups of 3-4 sows, the proportion of nursings occurring within 1 min
of each other amounted to 81% (Maletínská and Špinka, 2001). The acoustic contact is
sufficient for synchronization to occur also in sows housed in individual crates (Šilerová
et al. , 2013; Špinka et al. , 2004;). The possibility has been investigated to use this sensitivity
and induce more frequent nursings (and thereby higher piglet growth) through playbacks
of the nursing vocalizations. Although some studies found positive effects (Cronin et al. ,
2001), the majority found none (Farmer et al. , 2004; Špinka et al. , 2004; Widowski et al. ,
1984), so the method has not been practically applied.

Why does such a strong tendency exist among sows to synchronize their nursings? One
possibility is that when lactating sows roamed in small groups, nursing synchronization