Agriculture Reference
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creep feed as of day 22 of lactation. Alternatively, this period marks the declining phase
after peak lactation in sows. The declining phase of lactation in ruminants is associated
with a loss of mammary epithelial cells (Stefanon et al. , 2002).
4.6.2
During lactation
Unsuckled mammary glands in early lactation regress at a similar rate and follow a
similar pattern of regression than mammary glands that regress post-weaning (Kim et
al., 2001). Mammary gland tissue weight decreased by 2/3 within the first 7 to 10 days of
lactation when the gland was not suckled whereas the rate of regression was much slower
thereafter (Kim et al. , 2001). In accordance, mammary glands that are not suckled during
lactation do not show further loss of parenchymal tissue after weaning (Ford et al. , 2003).
When comparing litter sizes of 6 to 12 piglets, there were no differences in composition
or size of unsuckled mammary glands on day 21 of lactation indicating an absence of
litter size effect on regression (Kim et al. , 2001). On the other hand, the rate of regression
of unsuckled mammary glands is affected by dietary nutrient level during lactation. Kim
et al. (2001) reported that wet weight of unsuckled glands was 91% greater on day 5 of
lactation in sows fed a high energy (17.5 vs. 12 Mcal ME/day) high-protein (65 vs. 32
g of lysine/day) diet compared with sows fed a low energy-low protein diet. The rate of
regression of unsuckled glands therefore appears to be slowest under dietary conditions
which promote mammary growth. Nevertheless, the impact of the extent and rapidity of
regression of unsuckled mammary glands during lactation on their future development
and milk yield is not known.
The degree to which mammary gland involution is reversible is particularly important
in swine because of the common practice of cross-fostering piglets. Results from Kim et
al. (2001) showing extensive loss of wet weight and DNA in unsuckled glands suggest
that the loss of lactation function would not be reversible after the initial several days
of lactation. In 2005, Theil et al. specifically addressed that question by blinding teats
for either 24 or 72 h post-farrowing in order to prevent suckling by piglets. Mammary
development throughout lactation of glands blinded for 24 h was similar to that of
regularly-suckled glands, whereas that of teats blinded for 72 h was lesser as of day 6 of
lactation. Regression of unsuckled mammary glands during early lactation was found
to be reversible (gland rescue) during the first 24 h but to be irreversible after 3 days
(Theil et al. , 2005). Milk production from the rescued glands that were unsuckled during
24 h remained lower throughout lactation. This was further demonstrated by the fact
that piglets cross-fostered later than 24 h later the foster sow farrowed weighed 900 g
less at weaning than control piglets (Thorup, 1998). Suckling intensity is also important
for mammary regression. Theil et al. (2006) compared no suckling, transient suckling
(until 12 to 14 h postpartum) or regular suckling of mammary glands and observed
that regularly-suckled glands maintained lactation whereas transiently-suckled and non-
suckled glands regressed during lactation.
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