Agriculture Reference
In-Depth Information
2 piglets on to 2-week lactating sows resulted in a decrease in milk yield compared with
controls. Accounting for the close relationship between milk yield and mammary cell
number (Boutinaud
et al.
, 2004), those observations indicate that the greater piglet size
on the day 2 lactating sows stimulated a greater mammary development during the initial
days of lactation to meet the demands of the piglets, while fostering smaller piglets onto
an established mammary gland may result in regression of the tissue until it balances with
the demands of the smaller piglets. In both cases, the effects on milk yield disappear by
about 2 weeks, suggesting that the gland has reached a point of balance with the demands
of the piglet. Another study of this effect used sows overexpressing a mammary-specific
transgene (bovine alpha-lactalbumin; Marshall
et al.
, 2006). Fostering day 7 piglets on to
day 2 lactating sows resulted in daily milk yield of transgenic sows increasing rapidly to
a peak at day 9 and remaining higher than that of controls through day 15 of lactation.
Fostering day 7 piglets onto day 2 non-transgenic sows had a more limited effect on milk
yield.
Another example of how size of the piglet impacts development of the gland it suckles
may be seen when comparing the mass of mammary glands at farrowing in first-parity
gilts with the mass of glands at day 5 of lactation. An examination of data representing the
mean DNA content of mammary tissue by gland location at day 5 of lactation (Kim
et al.
,
1999a) and the mean DNA content by gland location at day 0 (Ji
et al.
, 2006; Kim
et al.
,
2000) suggests that the gland develops over that 5-day period at the start of lactation by
responding directly to the level of suckling demand of the piglet. In the case where piglet
birth weight was held constant at the start of lactation and therefore the suckling intensity
was similar across gland locations, the variation in mass among glands was reduced at
day 5 compared with farrowing. Glands that are largest at farrowing (typically the middle
glands; Ji
et al.
, 2006) may have excess tissue mass relative to the ability of the piglets to
remove the milk and may undergo regression during the initial 5 days of lactation. In
contrast, glands that are the smallest at farrowing (typically the posterior glands; Ji
et al.
,
2006) grow the most rapidly during the initial 5 days of lactation, although still remaining
smaller than the anterior glands.
4.5
Nutritional impact on mammary development
Nutrition of swine in the growing, gestating or lactating periods can affect mammary
development. A 34% feed restriction of growing gilts from 28 days (weaning) to 90
days of age had no significant impact on mammogenesis, whereas a 20% (Farmer
et al.
,
2004) or 26% (Sorensen
et al.
, 2006) feed restriction from 90 days of age until puberty
reduced mammary parenchymal mass by 26.3 and 34.2%, respectively. The effect of feed
restriction on mammogenesis is only seen as of 90 days of age, being the first period of
rapid mammary development. High feeding levels from 90 days of age until puberty are
therefore recommended to ensure optimal mammary development of growing gilts. On
the other hand, reducing dietary crude protein from 18.7 to 14.4% in that same period
does not affect mammogenesis (Farmer
et al.
, 2004) suggesting that total feed intake
is more important than protein intake per se for mammary development of growing
gilts. The impact of feeding flaxseed on mammary development of gilts was investigated
