Agriculture Reference
In-Depth Information
development. Furthermore, reproductive processes may still have been affected when
oestrus was not delayed. For example, Knox and Rodriguez Zas (2001) noted that in
sows with short lactation (<16 days), the percentage of sows showing oestrus within 8
days after weaning was reduced (35 vs. 94-98% for sows with lactation lengths of more
than 17 days), and the percentage of these sows that ovulated was also reduced (78 vs.
98% for sows with a lactation length of 25-31 days). Sows that failed to ovulate (2%)
either showed a short/intermittent oestrus with only small or medium sized follicles or
ovarian cysts at that time, or had a long oestrus period with normal sized pre-ovulatory
follicles that did not ovulate within the first 5 days of oestrus. In ovulating sows, lactation
length did not affect follicle size at ovulation. Thus, even though oestrogen production
in sows on short lactations is sufficient to induce oestrous behaviour, the immaturity of
the positive feedback system may prevent/disable the occurrence of an LH-surge (Sesti
and Britt, 1993) and, consequently, these sows fail to ovulate. If the LH surge fails in sows
with normal sized pre-ovulatory follicles, these follicles may become cystic. There are no
recent reports outlining the influence of lactation length on the development of cystic
follicles or cystic ovaries. In an early report, Svajgr
et al.
(1974) found that the number of
cystic follicles doubled for sows with a lactation length of 13 days compared with 24 days
(1.3 vs. 0.6). Consistent with the earlier review by Varley (1982), Willis
et al.
(2003) did
not find any effects of lactation length on ovulation rate.
Lactation length may also affect subsequent farrowing rate and litter size, as reviewed
by Varley (1982). In the more recent literature, short lactations (less than 3 weeks) were
found to negatively affect subsequent litter size and farrowing rate (Koketsu
et al.
, 1997;
Le Cozler
et al.
, 1997). The limited information available on reproductive performance
of sows with long lactations indicates a positive influence of lactation lengths above 4
weeks on both farrowing rate (+3%) and litter size (+0.6 piglets) (Gaustad-Aas
et al.
,
2004), although such effects are not always found (Tummaruk
et al.
, 2001) and may not
result in a greater number of piglets per sow per year (Xue
et al.
, 1993). On the other
hand, in systems where extended lactation periods are demanded (like organic farming
systems), older parity sows may have very high litter sizes. Leenhouwers
et al.
(2011)
showed that sows in organic farming systems farrowed larger litters than conventional
sows and Wientjes
et al.
(2012) reported litter sizes of as many as 17.0 to18.8 piglets in
sows on organic farms with an average lactation length of 41 days. Such high litter sizes
may be related to the fact that these sows became anabolic during the 6-week lactation
period, which presumably restored follicle development.
In summary, short lactation lengths (<3 weeks) have a clear negative effect on post-
weaning follicular development, subsequent interval to oestrus, ovulation response, and
even farrowing rate and litter size. Effects of short lactation lengths on weaning-to-oestrus
interval seem less evident in the recent literature, which may be related to the ongoing
genetic selection for short weaning-to-oestrus intervals. Extended lactations (up to 5 or 6
weeks) may increase subsequent litter size, as it restores body condition of the sow during
the ongoing lactation. A possible disadvantage of longer lactations on reproduction is
the increased number of sows with lactational ovulation. This may especially occur in
multiparous sows from specific prolific breeds, but may occur in any sow with a low
number of suckling piglets or a high feed intake during lactation. Lactational ovulation is
