Agriculture Reference
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accretion may be influencing the occurrence of puberty. There have been no recent
studies refuting earlier work (Beltranena et al. , 1993; Patterson et al. , 2002; Rozeboom et
al. , 1995) which has affirmed that the occurrence of puberty in gilts is not controlled by
a specific body composition or specific rates of fat or protein accretion. Opportunities
to improve the efficiency of introducing gilts into the breeding herd successfully still
exist. Gibson and Jackson (2012) reported that about 7.9% of gilts never cycle. Spörke
(2007) reported that 1 out of every 1.5 weaned gilts is successfully bred for her first litter.
Research therefore does not suggest that body condition will become more important in
the future for the initiation of estrus.
1.3
Gilt body condition and reproductive performance after
first service
The relationship between body condition and lifetime reproductive performance of sows
has been extensively studied (Kirkwood et al. , 1988; Newton and Mahan, 1993; Rozeboom
et al. , 1996; Young et al. , 1990). Conclusions have varied, yet body composition at first
service was described as poorly related to subsequent reproduction. Researchers making
recommendations for first breeding targets most often suggest a target for body weight,
which is a safe recommendation as it represents all body tissue stores. The adequacy of
body protein stores at first service has received greater attention than body fat in recent
discussions. Frequently, however, body protein stores are estimated using a prediction
equation based on body weight and backfat depth. In a few instances, longissimus thoracis
muscle diameter has been measured and used in estimating total body protein. Growth
rate has been studied because it might provide a better target as an indication of some
metabolic state that enhances sow performance.
1.3.1
Body weight
In 2001, Foxcroft and Aherne stated that gilt pool management must implement nutrition
programs that meet targets for age, weight and fatness at breeding (Foxcroft and Aherne,
2001). Breeding targets of 125 kg of bodyweight and 15 mm of backfat were proposed.
In 2002, Foxcroft changed this recommendation and stated that 135 kg was better than
125 kg as breeding weight. This change was based on the work of Clowes et al. (2003a,b)
which included primiparous sow lean mass loss in lactation. Working backwards from
the relationship between weaning body composition and post weaning ovarian function,
these researchers considered weight at farrowing and weight gained in gestation to derive
a suggested weight at first breeding of 135 kg minimum. Support for this first service
target came from the data of Williams et al. (2005) which showed a maximal number of
pigs born in three parities for gilts bred at a weight range of 135 to 150 kg.
The recommendation of Foxcroft (2002) and Williams et al. (2005) has become possibly
the most well accepted and adopted targets of the modern swine era and is supported by
other research (Amaral Filha et al. , 2009). Hoving et al. (2010) confirmed the importance
of gilt weight at first breeding, showing that reproductive performance in second parity
was inferior when gilts were first inseminated at lighter weights (group average of 124
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