A lot of strides have been made in the improvement of pigs in the past few decades. The advent of the use of BLUP in the early 90's in particular gave a boost to the slow progress seen in those traits with low heritability, particularly reproductive traits. BLUP gave breeders increased accuracy in the estimation of genetic values, and therefore higher reliability of selection decisions. Hence, as will be observed in many breeding programs litter size, a trait that was previously very slow to improve, started going up relatively quickly. Some breeding programs reported annual rates of improvement of as high as 0.3 pigs per sow per year in total piglets born. But with this improvement comes an important question; are there any drawbacks?
Indeed, if the breeding program is not well balanced there can be downsides to this improvement and these have been very well documented. One can cite such problems as reduced birth weight, an increase in the number of unviable piglets, an increase in number of still births, increased piglet mortality, an increase in number of runty pigs in nursery/grower, reduced carcass quality of runty pigs, shorter sow productive life, overall sow weakness, etc. These problems arise when the program fails to take into account the relationships between the different traits that are critical to having a balanced approach. When a breeding program is too heavily weighted towards prolificacy, some of the problems cited occur because litter size is negatively correlated to birth weight and consequently piglet viability. Some of the research done by people like Dr. George Foxcroft has shown that as litter size goes up average birth weight of the piglets' regresses.
Another trait that easily gets ignored as programs over-focus on prolificacy is the physical aspects of the selection candidates. Ignoring this aspect will eventually result in a line that lacks physical strength and hence sows that have very poor longevity. A prolific sow that lasts only one parity cannot recoup the investment of its purchase price.
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