Selection for Growth Rate of Pigs and Productivity of Sows

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Abstract
The effectiveness of different methods of selecting for improved growth rate of pigs and productivity of sows is compared. Data used are from the Iowa and Nebraska Station swine herds maintained in cooperation with the Regional Swine Breeding Laboratory. From 8 to 10 times as many boars, and about 3 times as many gilts, as are needed for breeding should be retained long enough after weaning to obtain a measure of growth rate that is more reliable than weaning weight. The rest may be culled at weaning without reducing the effectiveness of selection appreciably. Several plans for culling sows, differing in the number of litters produced before and after culling and in the number of farrowing seasons each year, are compared. Yearly progress from selection is greatest when sows are culled after the first litter, the best ⅓ to ½ being kept for a second litter 6 months after the first. A plan almost as good is that of delaying culling until after the second litter and keeping the best ⅕ to ¼ of the sows for a third litter at 2 years of age. Progress is retarded by retaining more than the optimum proportion of older sows, because the less intense culling of sows and the longer interval between generations is only partly offset by the more severe culling of gilts and the greater accuracy of sow culling. Having sows farrow two litters per year is definitely advantageous. It permits the accuracy of selecting boars and gilts to be improved by basing the dam's productivity on two litters instead of one. It also means that the more productive sows may be kept for additional litters with a minimum increase in average interval between generations.

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