Interactions between Genotype and Plane of Nutrition in Selection for Rate of Gain in Swine

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Abstract
A long-term project was initiated in 1947 to study whether selection of breeding animals is as effective when they are fed on a low plane of nutrition as when they are full-fed. Swine were selected for the experiment, and a total of 1705 animals contributed to the data through nine generations. Analysis was carried out on the interaction between genotype and plane of nutrition in selecting for rate of gain. A cross bred foundation stock was divided equally and at random into two lines. One line was full-fed from weaning to 150 lb., limited-fed from 150 lb. to parturition, and again full-fed during lactation. The other line received 70% as much feed as the high-plane group throughout. All selections were made as each pig reached 150 lb. live weight, using the following index: I=B+2W+35G, where B is the number of pigs farrowed alive in the litter in which the individual pig was born, W is the number weaned in the litter, and G is the average rate of daily gain of the individual pig from weaning to 150 lb. After six generations, each line was divided into two sub-groups. These sub-groups were designated HH, high plane remaining on high plane; HL, high plane shifted to low plane; LL low plane remaining on low plane; LH, low plane shifted to high plane. The four sub-groups were carried through the ninth generation to evaluate the performance of each line on the other nutritional level. Average daily gain increased under selection in both lines; the average increase in the full-fed line was slightly greater than in the limited-fed line—0.043 lb ./generation versus 0.034 lb. Realized heritability estimates were not materially different between the two lines, being 52% and 49% for high and low planes, respectively. Intra-sire regression of offspring on dam yielded heritability estimates of 41% and 54% for high- and low-plane pigs, respectively. The average weighted selected differential for eight generations of selection under high plane was 0.08 lb./generation, and that for low plane was 0.07 lb. Thus, expected progress under high plane was 0.033 lb./generation, as compared with 0.937 lb. for low plane. With the exchange of environments, LH pigs showed the highest rate of average daily gain (1.41 lb.) in the 7th generation. They were followed in descending order by HH (1.26 lb.), LL (1.011b.) and HL (0.94). A genetic correlation between rate of gain on high plane and that on the low plane was 0.74 whereas the correlation calculated between rate of gain on low plane and that on the high plane was 0.67. The pooled estimate of rG, based on the responses on both high and low plane of nutrition, was 0.70. A definite genotype-environment interaction was found to exist; selection for increased rate of gain under the two nutritional environments was for two distinctly different genotypes. Superior gains in the low plane may be due to superior feed efficiency as a result of lower thyroid activity. Superior gains in the high plane may be due to genes conditioning appetite, feed consumption, metabolic rate and/or over secretion of growth hormone. The findings are discussed in relation to prevailing theories on the effectiveness of selection under different environments. In general, the results support the contention that breeding animals should be selected in the environment under which their progeny are expected to perform. Copyright © . .

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