Modeling growth of pigs reared to heavy weights

A series of studies were conducted to determine the relationship between live weight and growth performance measures of pigs reared to heavy weights in commercial facilities and to evaluate different methods of modeling these relationships. The first study was designed to develop growth curves of live animal growth performance and ultrasound measures for barrows and gilts from weaning to a pen mean live weight of 167.5 +/- 3.30 kg. The results of this study suggested that instantaneous ADG peaked at 78 and 77 kg live weight for barrows and gilts, respectively, and decreased thereafter, instantaneous ADFI peaked at 115 and 121 kg for barrows and gilts, respectively, and decreased thereafter, and instantaneous G:F decreased quadratically as live weight increased for both genders. In addition, backfat depth increased linearly and Longissimus muscle area increased quadratically as live weight increased. A second study was carried out with 7 different harvest live weights for individual pigs within pens ranging from 113 to 181 kg to determine the effects of increasing harvest weight on overall growth performance and carcass characteristics. The results of this study suggested that pigs could be reared to heavier harvest weights with relatively limited impact on overall growth performance or carcass leanness. A third study was carried out to evaluate the growth of individual pigs within a pen. Barrows and gilts were housed in single-gender pens of approximately 153 pigs from weaning to week 10 post-weaning and pens with approximately 73 pigs from week 10 post-weaning to a pen mean live weight of 135.2 +/- 0.76 kg. Pigs were weighed individually at birth and every 2 weeks from weaning to the end of test. The results of this study suggested that both gender and birth weight impacted the growth curves of individual pigs, that the within-pen standard deviation in live weight increased quadratically with increases in live weight, and that interim weights were relatively poor predictors of subsequent growth of individual pigs within a pen. The final study consisted of a series of analyses which were carried out to determine the most appropriate method of describing the relationships between days on test and live weight and between live weight and various growth performance measures, including ADG, ADFI, G:F, and within-pen variation in live weight. The results of the final study suggest that the relationship between days on test and live weight can be described accurately by a number of nonlinear equations. However, simple polynomial or logarithmic equations between live weight and periodic measures of growth performance provide just as, if not more, accurate estimates than more complicated nonlinear equations. Mixed models were developed with random effects for individual pigs to predict the live weight of individual pigs within a pen across a range of live weights; however, the predictions of within-pen variation were generally inaccurate and lower than the actual measures and, therefore, alternative methods of analysis should be evaluated. The results of these studies can be used in economic modeling to determine the optimum harvest weight in the US swine industry.;Key words: pigs; growth; heavy weights.