Protein and fat utilization in lactating sows: II. Challenging behavior of a model of metabolism.

Abstract

The objective was to challenge the behavior of an existing mechanistic model of sow metabolism using data from a study that determined effects of fat and amino acid intakes on milk production, body and mammary composition in sows nursing 10 to 12 pigs. Sows were second through fourth parity; litter size was standardized at 12 pigs within 3 d of farrowing, and lactation was for 20 d. Diets were formulated and fed to provide three different amounts of protein intake and two different amounts of fat intake. Data were used to challenge the behavior of an existing mechanistic, deterministic, dynamic model for describing variation in milk production and changes in body composition. Nutrient intake and initial body weights and compositions were used as direct inputs into the model. The model could describe milk production within 1 SD of the mean on all rations. The model described body fat within 1 SD of observed values, but overestimated final body protein by approximately 30% (or between 1 and 2 SD). The model described the change in body fat and protein in response to 5% additional fat in the ration within 1 SD of observed (simulated - observed = -1.1 kg for protein and -0.7 for fat). The simulated response to dietary protein was within 1 SD (2.6 kg): simulated response from medium to high protein was 1.0 kg, observed 1.2; simulated response from medium to low was -0.7 kg, observed 0.3 kg. Thus, the model was not as accurate in describing the body protein response to a low-protein diet and overall, overestimated the retention of body protein. This model has utility for setting specific research hypotheses and may be amenable to use by nutritionists in a variety of applications. Further improvement in the ability of the model to describe metabolism and production of lactating sows is limited by a severe lack of data on metabolic interactions. Data on rates of muscle protein synthesis and proteolysis and interactions between amino acids, fatty acids, and glucose are needed to improve this and other models of sow production.