Higher Dietary Protein Increases Nitrogen Retention and Improves Caloric Efficiency

Abstract Recent research on soybean meal (SBM) has suggested that the energy value may be higher than reference values. This finding is likely not limited to just SBM and may apply across protein sources due to genetic selection for increased lean deposition. Two experiments were conducted to determine the effect of higher protein on performance, nitrogen retention, and energetics. Both experiments utilized a corn/SBM control diet (CON), and diets that replaced 50% of the synthetic lysine with SBM (SBM), a mixture of canola meal and meat and bone meal (CM-MBM), or soy protein concentrate (SPC). In experiment 1, 216 pigs (11.4 ± 1.0 kg) were stratified by weight and sex and randomly allotted to the 4 treatments (n = 18, 3 pigs per pen). Diets were fed for 4 weeks. Response variables included ADG, ADFI, G:F, and caloric efficiency (CE). In experiment 2, 32 barrows (43.1 ± 6.8 kg) from 6 sequential groups of pigs were allotted to the 4 treatments (n = 9). Pigs were fed 2.5X maintenance requirements in 2 equal meals daily. After a week of acclimation to diets, they were moved into metabolism crates. Following 3 days of adaptation in the crates, total urine and feces were collected for 3 days. Response variables included nitrogen intake, excretion, and retention along with digestible energy (DE) and metabolizable energy (ME) of the diets. Experiments were analyzed as a general linear model with post-hoc analysis using Tukey’s method. A contrast was constructed to compare CON to treatments with increased protein. Results of experiments 1 and 2 are summarized in Table 1. In experiment 1, increasing dietary protein tended to reduce ADG (P 0.10) and reduced ADFI (P 0.05), but did not affect G:F. CE was improved (P 0.05) when protein was increased in the diet. Individual protein source did not differ for performance or CE. In experiment 2, increasing dietary protein increased nitrogen intake (P 0.01) and nitrogen excretion in the urine (P 0.01) but resulted in increased nitrogen retention (P 0.01). Only nitrogen retention for SBM and SPC were detected as different from CON. Increasing protein tended to increase DE (P 0.10), but not ME. No individual treatment differences were detected for ME or DE. All ME and DE values are higher than calculated values from NRC equations 1-3 and 1-5. The results of these experiments support that energy of ingredients high in protein may be underestimated, and this response is not just limited to SBM. Additionally, the increased nitrogen retention is likely a contributing factor. This may be observed more in measurements of net energy than DE or ME.