Soybean meal (SBM) is the primary plant-protein source in diets for pigs and provides both amino acids (AA) and energy to the diets. Current estimates for net energy in SBM are less than for cereal grains, based on the assumption that there is more nitrogen to be deaminated if ingredients are high in protein because deamination and excretion of nitrogen via the urea cycle are energy-requiring processes, and therefore, reduce energy efficiency. It has been suggested that pigs retain only 45 to 50% of absorbed nitrogen, which corresponds to 40 to 45% of ingested nitrogen. Modern genotypes of pigs, however, have improved the capacity for protein synthesis and may retain more nitrogen than older genotypes, which would result in less AA deamination and, therefore, less energy loss to deaminate AA and excrete nitrogen. Indeed, results of recent research indicate that pigs fed corn-SBM based diets retain more than 60% of ingested nitrogen, indicating that protein retention by modern genotypes of pigs is more efficient than by older genotypes. It is likely that as breeding companies have selected for leaner pigs, they have also selected genotypes that are more efficient in converting dietary protein into body protein. It is, however, not known if the greater nitrogen retention that has been recently reported is experienced by all pigs regardless of body weight (BW) and if it is true for all types of diets regardless of the dietary level of protein. Therefore, the objective of this experiment was to test the hypothesis that nitrogen retention, measured as a percent of nitrogen consumed, is greater than 50% regardless of the dietary protein level and the BW of pigs.

Experimental design

Diets were fed to pigs in three phases (i.e., weanling, growing, and finishing pigs), and there were three dietary treatments within each phase. Therefore, there were a total of nine diets within each phase. The first dietary treatment was a high-protein diet containing corn and SBM as protein sources, and this diet contained no crystalline AA. A medium-protein diet contained less SBM and more corn than the high-protein diet, and crystalline Lys, Met, and Thr were added to the diet to maintain the required levels of indispensable AA. A low-protein diet that contained less SBM and more corn than the other two diets was also formulated, and five crystalline AA (i.e., Lys, Met, Thr, Trp, and Val) were supplemented to this diet to maintain required levels of digestible AA. All diets were formulated to meet or exceed the estimated requirements for standardized ileal digestible AA, vitamins, and minerals for weanling, growing, and finishing pigs, respectively (NRC, 2012).

Animals, housing, feeding, and sample collection

Twenty-four weanling pigs (initial BW: 11.37 ± 0.62 kg), 24 growing pigs (initial BW: 27.38 ± 1.34 kg), and 24 finishing pigs (initial BW: 76.22 ± 3.10 kg) were allotted to the three dietary treatments using a randomized complete block design within each phase, for a total of eight replicate pigs per diet in each phase. Initial BW was the blocking factor. Pigs were housed individually in metabolism crates that allow for the total, but separate, collection of feces and urine. Daily feed was provided to be equivalent to 3.2 times the metabolizable energy requirement for maintenance (i.e., 197 kcal/kg × body weight^0.60; NRC, 2012). Water was available at all times.

Pigs were fed experimental diets for 13 days. The initial five days were considered the adaptation period to the diet, and urine and fecal samples were collected during the following five days according to the marker-to-marker method (Adeola, 2001). All samples were immediately stored at –20 °C after collection. At the conclusion of the experiment, urine samples were thawed and mixed within animal and diet. Fecal samples were thawed and then dried in a forced-air oven at 50 °C before analysis. All samples were analyzed for dry matter (DM) and nitrogen. Crude protein was calculated as analyzed nitrogen × 6.25. Diet and ingredient samples were also analyzed for DM, ash, nitrogen, and AA. The apparent total tract digestibility (ATTD) of DM and nitrogen, retention of nitrogen, and biological value of nitrogen were calculated using analyzed nitrogen in diets, feces, and urine, and feed intake, dried fecal weight, and urine weight during the collection period.

Results

Nitrogen retention (g/d), nitrogen intake, absorbed nitrogen, and nitrogen excretion in urine decreased as dietary protein was reduced, but the decreases were greater as pig BW increased (interaction; P < 0.05; Table 1). Nitrogen retention (% of intake) and biological value increased (P < 0.05) as dietary protein was reduced. All measured parameters except nitrogen retention (% of intake) and biological value increased (P < 0.001) as pig BW increased. Nitrogen retention (% of intake) tended (P = 0.056) to be reduced, and biological value was reduced (P < 0.001) in finishing pigs compared with weanling or growing pigs. Feed intake, dry feces output, and nitrogen excretion in feces were not affected by dietary protein concentration.

Key points

• Regardless of body weight, pigs retain between 60 and 70% of ingested nitrogen if fed a corn-SBM diet.
• Pigs fed low protein diets have reduced daily nitrogen retention (g/d) compared with pigs fed high protein diets
• Regardless of pig body weight, daily nitrogen retention (g/d) is greater if diets with only intact protein are provided compared with diets containing less protein and crystalline AA.

Table 1. Effects of body weight and dietary protein on apparent total tract digestibility (ATTD) of dry matter (DM) and nitrogen balance of growing pigs¹

^a-gWithin a row, means without a common superscript differ (P < 0.05).

¹Least square means represent eight observations for all treatments, except for the low-protein diet of weanling pigs and the high-protein diet of finishing pigs (n = 7).

²Initial weights were 11.37, 27.38, and 76.22 kg for weanling, growing, and finishing pigs, respectively.

³Diets with high levels of protein (High) were based on corn and soybean meal and no synthetic AA; diets with medium levels of protein (Med) were based on corn, soybean meal, and synthetic lysine, methionine, and threonine; and diets with low levels of protein (Low) were based on corn, soybean meal, and synthetic lysine, methionine, threonine, tryptophan, and valine. All diets met minimum requirements for standardized ileal digestible amino acids according to NRC (2012).