Two studies were conducted to assess the effect of dietary protein reduction on N utilization, N excretion, and AA digestibility in growing pigs. The objective was to determine whether pigs fed diets with a reduced CP concentration could maintain the same N retention as pigs fed an adequate diet. The second objective was to test whether reducing dietary CP concentration decreases AA digestibility. In each study, six barrows were allotted to one of six dietary treatments in a Latin square design. Treatments consisted of four corn-soybean meal-based diets containing 15, 12, 9, and 6% CP, a casein-based diet containing 15% CP, and a protein-free diet. Crystalline AA were included in the 12, 9, and 6% CP diets. The indispensable:dispensable AA ratio was maintained at 45:55 with the addition of L-glutamic acid to the 9 and 6% CP diets. The casein-based and protein-free diets were used to determine endogenous total tract N and ileal AA losses. In the first study, total N losses and N absorbed decreased linearly (P < 0.001) as dietary CP concentration decreased from 15 to 6%. Both a linear (P < 0.001) and a quadratic (P < 0.05) decrease in N retention were found with decreasing dietary CP concentration. Nitrogen retained as a percentage of intake and absorbed increased (P < 0.001) as dietary CP concentration was reduced from 15 to 6%. In the second study, six barrows were surgically fitted with a T-cannula at the terminal ileum to determine ileal AA digestibility. For all dispensable and most indispensable AA, apparent and standardized ileal digestibility increased linearly (P 0.01, and for arginine, P < 0.05) as dietary CP concentration decreased. These results indicate that dietary CP concentration can be decreased from 15 to 12% with crystalline AA supplementation to meet an ideal AA profile without adversely affecting N retention, and that decreasing dietary CP concentration from 15 to 6% increases both dispensable and indispensable AA ileal digestibility.

The objective of this study was to investigate whether reducing dietary CP concentration decreases fecal VFA, manure ammonia (NH3) emission and odor, and urinary phenolic metabolites. Six barrows were allotted to one of six dietary treatments in a Latin square design. Treatments consisted of four corn-soybean meal based diets containing 15, 12, 9, and 6% CP, a casein-based diet containing 15% CP, and a protein-free diet (0% protein). Crystalline AA were included in the 12, 9, and 6% CP diets. The casein-based and protein-free diets were used to determine basal endogenous contribution of VFA, phenolics, NH3, and manure odor. Pigs were housed individually in metabolism cages to allow total collection of feces and urine. Feces and urine were collected and pooled within pig and period. Feces and urine were analyzed for VFA and phenolic metabolite concentrations, respectively. Feces and urine were then mixed, stored, and fermented at room temperature for 30 d. For NH3 determination, headspace air was sampled from manure slurries at 24, 48, and 72 h after fermentation. Slurry samples were placed into vials, capped, and randomized before odor panel evaluation. Odor offensiveness was classified on severity: 1 = non-offensive; 2 = mildly offensive; 3 = moderately offensive; 4 = strongly offensive; and 5 = extremely offensive. Reducing dietary CP increased (P < 0.05) fecal VFA concentrations but did not affect phenolic concentrations in urine. Manure NH3 emission was reduced (P < 0.05) as dietary CP concentration decreased from 15 to 0%. The 15% diet had the least offensive manure slurry with odor qualitative ranking of 2.58 (i.e., mild-moderately offensive). Compared with the 15% CP diet, manure from the 9 and 6% CP diets was found to be more offensive (P < 0.05), with qualitative rankings of 2.92 and 3.10, respectively. Odor qualitative rank for the 12% CP, protein-free diet, and casein-based diet did not differ from that of the 15% CP diet. These results indicate that reduction in dietary CP concentrations decreases manure NH3 emission, but it does not diminish manure odor offensiveness and fecal VFA concentrations.

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These investigations show that daily retention in the course of growth is subject to dynamic changes specific for each nutrient. While there are only insignificant differences between daily live weight growth as the practically usable parameter and body weight growth, daily dry matter deposition takes a comparatively different development. The specific development of dry matter deposition depends on the daily retention of protein and fat. Whereas in early age protein retention is higher, fat retention increases in the course of live weight development in dependence on the energy level. Different energy supply results in distinctly differentiated protein, fat and energy retention per day. The highest variation in daily retention could be ascertained for fat retention in dependence on energy supply. A comparison of the cumulated experimental results with model values generalised from the growth function corresponded well.

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The results of the carcass analysis show a distinct influence of the duration of keeping on protein retention. The protein percentage was highest with those animals whose lysine requirement was met 100%. A lysine supplement of 20% of the valid norm had no infuence on the percental protein content. In both cases (100% and 120%) 17.1% protein were determined in the carcass. Concerning the daily protein retention, boars whose protein requirement wes met 120% and with a live weight of more than 90 kg achieved more favourable results than those of the norm variant. Up to 90 kg live weight no difference between the two variants could be detected. Animals which received feed rich in energy showed a very high daily protein retention during the first third of the fattening period, which decreased relatively much, however, in the course of growth.

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