By Jessica Smallfield, Mike Tokach, Jason Woodworth, Robert Goodband, Joel DeRouchey, Katelyn Gaffield and Jordan T. Gebhardt, Kansas State University

The addition of dietary feed-grade amino acids has significantly benefited the swine industry by reducing feed costs, nitrogen excretion and environmental impact. Extensive research has shown that pig performance, specifically feed efficiency, can either be maintained or enhanced by substituting a portion of intact protein sources, like soybean meal, with feed-grade amino acids. However, pig performance may be compromised when high amounts of feed-grade amino acids are used in the diets. This decrease in performance is not entirely understood, but it has been hypothesized that the amount of nitrogen needed to synthesize non-essential amino acids may become limiting.

Using nitrogen requirements suggested by the NRC (2012), the standard illeal digestible SID Lys:CP ratio should be lower in older pigs because of their increased maintenance requirement and lower needs for protein synthesis than in young pigs. The NRC suggests that a SID Lys:CP ratio of 6.5 is optimal for pigs weighing between 25 to 55 pounds. Previous research showed that ammonium phosphate can serve as a nitrogen source in diets deficient in non-essential amino acids. Therefore, the objective of this study was to determine the impact of a NPN source (Di-ammonium phosphate;  DAP) or amino nitrogen source (glycine) on growth performance, feed efficiency, digestibility and blood urea nitrogen of 25 to 50 lb pigs fed low protein, amino acid fortified diets.

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This study was conducted at the New Fashion Pork research nursery facility in Round Lake, Minnesota. The facility contained 52 pens and is completely enclosed, environmentally controlled and mechanically ventilated. Each pen contained a three-hole, dry self-feeder and a bowl waterer for ad libitum access to feed and water.

Animals and diets

A total of 981 pigs ([Fast LW "×" PIC L02] "×" PIC 800; initially 22.8 "±" 0.42 lb) were used in a 21-day growth trial. Pigs were housed in mixed-gender pens with 19 to 20 pigs per pen and 10 replications per treatment in a randomized complete block design with BW as a blocking factor. Pens of pigs were randomly allotted to one of five dietary treatments (Table 1). Diets were corn-soybean meal-based and consisted of

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The net energy of soybean meal was considered to be 100% of corn net energy in diet formulation. Prior to diet formulation, samples of corn and soybean meal were submitted to Minnesota Valley Testing Laboratories, Inc. (New Ulm, Minnesota) for a complete mineral panel analysis and to Market 1, Inc. (Ag State; Cherokee, Iowa) for proximate and AA analysis. Dietary additions of feed-grade AA were adjusted to meet or exceed AA requirements in relation to Lys for Met and Cys, Thr, Trp, Val, Ile, and His. Pigs were fed treatment diets in meal form. Treatment diets were manufactured at the New Fashion Pork feed mill in Round Lake, Minnesota. Titanium dioxide was included at 0.4% in the treatment diets as an indigestible marker to determine apparent total tract digestibility of DM and CP.

Measurements and sampling

Pig weights and feed disappearance were measured on day 0, 13 and 21 to determine average daily gain, average daily feed intake and feed-to-gain ratio. Fecal samples were collected on d 21 from three pigs per pen to determine percentage dry matter. Fecal samples were analyzed separately for each pig and the average of the three samples from each pen was then used for statistical analysis. Blood samples were collected on d 21 from four pigs per pen (two barrows and two gilts of medium size) to measure serum blood urea nitrogen.

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Results and Discussion

Growth performance

Overall (d 0 to 21), there was no evidence (P > 0.05) for ADG to be affected by treatment. Feed efficiency worsened (linear, P = 0.002; quadratic, P = 0.054) with the greatest change occurring as the SID Lys:CP ratio increased from 6.5:1 to 7.0:1 corresponding with the addition of increased feed-grade AAs. Incorporating either DAP or glycine to the high feed-grade AA diet led to an improvement (P "≤" 0.003) in feed efficiency compared to the pigs fed the high feed-grade AA diet due to a reduction (P = 0.007) or numerical reduction (P = 0.109), respectively, in ADFI.

Blood urea nitrogen analysis

There was a decrease (linear, P < 0.001) in serum blood urea nitrogen as the SID Lys:CP ratio increased, indicating that the pigs were more efficient at retaining dietary nitrogen. Adding nitrogen to the high feed-grade AA diet in the form of DAP or glycine increased (P = 0.038) or tended to increase (P = 0.091) blood urea nitrogen, respectively.

Fecal dry matter analysis

There was a tendency (P = 0.051) for a quadratic effect in fecal DM with pigs fed the moderate feed-grade AA diet having the lowest fecal DM. The pigs fed the diet containing DAP had increased (P = 0.005) fecal DM compared with the pigs fed the high feed-grade AA diet, which could be due to the differences in N in the two diets or feed intake.

DM and CP digestibility

There was a tendency for improvement (P = 0.060) in ATTD of DM for the pigs fed the diet containing DAP compared to the pigs fed the high feed-grade AA diet without DAP or glycine. Additionally, the ATTD of CP decreased (linear, P = 0.048) with increasing feed-grade AAs. Pigs fed the diets containing either DAP or glycine had increased (P "≤" 0.026) CP digestibility compared to pigs fed the high feed-grade AA diet. Pigs fed the diet containing DAP had increased (P = 0.005) CP digestibility compared to the pigs fed the moderate feed-grade AA diet.

In summary, these data suggest that diets for 25 to 50 lb pigs should be formulated to a SID Lys:CP ratio of 6.5 or lower and that adding a non-protein nitrogen source or non-essential AA to diets formulated above this ratio can improve feed efficiency and CP digestibility.

The full research report and other swine nutrition and management research was presented at the 2024 K-State Swine Industry Day.

Acknowledgments

Appreciation is expressed to New Fashion Pork for providing the nursery facility and CJ America support of this trial.