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Effects of balancing low protein diets with supplemental amino acids on performance and carcass composition of growing-finishing pigs

Published on: 12/14/2018
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  • Reducing the dietary crude protein (CP) level and balancing with supplemental amino acids (AA) maintained optimal performance and carcass quality of growing-finishing pigs compared with feeding high CP diet.
  • Plasma urea nitrogen concentration decreased linearly when dietary CP was reduced, indicating a better balanced AA supply with less excessive AA being Nitrogen (N) retention (g/d) was not affected but the urinary and total N excretions were markedly reduced by 32 and 28 %, respectively by lowering the CP level from 16.1 to 12.4 % in late Finisher diet.
  • Lowering dietary CP level by replacing soybean meal with supplemental AA is possible to maintain optimal pig performance and carcass quality when diets are balanced for adequate levels of standardized ileal digestible AA, essential AA:total N ratio and net energy.

Introduction and Objective

Increasing public awareness of environmental pollution demands that today’s animal producers are paying attention not only to optimal productivity but also minimizing N excretion. Research showed that 1 %-unit dietary CP reduction results in roughly 8 % less N excretion in growing-finishing pigs (Kerr, 2003). Thus, lowering dietary CP level is an effective means to improve the efficiency of N utilization. An unwanted impact of feeding high CP diets is that it increases hindgut microbial fermentation and proliferation of pathogenic bacteria which promotes the incidence of diarrhea in weaned pigs. Thus, reducing dietary CP can improve the gut health of weaned pigs.

Currently, L-Val in a feed-grade form is available in the market (in addition to Lys, Met, Thr and Trp), and L-Ile has been permitted for use in the European Union. This gives nutritionists more flexibility to further reduce the dietary CP level, and take advantage of using less costly raw materials to improve overall profitability. Research has proven that reducing the CP level up to 4 %-units from 20.0 to 16.0 % CP (Le Bellego et al., 2002) and balancing the diets to meet all essential AA (EAA) on standardized ileal digestible (SID) and net energy (NE) basis had no detrimental effect on performance of grower-finisher pigs. However, a reduced performance was observed when the dietary CP level was reduced greater than 4 %-units (Guay et al., 2006; Yue and Qiao, 2008) although diets in both studies were balanced to meet SID EAA requirements.

Sufficient intake of non-essential AA (NEAA) is a quantitatively important part of total protein intake and the proportion of NEAA N:total dietary N can significantly affect animal performance (Heger et al., 1998). Containing an insufficient amount of N from NEAA may be one of the limiting factors affecting the performance of pigs when extremely low CP diets are fed. Thus, the objective of this experiment, conducted at the swine research facility of the University of Applied Sciences Bingen, Germany, was to evaluate the effects of balancing low CP diets with supplemental AA and maintaining optimal essential AA:total N ratio on performance and carcass composition of growing-finishing (24-110 kg) pigs.

Experimental design

A 83-day (d) growth trial was conducted with 180 TOPIGS pigs [initial body weight (BW) of 24.4 ± 2.0 kg] with 2 pigs per pen and 18 pens (9 barrow pens and 9 gilt pens) per treatment. Pigs were blocked by BW and assigned to 5 diets based on corn, barley, soybean meal (SBM) and rapeseed meal for each of the Grower 1 (24-40 kg, d 0-18), Grower 2 (40-60 kg, d 19-39), Finisher 1 (60-80 kg, d 40-61) and Finisher 2 (80-110 kg, d 62-83) phases. For each phase, the high CP diet (diet A) and low CP (diet E; without SBM) were initially formulated without inclusion of free AA. Diets B, C and D were produced by blending diets A and E in varying proportion (75, 50 and 25 % of diet A).

Ingredients and all diets were analyzed for total AA content, and supplemental AA were then added to the diets on the basis of SID AA (AMINODat® 4.0) corrected for the analyzed AA content to meet AA recommendation (Table 1; Evonik, 2009). As shown in Table 2, dietary CP was reduced linearly from diet A through E by step-wise reduction of SBM inclusion (0, 25, 50, 75 and 100 %). Additional AA (Gly, Pro, Glu, His, Phe) were supplemented in diets D and E of Grower 1 and 2 phases to balance the EAA:total N ratio to be approximately 50 % (Heger et al., 1998). Additional L-Arg was added in diet E to contain a similar Arg level like in diet C of Grower 1 and 2 phases (Tables 1 and 2). All diets were balanced for a similar NE content (Table 1).

The analyzed AA contents in the experimental diets were close to the calculated values indicating that diets were mixed properly (Table 3). Pig BW and feed intake were recorded during each phase. After 83 d of growth trial, pigs further received Finisher 2 diets until they reached slaughter BW of approximately 117 kg. Pigs were slaughtered at a commercial slaughter facility, and 10 pigs from treatment A, B, D and E (6 barrows and 4 gilts) and 9 pigs (5 barrows and 4 gilts) from treatment C were selected for carcass evaluation. On trial d 95, 6 barrows per treatment were selected to collect blood to determine plasma urea N (PUN) concentration.

Additionally, a 5-d N-balance trial was conducted by feeding Finisher 2 diets A (16.1 % CP) or E (12.4 % CP) to measure the effect of dietary CP on N excretion. Finisher barrows (average BW of 111.3 ± 3.4 kg; 3 pigs per treatment) were fed restricted at 2.0 kg/d. After a 7-d adaptation period, a total collection of feces and urine were undertaken for 5 days.

Results and discussion

During each phase and the overall 83-d period, average daily feed intake (ADFI), average daily gain (ADG) and feed conversion ratio (FCR) as well as final BW, were not affected by lowering the CP content and balancing with AA in the diets (P > 0.05; Table 4; Figure 1). Overall, feeding the lowest CP diet (diet E; without SBM) resulted in 30 g less ADG (886 vs. 916 g/d) but FCR was improved by 3.3 % (2.33 vs. 2.41) compared with the high CP diet (diet A).

In this study, inclusion level of SBM was gradually reduced from a “high” to “zero” inclusion level in diet A to E, and consequently lowered dietary CP by approximately 3 to 23 % on a relative basis, but pig performance was not affected when diets were balanced to meet requirement on SID AA and NE basis coupled with maintaining maximum EAA:total N ratio. A reduced performance was reported when dietary CP was reduced greater than 4 %-units from 16.0 to 10.1 % CP in 37-61 kg growing pigs (Guay et al., 2006) or from 23.1 to 17.2 % CP in 7-11 kg weaned piglets (Yue and Qiao, 2008) despite the fact that diets were balanced to meet for EAA on SID basis. A closer look at diet composition used in these studies revealed that lowering CP resulted in markedly reduced levels of NEAA which may have been insufficient to synthesis EAA.

Plasma urea N concentration (d 95) decreased linearly (P < 0.001) when dietary CP content was reduced from diet A to diet E (Table 5). This indicates that when dietary CP is reduced and AA are supplemented the dietary AA profile is closer to ideal protein ratio, and less excessive AA are being degraded in the body.

There were no differences (P > 0.05) among dietary treatments in terms of dressing percentage, lean meat percentage and back fat thickness (Table 5), which demonstrated that reducing dietary CP levels does not affect the carcass characteristics when diets are formulated on the same SID AA and NE basis.

As shown in Table 6, the daily N retained (g/d) was not affected (P > 0.05) but the urinary N excretion was reduced by 32 % from 32.8 to 22.2 g/d (P = 0.01) by lowering the CP from 16.1 to 12.4 % in Finisher 2 diet. As urinary N is the main route of N excretion in pigs, the total N excretion was also reduced by 28 % (P < 0.01) by lowering the CP level in Finisher 2. This equates to 7.4 % less N excretion by 1 %-unit dietary CP reduction confirming the previous estimate by Kerr (2003).

In conclusion, lowering dietary CP level by replacing SBM with supplemental AA allows to maintain optimal pig performance when diets are balanced for adequate SID AA and NE, and an EAA:total N ratio of not greater than 50 % is maintained to ensure that the diet is not deficient in NEAA. Furthermore, low CP, AA-supplemented diets deliver abetter balanced AA supply to pigs resulting a markedly reduced N excretion and reduced impact of live production on the environment.


Htoo, J. K., J. Trautwein, J. Gao, and G. Dusel. 2013. Feeding low protein, AA-fortified diets did not affect performance and carcass composition of growing-finishing pigs. The 4th International Symposium on Energy and Protein Metabolism and Nutrition. September 9-12, Sacramento, USA.

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