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The effect of feeding low-phytate barley-soybean meal diets differing in protein content to growing pigs on the excretion of phosphorus and nitrogen^1,2

J. K. Htoo^*, W. C. Sauer^*,, Y. Zhang^*, M. Cervantes^,3, S. F. Liao^*, B. A. Araiza, A. Morales and N. Torrentera

^* Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, AB, Canada T6G 2P5; and and Instituto de Ciencias Agrícolas, Universidad Autónoma de Baja California, Mexicali, México 21100

	Abstract

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An experiment was conducted with growing pigs to determine the excretion of P and N in 4 barley-based diets formulated to contain 18 or 15% CP by using a normal barley (NB) or a low-phytate barley (LPB). The NB contained 0.31% total P and 0.19% phytate P; the LPB contained 0.32% total P and 0.01% phytate P. The diets were supplemented, when so required, with lysine, methionine, threonine, and tryptophan to meet their apparent ileal digestible supplies according to the NRC (1998). The diets containing NB were supplemented with inorganic P to meet the NRC (1998) recommendation for available P (0.23%). The diets containing LPB were not supplemented with inorganic P because these contained sufficient available P (0.27%). Eight barrows with an average BW of 20.9 kg were assigned to the 4 dietary treatments according to a repeated 4 x 4 Latin square design. The diets were fed at a rate of 2.5 times the ME requirement for maintenance. The barrows were fed twice daily, at 0800 and 1500, equal amounts each meal. Water was added to the feed at a ratio of 2.5:1. Each experimental period consisted of a 7-d adaptation period followed by a 5-d collection of feces and urine. The substitution of NB with LPB decreased (P < 0.001) the total P excretion by 38 and 43% for the 18 and 15% CP diets, respectively. Reducing the CP content from 18 to 15% decreased (P < 0.001) the N excretion by 29 and 32% for the NB and LPB diets, respectively. With the reduction in CP content, there was a decrease (P < 0.001) in the amount of N retained. The N:P ratio in manure of pigs fed the LPB diets was greater (P < 0.001) than from pigs fed the NB diets. These data indicate that P and N excretion can be greatly reduced by substitution of NB by LPB, and also by the reduction of the CP content, in diets for growing pigs.

Key Words: low-phytate barley • nitrogen • phosphorus • pig

	INTRODUCTION

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Barley is a commonly used feed ingredient in diets for growing and finishing pigs in western Canada. It ranks fourth after wheat, rice, and corn in world cereal production (FAO, 2002). In Canada, barley production ranks second after wheat. It is grown mainly in the prairie provinces of Alberta, Saskatchewan, and Manitoba.

Approximately two-thirds of the P in commonly used feed ingredients of plant origin, including barley, is present in the form of phytate (Ravindran et al., 1995). Phytate P is poorly utilized by nonruminant animals such as pigs and poultry, due to insufficient activity of endogenous phytases (Nelson, 1967). Inorganic P (iP) sources are routinely added to the diet to meet the P requirements. Most of the phytate P is excreted, which can lead to environmental pollution. Phosphorus from animal manure is becoming an increasing environmental concern in many countries (Kornegay, 2001). One method to reduce P excretion, besides the use of microbial phytase, is the use of low-phytate crops in pig diets. In recent years, low-phytate crops have been developed, including low-phytate barley (LPB) that is phenotypically identical to normal barley (NB). There is little difference in the total P content between NB and LPB.

There are also concerns about environmental pollution resulting from N excretion in manure, especially in areas of intensified swine production (Otto et al., 2003). By reducing the dietary CP content by 3 to 4 percentage units (pu), with appropriate AA supplementation, the total N excretion decreased by 24 to 40% (Sutton et al., 1999; Shriver et al., 2003). On the other hand, protein digestion can be reduced because phytates posses the ability to form complexes with proteins (Selle et al., 2000).

Therefore, the first objective of this study was to determine the apparent total tract digestibilities (ATTD), retention, and excretion of P and N in growing pigs fed LPB-based diets compared with those fed NB-based diets. The second objective was to determine if a 3 pu reduction in the dietary CP content in LPB- and NB-SBM diets further reduces N excretion.

	MATERIALS AND METHODS

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Animals and Dietary Treatments
The experimental proposal, surgical procedures, and procedure for care and treatment of the pigs were reviewed and approved by the Animal Care Committee of the University of Alberta in accordance with the guidelines of the Canadian Council on Animal Care (CCAC, 1993).

Nine crossbred (Large White x Landrace) barrows with an average initial BW of 20.9 kg were obtained from the Swine Research Unit of the University of Alberta and housed individually in stainless steel metabolic crates (height = 82 cm; length = 124 cm; width = 76 cm) in a temperature-controlled room (22 ± 1°C). During a 7-d adaptation period to the crates, the barrows were fed ad libitum an 18% CP (as-fed basis) grower diet. Water was freely available from a low-pressure drinking nipple.

Based on feed intake, 8 barrows were selected and fed 4 diets according to a repeated 4 x 4 Latin square design. Each of the 4 experimental periods was 13 d. The barrows were fed twice daily, equal amounts each meal, at 0800 and 1500. The diets were fed at a rate of 2.5 times the maintenance requirement for ME (i.e., 106 kcal/kg of BW^0.75) based on the average BW of the pigs, which was recorded at the beginning of each experimental period. Four barley-based diets were formulated to contain 18 or 15% CP by using a NB (cultivar Metcalfe) or a LPB (mutant line 99048). Although the total P contents were similar, the phytate P content was lower in the LPB compared with the NB (0.01 vs. 0.18%), resulting in a greater content of available P (aP) in LPB (Table 1). The 4 experimental diets were an 18% CP, NB-based diet (NB-18% CP); a 15% CP, NB-based diet (NB-15% CP); an 18% CP, LPB-based diet (LPB-18% CP); and a 15% CP, LPB-based diet (LPB-15% CP).

Sample Collection and Chemical Analysis
Samples of the major feed ingredients were taken before diet formulation. Samples of the diets were taken at the time the meal allowances were prepared. The collection of feces was initiated at 0800 on d 8 of each experimental period and continued for 120 consecutive hours. Feces were collected at 0800 and 1600 and immediately frozen at –28°C. Feces contaminated with feed and urine or trampled on were discarded. A total collection of urine was carried out during the same time during which the feces were collected. Urine was collected into a container through glass wool and measured volumetrically, and a subsample (20% of the total volume) was stored at –4°C. Before collection, 5 mL of 10% (vol/vol) formic acid solution was placed into each container.

Before chemical analyses (Table 2), feces and urine were pooled for each pig in each experimental period. Feces were dried to a constant weight in a forced-air oven at 55°C. Urine samples were filtered through Whatman #2 filter paper and then dried in a forced-air oven before analysis. Samples of the major ingredients, diets, and dried feces were finely ground through a 1-mm screen in a Thomas-Wiley Laboratory Mill (Arthur H. Thomas Co., Philadelphia, PA).

Calculations and Statistical Analysis
The ATTD of DM, OM, CP, and P were calculated by using the following equation:

where D_D is the ATTD of a parameter in the assay diet (%), A_F is the concentration of a parameter in feces (%), I_D is the chromic oxide concentration in the assay diet (%), A_D is the concentration of a parameter in the assay diet (%), and I_F is the chromic oxide concentration in feces (%). The total N and P excretions in feces were estimated taking into account the intake of N and P and their digestibility coefficients (determined by aid of chromic oxide). The data were subjected to statistical analyses for a repeated 4 x 4 Latin Square design using the GLM procedure (SAS Inst. Inc., Cary, NC). The main effects of diets (n = 4), pigs (n = 8), and experimental periods (n = 4) were included in the model. Three orthogonal contrasts were used to test the effect of dietary protein content (C₁ = 18% CP vs. 15% CP), barley type (C₂ = normal vs. low-phytate barley), and their interaction (C₃ = CP content x barley type). A probability level of P 0.05 was defined as a significant difference.

	RESULTS

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All pigs remained healthy and usually consumed their meal allowances within 30 min after feeding throughout the experiment. The average BW of the pigs were 22.1, 27.5, 35.1, and 42.4 kg at beginning of periods 1, 2, 3, and 4, respectively. The average BW of the pigs was 50.8 kg at conclusion of the experiment. There were no differences in the ATTD of DM (P > 0.05) and OM (P > 0.05) among the diets (results not shown). The ATTD of DM ranged from 88.6 to 88.8%. The ATTD of OM ranged from 90.3 to 90.7%.

The P balance results are presented in Table 3. The excretion of P in feces, as well as in urine, was lower (P < 0.001) in pigs fed the LPB compared with the NB diets. Consequently, the ATTD and retention (%) of P were greater (P < 0.001) in pigs fed the LPB compared with the NB diets. The amount of retained P (g/d) was slightly lower (P = 0.026) in pigs fed the LPB compared with the NB diets. The excretion of P in feces was lower (P < 0.001) in pigs fed the 15 compared with the 18% CP diets. As such, the ATTD (P = 0.036) and retention (P = 0.038) of P were greater in pigs fed the 15% CP diets. There were no differences (P = 0.095) in the amount of P retained (g/d) between pigs fed the 15 and 18% CP diets. In addition, there were no interactions (P = 0.208) between CP content and barley type for all parameters measured for P.

	DISCUSSION

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The ATTD and retention (%) of P were greater for pigs fed the LPB compared with the NB diets (Table 4). This is a result of the lower content of phytate-P and a greater content of aP (Table 3). Thacker et al. (2003, 2004) also reported a greater ATTD of P in finishing pigs fed a LPB compared with a NB diet. Although the ATTD and retention (%) of P were greater, the amount of P retained (g/d) was slightly lower for pigs fed the LPB compared with the NB diets. Similar results were reported by Veum et al. (2002) in growing pigs fed 16.8% CP LPB and NB diets. The urinary P excretion was lower in pigs fed the LPB diets. As reported by Veum et al. (2002) and Liao et al. (2006), the excretion of P in urine is of a very small magnitude. The substitution of NB by LPB decreased the total P excretion by 38 and 43% for the 18 and 15% CP diets, respectively. These results are in agreement with previous studies that evaluated low phytate corn (Spencer et al., 2000; Veum et al., 2001; Bohlke et al., 2005), SBM (Sands et al. 2003), and barley (Li et al. 2001a,b; Veum et al., 2002) in poultry and swine diets. It should be mentioned that pigs had restricted access to feed in the current study. Therefore, the daily intake of P and aP (g/d) ranged from 44 to 59% and 56 to 69% of the NRC (1998) recommended intake, respectively.

The ATTD of P as well as the retention were slightly greater in the 15 than in the 18% CP diets (Table 3). The greater ATTD of P may result from the slightly lower content of phytate-P and greater content of aP (Table 2). This is likely a result of partial replacement of SBM by barley indicating that the content of phytate-P in barley was lower than in SBM. Reducing the dietary CP content from 18 to 15% resulted in a decrease in P excretion by 7.7 and 14.7% for the diets containing NB and LPB, respectively (Table 3). The least amount of P for all diets was excreted by pigs fed the 15% CP LPB diet.

The ATTD of CP was lower in the 15% than in the 18% CP diets (Table 4). The lower ATTD may be a direct result of changes in the ratios of ingredients. The content of SBM was lower and the content of barley was greater in the 15% than in the 18% CP diets. The ATTD of CP in SBM is greater than in barley (CVB, 1994).

In agreement with other studies (Zervas and Zijlstra, 2002a; Otto et al., 2003), the retention (%) of N increased as the dietary CP content was decreased (Table 4). This was mainly due to a reduction in the excretion of N in urine. The urinary N excretion decreased by 40% when the CP content was reduced from 18 to 15% in the diets containing NB, and by 42% when the CP content was decreased for the LPB-containing diets. These results are in agreement with studies by Otto et al. (2003) and Shriver et al. (2003) who reported an approximately 50% decrease in urinary N excretion of pigs fed low CP diets supplemented with AA. Reducing the CP content from 18 to 15% decreased the total N excretion by 29 and 32% for the NB and LBP diets, respectively.

Reducing the dietary CP content from 18 to 15% and supplementing with limiting AA decreased the daily amount of N retained (g/d) by 10% for the NB and LBP diets. These results are in agreement with those of Zervas and Zijlstra (2000b) and Shriver et al. (2003). Zervas and Zijlstra (2002a) also observed a 12% decrease in the retention of N in 30-kg pigs offered restricted access to a 15.6% CP diet compared with that in pigs fed an 18.5% CP diet, but the retention of N decreased by only 7% when pigs were offered unlimited access to those diets. Kerr and Easter (1995) reported that pigs fed a 12% CP diet supplemented with lysine, tryptophan, and threonine retained less N than pigs fed a 16% CP diet. The further addition of dispensable AA to the 12% CP diet increased the amount of N retained. Shriver et al. (2003) also reported that reducing the dietary CP content from 18 to 15% with appropriate AA supplementation in 36-kg pigs offered unlimited access to feed decreased the retained N by 15%, but the growth performance was not affected.

In the current study, the diets were formulated to meet the NRC (1998) recommendations for apparent ileal digestible AA for growing pigs. However, due to the restricted feeding design, the daily intake of total lysine, methionine plus cystine, and threonine (g/d) of pigs fed the 15 and 18% CP diets were only 62, 60, and 66%, and 62, 67, and 72% of the NRC (1998) recommended intake, respectively. The reduction in the amount of N retained may be attributed to inadequate daily intake of one or more essential AA, different digestible AA levels, AA imbalances (Kerr and Easter, 1995), or different efficiencies of AA utilization among diets (Zervas and Zijlstra, 2000b). Otto et al. (2003) suggested that endogenous N losses contribute to a substantial portion of total N losses, and this contribution relatively increases as the dietary CP concentration decreases. Therefore, an increased level of endogenous N losses may also lead to a reduced N retention (g/d) with low CP diets.

Reducing the CP content of the diet with AA supplementation will decrease N excretion (e.g., Kerr and Easter, 1995; Shriver et al., 2003). Kerr and Easter (1995) reported that for every 1 pu decrease in CP content there was a 10% reduction in total N excretion. Shriver et al. (2003) reported a reduction of 8% in total N excretion for every 1 pu decrease in CP content. In the current study, there was a 9% reduction in total N excretion for every 1 pu decrease in dietary CP content. The N:P ratio in manure of pigs fed the LPB diets, regardless of dietary CP content, was greater than in manure of pigs fed the NB diets (Table 4). Spencer et al. (2000) also reported that feeding a low-phytate corn increased the N:P ratio in manure, becoming more environmentally friendly and more suitable to use as a fertilizer.

In conclusion, the excretion of P can be greatly reduced by substitution of NB by LPB in diets for growing pigs. The excretion of N was considerably reduced when the 18% CP diet was replaced by a 15% CP diet supplemented with AA, and there was a further reduction in P excretion. However, the reduction in N excretion was compromised by a decrease in the amount of N retained.

	IMPLICATIONS

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The substitution of normal by low-phytate barley in an 18% crude protein barley-soybean meal diet for growing pigs will greatly reduce the excretion of phosphorus in manure. There will also be an increase in the nitrogen:phosphorus ratio of manure making it more suitable as a fertilizer. Reducing the crude protein content of barley-soybean meal diets, from 18 to 15%, and supplementing the low crude protein diet with amino acids will also greatly reduce the excretion of nitrogen. Lowering the dietary crude protein content should not affect the amount of nitrogen retained. More research may be warranted in the area of reducing the dietary crude protein content and supplementation with dispensable amino acids so that performance will not be compromised.

	Footnotes

 
¹ The authors thank K. Sauer and J. Hairong for their technical assistance.

² J. Helm (Field Crop Development Centre, 5030 – 50 Street, Lacombe, AB T4L 1W8, Canada) is acknowledged for supplying the barley cultivars used in this experiment.

³ Corresponding author: miguel_cervantes{at}uabc.mx

Received for publication April 19, 2006. Accepted for publication October 27, 2006.

	LITERATURE CITED

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This Article Abstract Full Text (PDF) All Versions of this Article: jas.2006-249v1 85/3/700    most recent Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Htoo, J. K. Articles by Torrentera, N. Search for Related Content PubMed PubMed Citation Articles by Htoo, J. K. Articles by Torrentera, N.