The effect of commercially formulated, reduced crude protein diets, formulated to 11 apparent ileal digestible essential amino acids, on nitrogen retention by growing and finishing boars

Livestock Production Science 81 (2003) 89–98 www.elsevier.com / locate / livprodsci

The effect of commercially formulated, reduced crude protein diets, formulated to 11 apparent ileal digestible essential amino acids, on nitrogen retention by growing and finishing boars P.A. Lee*, R.M. Kay ADAS Pig Research Unit at Terrington, Terrington St. Clement, King’ s Lynn, Norfolk PE34 4 PW, UK Received 17 August 2000; received in revised form 21 August 2002; accepted 21 August 2002

Abstract Two experiments were carried out to determine the nitrogen retention (NR) and gross efficiency of NR (NR / NI) of boars offered diets with reduced crude protein (CP) levels designed to decrease nitrogen excretion. Six diets were compared which differed in CP content, from 177 to 253 g / kg and from 151 to 229 g / kg for growing and finishing boars, respectively. All diets were isocaloric and formulated to minimum amounts of all 11 essential amino acids on a determined apparent ileal digestible basis (IEAA), calculated from the lysine:IEAA ratio required for Ideal Protein (IP). Formulating the diets to all 11 IEAA was not sufficiently accurate to maintain dietary IP content at the lowest CP levels (177 and 151 g / kg). These two diets significantly increased NR / NI for the boars (P , 0.01) but this was not sufficient to maintain NR which was reduced in both the growing (13%; P , 0.05) and the finishing (26%; P , 0.001) boars. The two lowest CP diets also had a lower dietary electrolyte balance (dEB) which could have contributed to the reduction of NR.  2002 Elsevier Science B.V. All rights reserved. Keywords: Boar; Nitrogen balance; Low crude protein; Ileal digestible amino acids; Ideal protein; Acid–base balance

1. Introduction Several possible methods of dietary manipulation to reduce nitrogen excretion (Nex) by pigs have been discussed (Lenis, 1989; Franz et al., 1989; Schutte ¨ and Bosch, 1990; von Essen and Gunther, 1990; Jongbloed and Lenis, 1992). One method is based on improving the protein quality of diets by formulating to ideal protein (IP) as opposed to crude protein *Corresponding author. Tel.: 144-1553-828-621; fax: 1441553-827-229. E-mail address: [email protected] (P.A. Lee).

(CP). IP can be defined as the protein which contains all essential amino acids (EAA) and non-essential amino acids (non-EAA) in a ratio which resembles the amino acid composition of pig tissue (Fuller and Chamberlain, 1985). In this way it is more directly aligned to the pigs’ dietary requirements for lean gain. The ratios for the amino acids are calculated with lysine as a base of 1 on the assumption that it is the first-limiting amino acid in pig diets. Formulating to IP reduces the level of excess amino acids and, therefore, the level of CP in the diet, and so reduces the level of nitrogen excreted. Published data have shown that Nex can be

0301-6226 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0301-6226(02)00193-8

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significantly reduced (Franz et al., 1989; Gatel and Grosjean, 1992; Jondreville et al., 1993; Roth et al., 1994) and that daily live weight gain (DLWG) can be maintained (Franz et al., 1989; Fremaut and De Schrijver, 1990; Gatel and Grosjean, 1992) when gilts and castrates are offered diets lower in CP than those available commercially. However, a number of other studies have shown that diets with lower CP levels than those in commercially available diets result in lower levels of nitrogen retention (NR) and reduced carcass lean percentage in both these classes of stock (Franz et al., 1989; Fremaut and De Schrijver, 1990; Spiekers et al., 1991; Heinrichs and Oldenburg, 1993; Roth et al., 1994). Results from a series of nitrogen balance studies carried out in the UK (Lee et al., 1993) showed that when reduced CP diets were offered to boars, which in general require higher levels of protein than gilts and castrates to optimise performance (English et al., 1988), there was a reduction in NR. This occurred even though the diets were specified to minimal requirements of all 11 EAA, as per the ratios in IP, and formulated with the inclusion of crystalline amino acids. One of the reasons for the reduction in NR could have been the use of total EAA (TEAA) values in the diet specifications, as opposed to ileal digestible EAA (IEAA) values. The superiority of using diet specifications based on IEAA was shown in the work carried out by Wiseman et al. (1991). Their results showed an increase in pig performance over the weight range 27.5–52.5 kg, with diets containing heat-treated fish meals formulated to IEAA values compared with similar diets formulated to TEAA values. Studies have also shown that pigs offered diets formulated to IEAA levels had a better feed to gain ratio and gave a better economic return than pigs offered diets based on TEAA levels (Yin et al., 1993). The primary objective of the study described was to determine if reduced CP diets could be commercially formulated and manufactured to an IP ratio based on ileal digestibile values for all 11 EAA, to an accuracy of IP content that would promote levels of NR in growing and finishing boars equivalent to those achieved by boars given diets with normal commercial levels of CP. The hypothesis was tested with diets which were formulated using actual

determined values of IEAA for the raw materials used and were as low as possible in CP but formulated to maintain a level of IP required for optimum performance. The secondary objective was to determine if maximum reductions in Nex, which could be expected when feeding diets based on IP as opposed to CP, could be achieved with the lowest CP diets without reducing the levels of NR.

2. Materials and methods

2.1. Animals A total of nine growing and nine finishing boars of commercial stock, averaging around 58% carcass lean, were used in the study. Their starting weights were approximately 33 and 64 kg, respectively. Prior to these experiments, the boars were housed in groups and fed ad libitum on a commercial grower diet containing 13.75 MJ DE, 24% CP and 11 g total lysine / kg.

2.2. Experimental design and procedure The study comprised two nutrient balance experiments, one with growing and one with finishing boars. In Experiment (Expt.) 1 three diets with either a high, medium or low CP content were offered to growing boars and in Expt. 2, a further three diets with either a high, medium or low CP content were offered to finishing boars. Both experiments followed a complete standard 3 3 3 Latin Square design with three periods and three pigs. The pigs were allocated to the diets within the Latin Square design such that each diet was offered during each period and each pig received each diet only once over the three periods.. The Latin Square was replicated three times with different pigs giving a total of nine sets of data per diet. For the balance periods the pigs were given a 2-day acclimatisation period to the diets followed by a 5-day total collection period. These lengths of acclimatisation and collection were found to be sufficient to reflect the diet changes in previous unpublished work. Faeces were collected daily,

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weighed and stored in air-tight bags at 4 8C. At the end of the 5-day collection period, all the faeces produced by each pig were mixed and representative samples taken for analysis. The urine was collected daily in 25 ml of 25% v / v sulphuric acid to prevent volatilisation of the nitrogen fraction. Sub-samples of the daily collections were stored in air-tight containers at 4 8C until the end of the 5-day collection period when the samples for each pig were mixed and representative samples taken for analysis. The amount of feed offered to the pigs was 95% of the calculated maximum DE intake, on a dry matter basis, calculated from 4.1 3 W 0.5 (determined by the breeding company), where W was equal to the live weight 1 3 (kg) for each pig at the start of each acclimatisation period. Samples of feed, urine and faeces were analysed for dry matter, nitrogen and gross energy (GE) content.

2.3. Experimental diets The aim of the study was to use diets which were formulated using a commercial best cost diet formulation programme working on a set of pre-determined minimum and maximum constraints. There were six experimental diets, three for the growing and three for the finishing boars. For each class of pig there was one diet with a commercial level of CP (GH and FH), one which was as low as possible in CP (GL and FL) whilst maintaining the same level of IP as the high CP diet, and a third which had the same IP content as the other two with a level of CP between the two (GM and FM). A small number only of raw materials were used for which TEAA levels and actual apparent ileal digestibility coefficients for the EAA had been determined. Within the diet formulation programme the minimum and maximum values for DE and ileal digestible lysine (Ilys) were set to be equal, for the remaining IEAA the minimum level was given a value based on the IP ratio and the maximum level was 10% above. The CP was set a maximum level, based on a ratio of CP to Ilys but the minimum level was given no restraint. All other parameters on the diet formulation programme were set as for a commercial diet. Thus the aim was to produce two series of diets which were

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all as close as possible to IP and which differed mainly in the amounts of non-EAA present. The DE and Ilys values used in the formulation programme were 13.9 MJ / kg DE and 10.4 g / kg Ilys and 13.9 MJ / kg DE and 9.2 g / kg Ilys for the grower diets (GH, GM, GL) and finisher diets (FH, FM, FL), respectively. The minimum levels specified for the IEAA to fulfil IP requirements for all diets were calculated from the ratios of the individual IEAA to the Ilys content as follows: threonine 0.65; methionine 0.35; methionine 1 cystine 0.60; tryptophan 0.18; histidine 0.33; leucine 1.10; isoleucine 0.60; phenylalanine 0.72; phenylalanine 1 tyrosine 1.20; valine 0.75. (Eurolysine and Finnfeeds International, unpublished data). The maximum level specified for the CP for each diet was calculated using the Ilys to CP ratio (Ilys:CP) of 0.04, 0.05 and 0.06 for the GH and FH, GM and FM and GL and FL diets, respectively. This resulted in specified maximum CP levels of 260, 208, 173 g / kg for the GH, GM and GL diets and 230, 184 and 153 g / kg for the FH, FM and FL diets. The Ilys:CP of 0.04 for the GH and FH diets was chosen to represent a standard, commercial level of dietary CP. The diet formulation programme was limited to four major raw materials, wheat, wheatfeed, soyabean meal, and skimmed milk. Discrete batches of these raw materials were sampled and the samples chemically analysed for TEAA. Further large samples were fed to pigs to determine the apparent ileal digestibility coefficients for the EAA (courtesy of Eurolysine). The IEAA contents of the raw materials were calculated and the values used in the raw materials database of the formulation programme. All the crystalline EAA that were available commercially, lysine, methionine, threonine, histidine and valine, were included in the raw materials database to be available for inclusion in the diet formulations. On the initial run the formulation programme was unable to reach a solution for the diets with the lower CP restraints, GL, FH, FM and FL with the stringent constraints and the small number of ingredients available to it. To allow a solution to be reached for these diets a kaolinite clay filler was included in the raw materials database. Although this was also available for the formulations of the GH and GM diets the programme did not choose to include it in

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Table 1 The ingredient formulation (g / kg) of the six experimental diets: GH; GM; GL; FH; FM; and FL Raw materials Wheat Wheatfeed Hi-pro Soya Skim Milk Lysine HCL Threonin Methionine Histidine Valine Glycine Soya Oil Tallow grade 6 Kaolinite clay Limestone flour Dical phosphate Salt Vit / TE suppl Total

GH

GM

GL

FH

FM

FL

337.65 310.73 250.00 61.43 0.00 1.02 2.60 0.00 0.00 5.03 6.85 8.18 0.00 6.90 3.28 2.33 4.00

536.13 200.07 220.00 0.00 2.93 2.15 0.83 0.39 0.56 0.00 7.96 8.09 0.00 6.29 7.37 3.23 4.00

534.73 0.00 96.00 183.49 2.30 2.33 1.29 0.64 0.24 0.00 11.72 43.28 99.95 12.71 6.42 0.90 4.00

387.60 299.67 250.00 0.00 0.34 0.83 0.22 0.00 0.00 0.00 4.73 25.27 1.50 16.47 6.24 3.13 4.00

509.77 130.94 215.29 0.00 2.06 1.62 0.62 0.19 0.20 0.00 6.18 48.82 53.03 15.50 8.59 3.19 4.00

577.37 0.00 5.32 235.22 1.94 2.02 1.01 0.49 0.00 0.00 12.17 42.83 99.21 11.76 6.38 0.28 4.00

1000.00

1000.00

1000.00

1000.00

1000.00

1000.00

these diets. The diet ingredient formulations generated by the computer programme are given in Table 1.

3. Statistical analysis Analysis of the data was carried out using an ANOVA for a Latin Square design in the Genstat V computer package. The model has the form Yij 5 m 1 Cj 1 R i 1 T k 1 ´ijk where m is the overall mean variate and ´ijk is the random experiment error and C the period, R the diet and T the pig.

4. Results

4.1. Diet nutrient content The energy contents of the diets are presented as MJ / kg DM in Table 2. The GE contents decreased with the decrease in CP content from high to low across both the grower and the finisher diets, but this was concomitant with an increase in the value of the digestibility coefficients. Although all six diets were formulated by the computer programme to the same level of DE (13.9 MJ / kg), when the DE was converted to a dry matter basis the low CP diets (GL and FL) had lower formulated DE contents than the high and medium. However, the determined DE contents were all within 5% of the target DE value

Table 2 Gross energy (GE), digestibility (D) coefficient and formulated and determined digestible energy (DE) contents of diets J to L offered to growing boars and M to O offered to finishing boars Diets

GH

GM

GL

FH

FM

FL

Energy (MJ / kg dm) GE D Coefficient Determined DE

19.11 0.843 16.11

18.94 0.852 16.14

17.23 0.903 15.56

19.17 0.810 15.53

18.93 0.842 15.94

17.79 0.866 15.41

Target energy content515.98 MJ DE / kg DM.

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calculated as 15.98 MJ / kg on an 87% dry matter basis. From the print-out of the diet formulation programme all the diets fulfilled the minimum and / or maximum specifications for DE, Ilys, CP and IEAA. However, there were differences in the moisture contents of the diets as wider commercial limits had been set on this parameter. The values were 122, 127, 93, 125, 115 and 89 g / kg for GH, GM, GL, FH, FM and FL, respectively. The lower values for the low CP diets (GL and FL) reflected the larger amounts of high dry matter ingredients, e.g. skimmed milk and clay binder in these diets. The determined DE, proximate and mineral analyses for the diets are presented in Table 3. Also

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presented are the IEAA contents taken from the diet formulation programme and the dEB values calculated from the sum of the monovalent ions as follows: (Na g / kg423)1(K g / kg439)2(Cl g / kg435.5)31000 mEq / kg. This parameter did not form part of the formulation programme. As there was a difference in the moisture contents of the diets the values in Table 3 are presented on a dry-matter basis to allow a comparison between the diets. The reduction in CP between the high and the medium and the high and the low diets was 12 and 32% and 13 and 37% for the grower (GH, GM, GL) and finisher (FH, FM, FL) diets, respectively. There was a slight reduction in the level of Ilys in the dry matter content of the two low diets (GL and FL)

Table 3 The nutrient composition (g / kg dry matter) of the six experimental diets Diets

DE MJ / kg CP Lysine b Lysine:DE Lysine:CP

a

Dry matter EE % NDF % Ash % Calcium Phosphorus Sodium Sodium chloride Threonine b Methionine b Met1cys b Tryptophan b Histidine b Leucine b Isoleucine b Phenylalanine b Phe1tyrosine b Valine b

GH

GM

GL

FH

FM

FL

16.1 288 11.86 0.74 0.041

16.1 253 11.86 0.74 0.047

15.6 195 11.47 0.74 0.059

15.5 262 10.47 0.68 0.040

15.9 228 10.37 0.65 0.045

15.4 166 10.12 0.66 0.061

878 4.9 15.4 6.5 4.7 5.6 1.7 4.7

873 4.9 13.8 6.0 4.4 5.0 1.5 4.9

907 7.7 7.1 15.4 9.5 4.3 1.4 5.7

875 6.3 15.4 7.7 7.2 5.7 1.8 4.9

885 8.7 11.3 11.8 6.7 4.8 1.5 4.6

911 7.5 6.8 14.9 9.4 4.2 1.3 5.9

8.55 6.89 10.30 2.94 5.13 16.23 8.81 10.49 18.54 10.11

8.61 4.21 7.46 2.48 4.75 13.43 7.37 9.15 15.55 8.97

8.25 4.9 7.20 2.11 4.58 13.52 6.87 7.93 14.62 8.65

7.55 3.70 7.04 2.67 4.61 14.23 7.86 9.61 16.53 8.86

7.50 3.68 6.60 2.23 4.13 12.30 6.78 8.40 14.28 7.86

7.21 4.47 6.41 1.86 4.02 12.51 6.08 6.90 13.04 7.78

DEB (meq / kg)

310

251

159

259

213

118

Calculated IP

169

169

157

150

148

137

a b

Determined in energy balance studies. Figures are ileal digestible values derived from the computer formulations and taken from the formulation print-out.

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compared with the two high CP diets (GH and FH); this amounted to about 4%. Converting the IEAA to dry matter values resulted in the content of Iphenylalanine (Iphe) of the low CP diets being lower than the minimum levels required to fulfil the specified ratio of Iphe to Ilys of 0.72 for IP. The dry matter values were: 7.93 g / kg (specified 8.26 g / kg) for diet GL and 6.90 g / kg (specified 7.24 g / kg) for diet FL.

4.2. Nitrogen balance The results for the nitrogen balances for the growing boars in Expt.1 are presented in Table 4. The mean starting weight of the boars was 33.0 kg. Feed intakes were similar for all diets with a mean of 1357 g of dry matter per pig per day. This resulted in similar intakes of DE across the three treatments, 20.6, 20.7, 21.4 MJ DE / d (P.0.05). There was a significant reduction (P,0.001) in intakes of nitrogen (NI) as the CP level of the diets decreased, whilst the level of intake of Ilys (IlysI) was maintained over the three diets. There was little difference in daily faecal nitrogen output from the boars when offered either the GH or the GM diets but faecal nitrogen was reduced significantly (P,0.001) when the boars were offered the GL diet, reflecting the increase in digestibility of this diet. Urinary nitrogen content dropped significantly (P,0.001) from the GH to the GM diet but a similar large reduction was not seen between the GM and GL diets. When the data for faecal and urinary NEx are combined, there

were significant reductions in total NEx of 25.9 and 39.3% from the GH to the GM and from the GH to the GL diets, respectively. There was a significant difference between the three diets for urinary nitrogen as a proportion of total nitrogen excreted, but this parameter did not appear to be linearly related to the CP content of the diet. Nitrogen retentions were similar for the GH and the GM diets but this parameter was significantly reduced (P,0.05) for the GL diet. The gross efficiency of NR, defined NR / NI was significantly (P,0.01) improved with both the reduced CP diets (GM and GL) but this improvement was not sufficient to maintain NR per se in the case of the lowest CP diet. The results for the nitrogen balances for the finishing boars in Expt. 2 are presented in Table 5. The mean starting weight of the boars was 64 kg. There were no feed refusals resulting in similar DMIs for all three diets, with a mean intake over the 3-week period of 1956 g per pig per day. As with the growing boars, there was no difference in DEI between the three diets at 29.4, 30.5 and 29.6 MJ DE / d (P.0.05). The NI fell significantly (P, 0.001) from the FH to the FL diet, whilst the IlysI remained similar, as intended. Nitrogen excretion in faeces and urine both fell significantly (P,0.001) with the decrease in CP content of the diet. These decreases resulted in significantly lower (P,0.001) total NEx over the three diets, with reductions of 25.3 and 41.1%, from the FH to the FM diets and from the FH to the FL diets, respectively. These decreases were similar in proportion to those found

Table 4 Intakes, g / day (DMI, NI IlysI,), nitrogen excretion, g / day (NEx-faeces, urine, total) and nitrogen retention, g / day (NR) of growing boars (33–55 kg) offered diets formulated to an ileal digestible Lysine:DE of 0.9, differing in crude protein (CP) content but with similar levels of ileal digestible essential amino acids (IEAA) Diets

DMI NI I-lysI NEx-faeces NEx-urine NEx-total NEx-urine / NEx-total NR NR / NI

GH

GM

GL

S.E.D.

FP value

1330 60.5 15.8 6.9 21.3 28.2 0.75 32.3 0.53

1329 54.1 15.8 6.6 14.3 20.9 0.69 33.2 0.61

1411 45.4 16.2 3.9 13.3 17.1 0.77 28.3 0.62

410.0 1.74 0.93 0.25 0.65 0.74 0.01 1.53 0.020

0.109 ,0.001 0.920 ,0.001 ,0.001 ,0.001 ,0.001 0.022 0.002

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Table 5 Intakes, g / day (DMI, NI IlysI,), nitrogen excretion, g / day (NEx-faeces, urine, total) and nitrogen retention, g / day (NR) of finishing boars (64–94 kg) given diets formulated to an ileal digestible Lysine:DE of 0.8 differing in crude protein (CP) content but with similar levels of ileal digestible essential amino acids (IEAA) Diets

DMI NI I-lysI NEx-faeces NEx-urine NEx-total NEx-urine / NEx-total NR NR / NI

GH

GM

GL

S.E.D.

FP value

1942 81.5 20.4 8.8 27.0 35.8 0.75 45.7 0.56

1961 69.9 20.3 7.1 19.6 26.8 0.73 43.1 0.62

1966 53.6 19.9 5.1 16.1 21.1 0.76 32.5 0.61

480.0 1.65 0.98 0.37 0.70 0.90 0.01 1.58 0.02

0.950 ,0.001 0.802 ,0.001 ,0.001 ,0.001 0.042 ,0.001 0.007

with the growing boars. There was a significant difference (P,0.05) found between the diets for the proportion of urinary nitrogen of the total NEx but, as with the growing boars, this did not appear to be related to the dietary CP contents. Nitrogen retention was maintained with the FM diet but was significantly reduced when the boars were offered the FL diet. The gross efficiency at which the nitrogen was retained (NR / NI) increased significantly with the initial drop in dietary CP content (to the Medium diet) but there was no further improvement when the CP was further decreased (to the low diet).

5. Discussion The objective of the work was to determine if NR of growing / finishing boars could be maintained when offered reduced CP diets that were formulated to all 11 EAA based on actual apparent ileal digestible values. The results from these two balance experiments indicate that high levels of NR can be maintained, in both growing and finishing boars, when offered reduced CP diets which are approximately 12% lower in CP than commercial levels and are formulated to IP using apparent IEAA values which were determined from pig digestibility studies. These results are in contrast to the reduction in NR found in boars offered diets containing similar reduced levels of CP but formulated to IP using TEAA levels (Lee et al., 1993). However, in the current experiments, when the CP levels of the diets

were reduced further, by approximately 34% of the level in commercial diets, to achieve a maximum reduction in Nex, then a reduction in NR was measured. This reduction in NR might suggest that the amount of one or more of the EAA that was available to the pigs was below the optimum requirement in the two low CP diets. This was despite their being formulated using the determined apparent IEAA contents of the raw materials. The experimental hypothesis, that the reduced CP diets (GM, GL, FM and FL) will promote a level of NR equal to that from the commercial, high CP diets (GH and FH), is based on three assumptions: (i) the intake of digestible energy is not limiting to NR; (ii) the reduced CP diets contain the same amount of IP as their respective high CP diets; and (iii) the level of intake of IP is similar for the high, medium and low CP diets. The results obtained would suggest that although these assumptions were probably correct in the case of the medium CP diets (GM and FM), it is likely that one or more were not fulfilled by the low CP diets (GL and FL). The results from the two balance studies show that there were no significant differences between the dry matter intakes of DE for either the three grower or the three finisher diets. It is, therefore, unlikely that energy intake was a factor per se limiting the retention of nitrogen from the GL and FL diets. However, it is still possible that the ratio of IP to DE intake was limiting the retention of nitrogen. In accordance with the specified EAA and nonEAA ratios, dietary IP contains 7% lysine and,

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therefore, the content of IP in a diet can be calculated if the dietary level of lysine is known. As stated earlier, the ratios of EAA to lysine which define IP are dependent on the fact that lysine is the firstlimiting EAA in the dietary protein, and that all the other EAA are present in excess to the level required with respect to lysine. The values for the IEAA contents of the dry matter (Table 3), showed that the content of Iphe for the GL and FL diets was below that required for IP. This indicated that phenylalanine, not lysine, was the first-limiting EAA in these two diets. In order to calculate the IP content of the diets based on the level of lysine as the first-limiting EAA the level of lysine concomitant with the level of the actual firstlimiting amino acid, phenylalanine, must be determined. This calculated lysine value (5dietary phenylalanine content / lysine:phenylalanine of IP) equals 11.01 g / kg for diet GL and 9.58 g / kg for diet FL. The results of these IP calculations, using the calculated Ilys value as above, presented in Table 3, revealed that the level of dietary IP was approximately 8% lower in the two low CP diets than in their two high and medium contemporaries. The assumption of IP equality over the diets was, therefore, not valid despite the fact that the diets had been formulated by a commercial feed formulation programme using data for all 11 IEAA contents based on determined apparent ileal digestibility values. The calculations of the IP intakes of the finisher diets were 291, 290, 269 g / day, for diets FH, FM and FL, respectively. These showed a 7.4% lower IP intake by pigs given the lowest CP diet than either the high or the medium CP diets. This difference in IP intake could account for the lower NR measured for this diet in the balance studies. However, the reduction in NR measured for the FL diet was 27%, approaching four times the reduction in IP intake. Thus this reduction in IP intake may not account for the entire reduction in NR measured. There was no similar reduction in IP intake with the low CP grower diet, 225, 225, 222 g / day, for GH, GM and GL, respectively. Although the IP content of the GL diet was lower than that of the other two diets the DMFI was higher, resulting in a similar IP intake. Therefore a lower intake of IP could not be responsible for the reduction in NR measured from the GL diet. A factor that could affect Nex and, therefore, the calculation of NR, is the electrolyte balance of the

diets. The dEB of the six experimental diets showed a decrease with the reduction in CP level, Table 3. The values for the GL, FM and FL diets were all below the optimum level of 250 mEq / kg (Austic and Calvert, 1981; Haydon et al., 1990). Diets which contain substantial levels of CP content of vegetable origin have high levels of potassium which protect the body against high inherent chloride concentration. However, there was only a small amount of vegetable protein in the GL and FL diets combined with a substantial amount of synthetic lysine–HCl. These two factors together would account for the reduction in the dEB value of these diets. The area of electrolyte balance has been comprehensively reviewed by Patience (1990). Urea, a neutral molecule, is the main excretion product for the nitrogen of excess absorbed amino acids and plays no part in the maintenance of the body’s acid–base homeostasis. Conversely, ammonium ions are excreted to remove excess protons from the body to maintain the acid–base balance (Welbourne et al., 1986). If the metabolic acid–base balance increases in acidity, which could occur if a diet with below optimum electrolyte balance is fed, there will be an increase in ammonium ion excretion (Hannaford et al., 1982). A change in the acid–base balance of the body giving a decrease in urine urea accompanied by an increase in urine ammonia concentration has been shown to occur in the pig (Patience and Wolynetz, 1987). The nitrogen in the ammonium ions is derived from the breakdown of glutamic acid, a non-EAA. If there is insufficient glutamic acid available to meet the demands of this process the body will breakdown other AA for this purpose. The result of feeding a low CP diet with a low dEB value could result in a low calculated NR either from an increase in ammonium nitrogen excretion or a deficiency in EAA required for NR per se. The dEB values of the experimental diets suggest that both the low CP diets (GL, FL) and possibly the medium CP diet offered to the finishers (FM) would produce an acidotic condition in the body when consumed. This could, therefore, account for some of the reduction in the NR achieved with these diets, due to the changes in amino acid metabolism which would occur resulting in a reduction in the levels of some amino acids available for protein synthesis. A shift in the N excretion process from urea N to

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ammonium N would increase the level of N in the urine. This could account for the fact that no decrease in urinary N, reflecting a reduction in excess EAA was found between the medium and low CP diets.

6. Conclusion In conclusion this work shows that formulating reduced CP diets to IEAA, as opposed to TEAA, levels will maintain NR in growing / finishing boars when the reduction in CP is from 24.5 to 21.0% CP and 22.0–19.0% CP (formulated values) for growing and finishing boars respectively. However, if the CP content is further reduced by more than 30% of commercial levels to 18% CP and 16% CP (formulated values) for growing and finishing boars, respectively, the level of NR achieved by the boars is not maintained. This was despite the diets being formulated to actual determined levels of IEAA. The maximum reduction of Nex obtained was about 40% for both the growing and the finishing boars, but NR was not maintained at these levels. It can be concluded that, even with the careful formulation of the diets, it was not possible to maintain NR whilst obtaining the maximum reduction in Nex. It is, therefore, unlikely that diets with the lowest CP levels, giving reductions of Nex in the region of 40%, would be used in a commercial situation.

Acknowledgements We would like to thank the UK Ministry of Agriculture, Fisheries and Food, Dalgety Agriculture Ltd., Eurolysine and Finnfeeds International for their funding of this work.

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