Valine requirement for maximal growth rate in weaned pigs
Introduction
During the last decade the excretion of nutrients (especially nitrogen and later phosphorus) and the impact on the environment have become a point of focus. The most potent way of reducing the nitrogen excretion is by reducing the level of crude protein (CP) in the pig diets and adding crystalline amino acids (AA) in the diet formulation to obtain the ideal AA profile Dourmad et al., 1999, Everts, 1994. Feeding growing pigs with a diet supplying an optimal AA composition (i.e. without excessive amounts of non-limiting AA's) will improve the utilisation of dietary protein by pigs and limit the incidences of diarrhoea (Eggum et al., 1987). As a consequence, the feed utilisation will be improved, and concomitantly the nitrogen wasting will be reduced Dourmad et al., 1999, Everts, 1994. From an environmental/economical/practical point of view it was of interest to study the response of pigs fed with a low protein diet supplemented with only first-limiting AA (lysine, threonine, methionine+cysteine, tryptophan), which are economically available.
According to the current Danish recommendations for nutrients for pigs (The National Committee for Pig Breeding, Health and Production, 2002), diets for weaned pigs (5–8 weeks of age) ought to supply 11.9 g CP, 0.82 g lysine and 0.59 g valine expressed as faecal digestible amounts per megajoule (MJ) of ME to ensure maximal weight gain. If the diet to weaned pigs is formulated to contain less CP than recommended and supplemented with crystalline AA (lysine, methionine, threonine, tryptophan), then it is expected that one or more of the secondary AA will become the limiting factor for growth. In most cases valine will be the first limiting AA, but leucine will follow closely.
The recommended level of CP is included in the Danish recommendations to ensure a sufficient supply of secondary AA (isoleucine, leucine, valine, histidine, phenylalanine and tyrosine), because at present they are too expensive to add as synthetic supplements. As a consequence, pigs are normally fed with excessive dietary protein. The excessive supply of protein consists of both essential amino acids (EAA; mainly secondary AA) and non-essential AA. AA originating from excessive dietary protein are oxidised, but this results in a lower net energy value than oxidation of dietary fat and carbohydrates, because substantial amounts of metabolisable energy are excreted as urea in the urine. Just (1982) found that growing pigs (20–90 kg) lost 4.9 kJ in the urine and utilisation of ME for production decreased by 6.6 kJ for each gram of catabolised protein. As a corollary, the highest efficiency of utilising ME for production and thereby the lowest feed conversion ratio (FCR) would be obtained by minimising the excessive amounts of protein in the diets.
Recently, the valine requirement was investigated for young pigs in the intervals of 5–10 and 10–20 kg (Mavromichalis et al., 2001). The valine requirement was found by broken line analysis and reported as 0.60 and 0.53 g ‘true’ ileal digestible valine per MJ ME, respectively, for the two weight intervals. Expressed on an apparent ileal digestible basis, this corresponded to 0.55 and 0.49 g apparent ileal digestible valine per MJ ME according to NRC (1998). These values agreed fairly well with NRC (1998), which recommend 0.54 and 0.46 g apparent ileal digestible valine per MJ ME for 5–10 and 10–20 kg pigs, respectively. It is generally agreed that the supply of AA should be evaluated on an ileal digestible basis (Mosenthin et al., 2000). We chose to express the digestibility on an apparent ileal basis, and the term digestibility will refer to this basis throughout the paper, if nothing else is mentioned. Here it should be emphasised that the Danish Recommendations are at present expressed as apparent faecal digestible amounts. In our case the diets were formulated on apparent ileal digestible basis, under the assumption that the amounts of AA supplied were equal to or greater than the Danish Recommendations (The National Committee for Pig Breeding, Health and Production, 2002).
The present study was undertaken in order to elucidate the adequacy of the current Danish recommendation for CP and valine for weaned pigs. A growth trial was performed to evaluate the effects of (1) reducing the dietary protein, (2) reducing the dietary valine or (3) reducing all EAA's relative to the supply of net energy on feed intake, weight gain and FCR.
Section snippets
Experimental plan
The experimental set-up consisted of six diets (Table 1) aiming at studying (1) the effect of reducing the dietary protein from 12.1 to 11.3 and 10.5 g apparently ileal digestible per MJ ME (diets 1, 2 and 3) without reducing the supply of EAA, (2) the effect of reducing the level of valine (100, 90 and 80% relative to diet 3: diet 3, 4 and 5), and (3) the effect of reducing the dietary protein level by 15% while maintaining the EAA profile (diet 1 and 6) or reducing the supply of all EAA by
Results
The analysed chemical composition of the experimental diets is given in Table 3. The analysed nutrient content was in accordance with the expected values given by the formulation. Consequently, it is assumed that the dietary levels of ileal digestible protein and ileal digestible AA in relation to ME levels are also in accordance with the formulation. The values of digestibility presented in Table 3 are a true expression of the experimental plan.
In general, the pigs were healthy throughout the
Discussion
The amount of digestible CP is included in the Danish recommendation to ensure an adequate supply of secondary limiting AA, which, at present, are too expensive to achieve by supplementation. Reducing the dietary level of CP from 12.1 to 10.5 g digestible CP per MJ ME did not affect the performance of the pigs, even though the protein supply was lower than the Danish recommendations (diets 1, 2, and 3). The reason was that when the CP level was reduced, the supply of all EAA was maintained by
Implications
Diets based on barley, wheat and soy protein concentrate have a low content of EAA relative to non-essential AA. Consequently, the level of dietary CP for pigs can be reduced with no negative effect on the performance, as long as adding crystalline AA to the diet formulation ensures the EAA supply. Further reduction of dietary CP is possible with marginal limitations of pig performance. Valine was found to be the first limiting AA for growth rate in young pigs after lysine, methionine and
Acknowledgements
We gratefully acknowledge the financial support of this study from Orffa Skandinavien A/S/Ajinomoto Eurolysine.
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