Valine requirement for maximal growth rate in weaned pigs

Abstract

The objective was to define valine requirements for weaned piglets in the context of reducing dietary crude protein (CP). A total of 120 pigs were fed six diets with varying content of ileal digestible CP, valine and essential amino acids (EAA). Littermate piglets with initial live weights (±S.E.) of 8.0±0.1 kg were weaned at day 28 and fed with experimental diets for 4 weeks. The highest dietary level of CP, lysine and valine (12.1, 0.82 and 0.59 g apparent ileal digestible per MJ ME, respectively) were in accordance with the Danish Recommendations. Lower levels of CP, valine and EAA were obtained by reduced amounts of fishmeal in the diet formulations, while diets were fortified with crystalline AA. Reducing the CP level did not affect daily gain or feed conversion ratio (FCR). Lowering the valine:lysine ratio from 72% to 59% reduced the daily gain from 457 to 361 g per day and concomitantly raised the FCR from 23.7 to 26.0 MJ ME kg⁻¹. Lowering the supply of CP and essential AA's by 15% reduced the growth rate by 8%. The requirement for valine to support maximal growth amounted to 0.59 g apparent ileal digestible valine per MJ ME.

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.