Manipulating dietary crude protein to reduce nitrogen excreted by pigs

Abstract

In the UK, pollution of the water system with nitrate nitrogen leaching from the soil is seen as a major problem and farm animal effluents have been identified as a major source of nitrate pollution. It would, therefore, be beneficial to the livestock producer and to the environment if the nitrogen excretion from animals could be kept to a minimum. To limit the excretion of nitrogen by the pig, it is necessary to supply amino acids in the diet in better agreement with its dietary requirements. This could be achieved either by feeding diets according to the pig's requirements based on age and/or weight (phase feeding) or by improving protein quality. The best protein quality would be that which has the same balance of essential amino acids (EAA) with respect to lysine as that in ideal protein. Diets formulated on the basis of total dietary EAA on an ideal protein basis, using crystalline EAA, could enable lower crude protein (CP) diets to be offered whilst maintaining nitrogen retention (NR). The majority of the experimental evidence to support the theory of low crude protein (CP) diets comes from either the use of cereals alone with crystalline amino acid supplementation or diets high in maize by-products and low in lysine : digestible energy (Lys : DE) which contrast with the type of diets commonly used in the UK. A series of experiments have therefore been carried out to investigate more thoroughly the possibility of using dietary manipulation, based on diet specifications and ingredients more appropriate to the UK, to reduce nitrogen excretion by the pig, whilst at the same time maintaining the high level of lean gain achieved by UK stock. The object of the present experiment was to determine the effect of isocaloric diets which differed in CP content but had similar levels of essential amino acids (EAA), on nitrogen excretion (NE) and retention (NR) by growing and finishing pigs; these diets being formulated on an ileal digestible EAA (IEAA) basis.