Effects of source and content of ash in poultry litter used in diets for beef cattle

Abstract

Two experiments were carried out in order to study the effect of the ash content of poultry litter (PL) on its nutritional value. In Expt 1, broiler PL was incorporated in the proportions of 30 and 45% into diets based on wheat straw supplemented with 20% maize grain. The PL was either fresh (containing 19% ash) or had been heated in a stack (giving 26% ash). There were also two additional diets containing 45% fresh PL to which either ground limestone or wood ash had been added to provide the same ash content as the diets containing 45% stored, high ash PL. An in vivo digestibility and nitrogen balance trial was carried out. Storing the PL, which resulted in heating and an increase in ash content, reduced dry matter intake significantly. The largest and most significant effect on the digestibility of dry matter, organic matter and crude protein can be ascribed to the freshness of the PL. Stored, heated PL had a highly negative effect on crude protein digestibility and on nitrogen retention. No negative effect of adding extrinsic ash was recorded. In Expt 2, the nutritional value of layer PL (LPL), compared with that of broiler PL (BPL), was examined using ten beef cows before and after calving. The diet was composed of 45% PL, 35% wheat straw (WS) and 20% maize grain. Daily dry matter intakes were 12·7 and 16·2 g/kg liveweight (LW) before calving and 15·5 and 22·8 g/kg LW after calving, for cows fed LPL and BPL, respectively. Owing to the combined effect of the lower dry matter intake, lower gross energy content and lower digestibility of LPL, the digestible energy intake of BPL diets was almost three-fold, and nitrogen absorption (g/kg LW0·75) on the BPL was 2·4-fold higher than on the LPL diets. Because the ash intake of both diets was similar, it is suggested that the main inhibiting factor of intake after calving was the rumen's ability to mobilize the large amounts of ash-rich complexes of ash and organic matter to the reticular omasal orifice and, consequently, out of the rumen.