In vitro determination of the ileal digestibility of protein and amino acids in New Zealand barleys : a thesis presented in partial fulfilment for the degree of Master of Agricultural Science at Massey University, Palmerston North, New Zealand
The aim was to evaluate a recently developed in vitro digestibility assay for predicting the apparent ileal digestibility of protein and amino acids in barley, and secondly to evaluate the statistical prediction of apparent ileal digestibility of protein in barley based on chemical and physical measurements. Seventeen barleys were collected from six growing regions from throughout New Zealand in 1995. Ten of these were selected to provide a range in crude protein from 8.5 to 13.3% (DM basis). The ten barleys were subjected to several physical and chemical measurements, and to the in vitro assay. The barleys were given as sole sources of protein to growing rats (n=6) and ileal digesta were collected at slaughter and nitrogen and amino acid digestibility determined with reference to the marker, chromic oxide. Six of the barleys were treated with O-methylisourea to convert lysine to homoarginine, a synthetic analogue of lysine, to allow determination of endogenous ileal lysine and protein flows. Mean in vivo apparent ileal digestibility of nitrogen ranged from 71.4% to 80.3%. In vivo true lysine digestibility ranged from 73.2% to 100% while endogenous protein loss ranged from 6.4 to 44.8 g/kg DMI. Physical measures made on the barley included grain bulk density (kg/hecto litre), screenings (%) and 1000 seedweight (g) and were highly variable. They provided no significant (p>0.05) predictive ability for protein digestibility or endogenous ileal protein loss. Chemical measures included CP (%), NDF (%), ADF (%), lignin (%), total β-glucans (%) and gastro-intestinal (GI) extracted β-Glucans (%) and were also highly variable. True lysine digestibility was able to be predicted based on the levels of GI extracted β-glucans and crude protein (r2= 0.97). In vivo endogenous ileal protein loss was predicted based on total β-glucans (r2= 0.77). In vitro protein digestibility was not significantly correlated with in vivo values. The in vitro technique requires more development before it can be used for the routine evaluation of digestible protein in barleys.