The constraining effect of feed bulk on the voluntary feed intake of laying hens.
1. Two experiments were designed to determine a suitable method of measuring and predicting feed bulk, such that this could be used to predict when the feed intake of a laying hen would be constrained by feed bulk. 2. In the first trial the diluents used were cellulose, plasterer's sand, sunflower husks, sawdust and vermiculite. These were included at 100, 250 and 500 g/kg into a commercial layer feed which was used as the basal feed. The trial was divided into three phases of 21 days each. After each phase, either the diluent fed was changed, or the inclusion level of the diluent was changed. 3. It was observed that as the water-holding capacity (WHC) of the feed increased, the feed intake decreased. The scaled feed intake (SFI) of the hens was fitted to the reciprocal of the WHC to give the relationship; SFI (g/kg body weight) = 313.6 (±8.9) x 1/WHC. This regression was the best fit and represents the maximum amount of feed that the laying hen can consume when the constraint measured is the reciprocal of WHC. 4. Trial 2 identifies the physical characteristics of the feed that best describe the bulkiness of the feed, and also determined the extent, and the rate at which, the laying hen can adapt to feeds that are high in bulk. The five diluents that were used were wheat bran, river sand, potter's clay, unexpanded polystyrene and sawdust, and the inclusion rates were 50, 100 and 150 g/kg. The hens were fed the feeds for six weeks. The equation from Trial 1 was fitted to the data from Trial 2 and few treatments were found to be constraining. 5. The constraining feeds from both trials and Williams (1993) were combined to obtain a more accurate assessment of the relationship between the SFI and the reciprocal of WHC. This relationship was represented by the equation; SFI (g/kg body weight) = 301.4 (±8.9) x 1/WHC. 6. The prediction of the effect of feed bulk on the voluntary feed intake of the hen is an important aspect for accurately predicting the feed intakes of the hen and formulating a "perfect" diet. The variation in constrained intakes was not accurately predicted by the WHC of the feed, although this measure of bulkiness was considerably better than any of the other measures applied.