Phytasic activity and phytic P content, which are important factors in determining the availability of P, were measured in 56 wheat samples. The different agronomic and genetic factors which could have an influence on these two variables were studied, specifically N and/or P fertilisation, date of harvest, preharvest sprouting and variety of French wheat. Phytasic activity ranged from 206 to 775mUg−1, with an average of 508mUg−1 and a standard deviation of 109mUg−1. Only the variety of wheat appeared as a significant factor explaining the endogenous phytasic activity (P=0·006). The phytic P content varied between 0·92 and 2·80gkg−1 DM, with an average of 2·18gkg−1 and a standard deviation of 0·35gkg−1 DM. None of the factors studied significantly affected the phytic P content of the wheat. This content was correlated with the total P content (r=0·56;P<0·05). The activity of the endogenous phytase was independent of the quantity of its substrate, the phytic phosphorus of the kernels of wheat.

In order to reduce the P concentration in manure, P availability in different plant feedstuffs should be determined. Wheat P availability has been studied by the digestive balance technique on broilers and pigs. Nineteen wheat samples were tested on broilers, giving a mean P retention of 57% (45–70%). P retention was observed to be linearly associated with the endogenous phytasic activity in wheats (r=0·58;P<0·05). This variability in P retention cannot be satisfactorily explained by the phytic P content of wheat. Four wheat samples were tested on pigs, giving a mean apparent P digestibility of 40%.