The effect of fertiliser nitrogen on the solubility and rumen degradability of dry matter and nitrogen in wheat grain

Seventy-four samples of wheat grain (c.v. Mercia) grown under a variety of different fertiliser nitrogen (N) regimes were milled through a 3 mm screen. The rumen degradation characteristics of dry matter (DM) and N were estimated using the polyester bag in situ technique and DM and N solubility in water was estimated in the laboratory. The values determined in situ for the immediately soluble (a) fraction for N (16.9 ± 3.81%) and the rate (c) of N degradation (0.128 ± 0.0178 h−1) were both considerably lower than in previous reports in the literature. The N content of the grain exhibited a strong curvilinear relationship with the total amount of N applied as fertiliser. Increasing fertiliser N and the resulting increase in grain N contents tended to reduce and increase (P < 0.01) the immediately soluble and insoluble but degradable N fractions respectively, although the relationships only accounted for about 10% of the variance. For both statistical and theoretical reasons these relationships may not be applicable to other samples of wheat. The work also showed that the water soluble DM measured in the laboratory was about ten times lower than the immediately soluble DM measured in situ. A similar, but less pronounced effect was seen for N where soluble N was on average only about 0.4 of the in situ zero hour loss (21.2 ± 4.49%) and about 0.5 of the fitted a value (16.9 ± 3.81%). This suggests that the in situ technique can seriously overestimate the immediately soluble DM and N fractions of wheat and probably other cereal grains.