Effect of nitrogen fertilization on chemical composition and rumen fermentation of different parts of plants of three corn hybrids

The experiment compared reductions in the level of N fertilisation of corn crop (i.e., no N fertilisation; 0-N, or 150 kg of N/ha; 150-N) with those currently adopted in the Po valley of North Italy (i.e., 300 kg of N/ha; 300-N) on chemical and nutritional characteristics of grain, stalks and leaves in corn hybrids selected for whole plant silage production (i.e., FAO 700 maturity). Grain composition and starch availability for digestion was examined at three stages of maturity of the kernels (i.e., ½ milk line, ¾ milk line and black layer). One hybrid had a lower grain yield (15.2 versus 16.0–16.8 tonnes DM/ha; P<0.05), while all hybrids were similar in whole plant yield. Levels of N fertilization of 150-N and 300-N resulted in similar yields of grain and whole plants, which were 17 and 33% higher, respectively (P<0.01) than 0-N plots. Hybrids were similar in chemical composition. Reduction of N fertilisation from 300-N to 0-N lowered (P<0.01) the crude protein content in grains, leaves and stalks (P<0.05 in apical part of stalks) and reduced the zein content in grains from 43 to 33 g/kg DM (P<0.01). Vitreousness of grains was not influenced by N fertilisation, while the highest (P<0.01) gas production was at the short incubation time of 2 h for grains with 300-N. Reduction of fertilisation levels (i.e., 150-N and 0-N versus 300-N) lowered (P<0.01) NDF degradation in the basal stalks (0.385 and 0.392 versus 0.412) and leaves (0.604 and 0.620 versus 0.622). Stage of maturity of grain did not interact with the N level of fertilization, and advanced maturity was associated with a marked increase (P<0.01) in zein content from 36 to 49 g/100 g CP and in grain vitreousness from 31 to 40 g/100 g endosperm and with a reduction of the gas production, which had the lowest values at 2 and 48 h (P<0.01) of incubation. Overall, reductions in N fertilisation of corn crop slightly lowered the crude protein content and fiber degradation of different part plants. In grains, the lowest N fertilisation (i.e., 0-N) reduced the zein contents and increased rumen fermentability.