Effects of irrigation and rates and timing of nitrogen fertilizer on dry matter yield, proportions of plant fractions of maize and nutritive value and in vitro gas production characteristics of whole crop maize silage

The objective of this study was to investigate the effects of contrasting levels of irrigation water and rates and timing of N fertilizer on yield, different fractions of maize plant and their subsequent effects on nutritive value of maize silage. Hybrid forage maize (Pioneer 31H50) was grown at Camden, Australia with two rates of pre-sown (0, 135 kg/ha), three post-sown (0, 79, 158 kg/ha) N fertilizer and four levels of irrigation (0, 153, 305, 480 mm). Each treatment was replicated four times (blocks) with n equals 96 (2 × 3 × 4 × 4 plots of 3.5 m × 3.5 m). Maize was harvested at physiological maturity, chopped at a particle length of 2.5 cm and ensiled in micro-silos. Silage samples were analyzed for dry matter (DM), pH, chemical composition, and metabolizable energy (ME) content. Gas production was measured up to 48 h of incubation and then pH of in vitro fermented end products was recorded. Additional plant samples from each replication were fractionated into leaf blade, leaf sheath, stem, cob structure and grain. Increase in irrigation water (0–480 mm) increased DM yield from 9.3 to 23.8 t/ha. Increase in irrigation also increased grain from 92 to 315 g/kg DM but decreased stover of plants from 907 to 685 g/kg DM. For silage, increase in irrigation increased neutral detergent fiber (NDF) from 524 to 555 g/kg DM, but decreased crude protein (CP) from 78 to 52 g/kg DM and water soluble carbohydrate (WSC) from 88 to 31 g/kg DM, which resulted in a decrease in ME from 9.82 to 8.81 MJ/kg DM. In contrast to irrigation, application of post-sown N fertilizer increased CP from 57 to 67 g/kg DM and ME from 9.03 to 9.47 MJ/kg DM. There was also an irrigation × pre-sown N fertilizer interaction for NDF and ME contents, but effects of these interactions were minimal compared to the main effects. Overall, increasing irrigation water had a negative effect on the nutritive value of silage by increasing NDF and decreasing CP and WSC and therefore, ME content. This was despite a substantial increase in grain in irrigated treatments. In contrast, application of N fertilizer, in general, increased ME content of silage, due mainly to an increase in CP content. Results indicate that maximization of forage maize yield through increased application of N and water may be compromised by a decreased nutritive value of the subsequent silage.