Effect of nitrogen application on dry matter yields, nutritive characteristics and mineral content of summer-active forage crops in southern Australia

In the dryland areas of southern Australia, summer-active forage crops have been traditionally grown to provide additional home-grown feed during a period when pasture is often in short supply. Dry matter (DM) yields are often variable and the use of nitrogen (N) fertiliser is considered a viable option to increase such yields. The effect of applying N fertiliser to the summer-active species, forage brassica cv. Hunter (Brassica campestris L. x Brassica napus L.), forage brassica cv. Winfred (Brassica napus L.), turnips (Brassica rapa L. cv. Barkant), chicory (Chichorium intybus L. cv. Grouse), plantain (Plantago lanceolata L. cv. Tonic), sorghum sudangrass hybrid cv. Sweet Jumbo [ Sorghum bicolour (L.) Moench x Sorghum sudanese (Piper) Stapf.], millet (Echinochloa utilis Ohwi & Yabuno) and a mixture of Winfred and millet (1 : 5) over two summer periods were measured. N was applied at 0, 40, 80, 120, 160 and 200 kg N/ha with half of the N being applied similar to 5-6 weeks after sowing for all crops and the remainder immediately following the first grazing of regrowth crops and at 9 weeks after sowing for turnips. DM yield responses to applied N were variable, with increases only being observed in the first year. For the regrowth forages (all forages except turnips), the DM yield increase occurred during the second growth period and reflects higher than average rainfall during that period. The crude protein content of all forages generally increased with increasing N application in both years, with the highest response being 0.14% per kg N applied. In contrast, the water-soluble carbohydrate content of forages decreased with N application. The results from this study indicate that the effect of applying N fertiliser to summer forages is limited by available moisture and subsequent active growth of the forage. Where there is adequate moisture, N application can improve DM yield and also the crude protein content of most summer forages. Application of N to summer forages should be seen as a tactical decision based on available soil moisture and active plant growth to ensure N uptake and subsequent DM accumulation.