Irrigation and nitrogen fertiliser effects on dry matter yield, water use efficiency and nutritive characteristics of summer forage crops in south-west Victoria

Summer forage crops form an integral component of feed rations to meet the dietary requirements of dairy cows in south-west Victoria. Irrigation of such crops has the potential to increase the availability of feed of high nutritive value. The effect of irrigation strategies and nitrogen (N) fertiliser on forage crop accumulation rates, dry matter yield, water use efficiency and nutritive characteristics was determined at each harvest over 2 summers. The crops used were Hunter ( Brassica campestris L. x Brassica napus L.), Graza ( a complex hybrid of Raphanus sativus L. with introgression from Raphanus maritimus L. and Brassica oleracea L.) and Shirohie millet (Echinochloa utilis Ohwi & Yabuno). Irrigation treatments were dryland control, weekly irrigation to 100% of estimated requirements, weekly irrigation to 50% of estimated requirements and 25% of estimated requirements every second week. Following sowing and after each harvest, N was applied at either 50 or 100 kg N/ha (brassica crops, 3 applications; millet, 2 applications). In both years, fully irrigated crops produced higher dry matter yields than the dryland crops for all species. For Hunter and Graza, full irrigation also resulted in higher dry matter yields than irrigating every second week. Nitrogen at the higher application rate led to higher dry matter yields for Graza in both years and for millet in year 1. Irrigation had inconsistent effects on the nutritive characteristics of all species. Metabolisable energy content ranged from 10.1 to 13.6, 9.8 to 13.3 and 8.2 to 11.3 MJ/kg dry matter for Hunter, Graza and millet, respectively. Nitrogen application at 100 kg N/ha resulted in higher crude protein content for Hunter and Graza at the second and third harvests and for millet at the second harvest at 50 kg N/ha in both years. Water use efficiencies ( irrigation plus effective rainfall) varied according to species with all dryland crops having higher water use efficiencies than the irrigated crops. Total water use efficiencies ranged from 21 to 55, 17 to 39 and 28 to 86 kg dry matter/ha. mm for Hunter, Graza and millet, respectively. In contrast, water use efficiencies from applied irrigation water ranged from 0 to 18 kg dry matter/ha. mm for Hunter, 5 to 18 kg dry matter/ha. mm for Graza and 3 to 33 kg dry matter/ha. mm for millet. Economic assessments indicated average costs for dryland Hunter, Graza and millet to be AU$94, $124 and $76/t dry matter and average costs for fully irrigated crops to be $57, $67 and $51/t dry matter, respectively. This study indicates there is potential to economically irrigate these species to provide additional dry matter of medium to high nutritional value to feed lactating dairy cows through late spring and summer. The data also indicate that for the irrigation of summer forage crops in this environment, the most efficient use of limited water supplies is likely to be a weekly application of water at 50% of the estimated perennial pasture requirements.