Efficient use of water on spring sown forage brassicas

In the cool temperate dairy regions of Australia, irrigation is used during spring and summer to increase feed supply and milk production. This use of irrigation produces low returns per megalitre of water, and with increasingly restricted water supply the dairy industry is at a competitive disadvantage compared to other agricultural sectors. Therefore the industry needs to improve feed production from the amount of irrigation water available. The potential of four brassica forage species, turnip (Brassica rapa var. rapa), pasja (Brassica campestris x B. napus), kale (Brassica oleracea), and rape (Brassica napus var. napus) to increase the water use efficiency (WUE) of feed supply under irrigation, without negatively affecting yield or nutritive value, was examined under varying irrigation regimes and rainfed conditions. Turnip was the highest yielding species at all irrigation levels. Its maximum yield under 100% irrigation was 15.4 t DM/ha which compared to the yields of pasja of 9.2t DM/ha, rape of 7.2 t DM/ha and kale of 5.4 t DM/ha. The utilisable dry matter (DM) yield of each species was found to be proportional to the amount of water applied prior to each harvest. The WUE (kg DM/ha/mm) varied significantly, both between species and harvests. Turnip had the highest WUE of 44 kg DM/ha/mm, at final harvest. Pasja and rape both has WUE of around 20 kg DM/ha/mm and kale 14 kg DM/ha/mm. The WUE of all species increased with time during the growing season. There were some differences in the nutritive value of the brassica species. All species had a high DM%, which decreased with increasing irrigation rates. The species were all high in energy, with metabolisable energy (ME) levels greater than 89%. Crude protein (CP) contents were marginal for animal production, especially in turnip bulbs where values were found to be around 14%. The level of fibre in the species was relatively low at approximately 22% in the shoots, but this should not present any problem provided that the crop was fed as part of a diet with another fibre source as is the usual practice. Overall it was concluded that there were no practical nutritional differences between the species with all species being considered suitable to maintain summer milk production. Turnip has a large bulb, a storage organ, which is not present in the other forage brassica species. The bulb is a major reason for the increased yield and superior WUE of turnip. Bulb expansion was most likely associated with the transfer and storage of plant reserves from the senescing leaves, rather than from current plant photosynthesis. This provides turnip with a photosynthetic advantage over the other species. The bulb may also buffer the water supply to the shoots of the plant enabling it to continue transpiration for longer than the other species. Monitoring the growth and development of turnip identified three growth phases: firstly, a period of slow growth after emergence; secondly, a period of exponential growth of shoots and bulb that produced a large increase in DM yield; and thirdly, a decline in overall growth rate, when bulb growth rate was maintained at a relatively constant level but leaf growth rate declined. The second growth phase was the period of maximum growth (up to 460 kg DM/ha/day) and water use efficiency (48 kg DM/ha/mm). Senescence of the cotyledons provided a simple visual indicator of the transition from the first to the second growth phase. This change also corresponded to accumulation of approximately 600 day-degrees. Identification of this change in growth stage could provide the opportunity for improved management, by targeting irrigation to achieve maximum turnip growth rate and water use efficiency.