Ground Cover Supplement : GC Supplement - Crop sequencing
8 Issue 133 | Mar – Apr 2018 | GRDC GROUNDCOVER SUPPLEMENT: CROP SEQUENCING GROUNDCOVER PULSES Simple ‘rules of thumb’ allow growers to predict the nitrogen benefit for their specific situation By Dr Mark Peoples n Growers with cereal-based crop sequences sometimes view legume break crops as too risky for the income they generate. However, a five-year research project across south-eastern Australia found that the benefits of including a legume break crop are more significant than anticipated. It found that legume break crops are as profitable, and in many cases more profitable, than wheat based on the grain prices and growing seasons experienced during the project, from 2010 to 2015. The GRDC research investment was led by CSIRO in conjunction with the NSW Department of Primary Industries, Agriculture Victoria, the South Australian Research and Development Institute and seven grower groups across the low, medium and high-rainfall zones and irrigation areas. By combining the data from numerous researcher and on-far m trials they found that cropping sequences that included at least one legume break crop were more productive and profitable than continuous wheat when best management practices were used. NITROGEN BENEFIT Without an easy way to predict the nitrogen benefit from including a legume break crop growers tend to focus on the simpler question ‘How much money can I earn in the year of the break crop?’ Yet the results from these trials, in combination with additional findings from more than 25 years of legume nitrogen research, revealed that growers often underestimate the nitrogen boost that legumes provide. Soil mineral nitrogen (nitrate and ammonium) measured in autumn to a depth of 1.2 metres (the expected rooting zone of a wheat crop) was, on average, 35 kilograms of nitrogen per hectare higher following a legume SEQUENCING TRIALS TALLY LEGUMES’ NITROGEN BENEFIT crop than following a wheat or canola crop. Results were even higher when the legume was brown manured. To help growers estimate the value of soil nitrogen benefits researchers have developed some potential predictive relationships that growers and their advisers can use as rules of thumb to assist in nitrogen decision-making when sowing a cereal crop after legumes (see ‘Rules of thumb’). As most growers routinely measure rainfall and grain yield the relationships 0.15kg of nitrogen per millimetre of fallow rainfall, and 18 × legume grain yield (tonnes per hectare) are perhaps the simplest for growers to use. The most reliable predictions were based on legume residue nitrogen. The analyses indicated that it explained the largest fraction (57 per cent) of the observed variation. However, residue nitrogen is a particularly difficult parameter for growers to measure directly. But since grain yield is usually closely related to above-ground residue biomass (shoot residue dry matter as about twice the legume grain yield), it was argued that grain yield might also provide a guide to the amount of total residue nitrogen remaining at the end of the legume growing season. Further analyses of experimental data confirmed this relationship and estimated the additional mineral nitrogen as 0.28 × [54 + (30 × tonnes of legume grain harvested per hectare)]. RECOVERY Subsequent wheat crops recovered an average of 30 per cent residual legume nitrogen, which is comparable to the amount recovered from fertiliser nitrogen applied at sowing, but lower than top-dressed fertiliser nitrogen (65 per cent of the 51 to 75kg fertiliser-N/ ha) applied at stem elongation just prior to the peak period of nitrogen demand. However, unlike fertiliser nitrogen, legumes contribute to a large pool of organic nitrogen that continues to benefit more than one subsequent crop and sustains soil fertility in the long term. o GRDC Research Code CSP00146 More information: Dr Mark Peoples, CSIRO Agriculture & Food, 02 6246 5447, email@example.com CSIRO’s Tony Swan compares the growth of wheat following either a previous wheat (left) or field pea crop (right) in the project’s Birchip Cropping Group trial at Hopetoun, Victoria. PHOTO:MARKPEOPLES RULES OF THUMB Rules of thumb for estimating the additional mineral nitrogen after legume break crops: n 0.15kg N/ha per millimetre of summer fallow rainfall; n 18kg N/ha per tonne of legume grain harvested per hectare; or n 0.28 × [54 + (30 × legume grain harvested (t/ha))].
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