Ground Cover Supplement : GC Supplement - Crop sequencing
6 Issue 133 | Mar – Apr 2018 | GRDC GROUNDCOVER SUPPLEMENT: CROP SEQUENCING GROUNDCOVER BREAK CROPS FAST FACT Canola is a good break crop for weed management, but legumes are better for soil nitrogen and disease management. Intensification of cereal cropping and a reliance on canola as the break crop of choice has led to a shift in root pathogen and nitrogen status of cropping soils in south-western Australia By Martin Harries n A survey, with GRDC investment, by Western Australian Department of Primary Industries and Regional Development (DPIRD) of 184 ‘Focus Paddocks’ from 2010 to 2014 found that far ming systems in WA have undergone significant changes in the past two decades and these changes are changing paddock biology. Across the south-western agricultural zone the trend has been towards a greater area of crop (87 per cent of the 877 different paddocks over the five years) at the expense of pasture (12 per cent). In cropping paddocks there was a greater number of cereal crops (66.5 per cent of paddocks) and canola (13.5 per cent), at the expense of grain legumes (8 per cent). Wheat was grown in 58 per cent of cropping paddocks, although it LEGUMES’ ROLE IN DISEASE AND N MANAGEMENT IN WA varied across the region, with more grown in the northern agricultural region (68 per cent) compared with the central (58 per cent) and southern (49 per cent). This shift in land use is having a profound impact on soil health and disease risk in the south-western agricultural zone. WEEDS AND DISEASES We sampled soil for PreDicta® B disease testing twice a year and found a steady increase in the incidence and severity of some major root diseases – root lesion nematode (Pratylenchus neglectus), take-all and crown rot – over the five years of the study (Figure 1). The increase in nematodes is a major concern, but not surprising given that 75 per cent of land in the southern region, for example, was sown to canola, barley or wheat, which are all hosts for this nematode. Grain legumes, such as lupins and field peas, provide a good break for these nematodes. Weeds were a key focus for the growers surveyed and were well controlled in most paddocks, but the number of grass weeds increased in wheat crops (Figure 2), with total populations spiralling when wheat was grown too often. Under wheat crops grass weed numbers increased from quite low levels (8.5 plants per square metre four weeks after establishment) to 13.6 plants/m2 in spring. Canola was a good break crop for weed control, with grass weeds being reduced from an average of 24 plants/m2 four weeks after crop establishment to three plants/m2 in spring. SOIL CONSTRAINTS Soil and tissue testing showed that the majority of crops had adequate nutrition. Phosphorus was generally found at high levels in the soil, indicating fertiliser budgets could be used for other nutrients, but in many paddocks the nitrogen balance was negative. Growers who have moved away from legumes need to monitor the nitrogen supply carefully to avoid reducing soil fertility over the long ter m. Soil acidity was a common constraint and is a major limitation to what break crops can be sown. Results of soil cores to one metre depth showed that 70 PHOTO:WADPIRD WA DPIRD Focus Paddock project manager Martin Harries. FIGURE 1 Root lesion nematode (P. neglectus) multiplication rates during the growing season for different land uses. Change in P. neglectus DNA (%) SOURCE: MARTIN HARRIES WheatBarleyCanolaPastureLupinsFieldpeas 250 200 150 100 50 0 FIGURE 2 In wheat crops the average number of grass weeds increased during the growing season, while other land uses reduced weed numbers. Change in grass weeds during the growing season (plants/m2) SOURCE: MARTIN HARRIES WheatBarleyLupinsCanolaOathay 10 0 –10 –20 –30 –40 –50 per cent of paddocks surveyed would benefit from a liming program. We also found that 15 to 20 per cent of canola crops were sown in paddocks where the surface pH was high enough, but the subsoil pH was too low for canola. This highlights the need to sample below 10 centimetres when selecting the best paddocks for acid-sensitive break crops. o GRDC Research Code DAW00213 More information: Martin Harries, WA DPIRD, 08 9956 8553, firstname.lastname@example.org Thank you to WA DPIRD, SARDI, WANTFA, FACEY, Mingenew Irwin and Liebe groups for testing and sample collection.
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