NORTHERN Australian graingrowers need to be aware that the inability of canola to host a beneficial root-dwelling fungus could have a detrimental effect on yields in the following wheat crop. However, trials in the southern wheatbelt are showing quite a different picture.
Arbuscular mycorrhizal fungi (AMF) are naturally occurring beneficial fungi that live in the roots of host plants. They send out hyphae (hair-like structures) from the roots into surrounding soil and can access nutrients such as phosphorus and trace elements, especially zinc. Studies have shown that, for each I cm of a plant root containing AMF, there can be 100 cm of hyphae attached.
At the recent GRDC-supported International Conference on Mycorrhizas in Adelaide, John Thompson of the Queensland Department of Primary Industries said earlier research had shown that clean fallows reduced AMF levels because the fungi needed living roots for their survival.
He said phosphorus and/or zinc deficiencies could result in following crops with a drop in AMF populations. Knowing that canola was a non-host of AMF and that interest in growing this oilseed was increasing in the north, Dr Thompson and his DPI colleagues set out to determine its effect on following wheat yields in their Darling Downs trials. Trial soils were dark cracking clays, also known as black earths, vertisols, or simply black soil.
In 1999 they planted 14 varieties of eight crop species, including AMF-host and non-host types and then in 2000, before sowing Batavia wheat across all the plots, sampled this alkaline soil for AMF and nutrient levels.
"Growing canola or having clean fallows instead of growing other crops species in 1999 resulted in lower numbers of AMF inoculum in the soil and in poorer AMF root colonisation of the 2000 wheat crop," Dr Thompson said.
''The largest difference was 0.8 tlha of wheat after Karoo canola compared to 1.9 tlha after a Yallaroi durum crop. The average wheat yield following all five canola varieties grown was 1.1 t/ha.
Strategy to deal with AMF decline
"What this trial shows is that, in the north, farmers growing canola have to be mindful of the likely run-down in AMF numbers and the potential yield-decreasing effects on the following wheat crop."
After growing canola, before the following crop, growers should make sure that their soil's phosphorus and zinc status is high, and add extra fertiliser if necessary.
However, in southern trials ...
Further south at Temora, NSW, and in the Victorian Wimmera, Megan Ryan of CSIRO Plant Industry and colleagues have conducted similar research spurred by the huge increase in area sown to canol a and the wheat--canola rotations being used by some farmers. They found reduced levels of AMF colonisation after brassicas or fallows had little effect on yield of wheat or field peas.
At Temora in 1999 she obtained high AMF levels through growing the host plants Linola and a pasture legume while AMF numbers were reduced through growing the non-host canola, and the establishment of a chemical and mechanical fallow. There were no root diseases such as Rhizoctonia and take-all present and so no opportunity for the biofumigation effects of canol a to come into play on these diseases.
In 2000 she sowed wheat and field peas over the 1999 treatments. There were two phosphatic fertiliser levels on each site ~ nil and 20 units Plha as triple superphosphate. Soil-available phosphorus was very low, around 10 mg/kg (Colwell) and crop biomass was greatly increased by the superphosphate.
"I expected that I would get increased wheat and field pea yields where AMF host crops had been grown the previous year but that didn't happen," Dr Ryan said.
Yields stay same across treatments, zinc levels up with AMF
"Wheat and field pea yields were basically the same across all treatments; if anything, they were higher after the fallows. The responses to high AMF colonisation were similar for wheat and peas with no effect on crop biomass or crop phosphorus uptake at any stage during season at either P level.
"What I did find, however, was that zinc concentrations in wheat grain grown after Linola and pasture increased by about 2S per cent at both P levels, providing grain of greater nutritional value.
"Similar results were obtained from trials where wheat was grown after AMF hosts, non-hosts such as canola, and fallows over a number of years at Longerenong in Victoria. Thus it seems that AMF playa far greater role in crop nutrition and growth in the northern wheatbelt than in the southern wheat belt."
Why the difference in AMF effects?
Dr Thompson and Dr Ryan say the reasons for the differing results between north and south are still speCUlative. Dr Thompson said a paper from Denmark at the Adelaide conference indicated that the AMF hyphae were rather restricted at soil temperatures below l5°C, leading to more measurable benefits in the north.
Dr Ryan said crops tended to grow more quickly in sub-tropical conditions and probably had a great demand for phosphorus early in the growing period, which was met by the synergistic relationship between AMF and host plants.
She said these results suggested that, in the southern grainbelt, low AMF numbers induced by canola crops were unlikely to effect phosphorus uptake or yield in wheat and, in southern NSW, in legumes.
"Farmers in the south experiencing poor growth of wheat after canol a should consider factors other than AMF. In particular they should be aware that modern phosphatic fertilisers have very low levels of zinc contamination compared to those in the past. Canol a uses a lot of zinc and thus much zinc is removed in canola grain depleting many soils.
"In addition, while canola does not host many of the root diseases in wheat, it will host the root lesion nematode Pratylenchus neglectus. Sulphonylurea herbicides may also affect the ability of roots to take up phosphorus and zinc."
Research supported by growers and the Federal Government on these relationships is ongoing.
Program 4.1.4 Contact: Dr John Thompson 07 4638 8806 or Dr Megan Ryan 02 6246 4911 email email@example.com or firstname.lastname@example.org