ABSTRACT

Field experiments with break crops in southern New South Wales in the 1980s suggested that wheat growing after Indian mustard (Brassica juncea) yielded more than wheat growing after canola (B. napus), which in turn outyielded wheat (Triticum aestivum) growing after non-Brassica crops. Indian mustard roots are known to contain a different profile of glucosinolates (GSLs) from canola roots. Since GSLs are precursors of isothiocyanates (ITCs) that are known to suppress cereal root pathogens, there may be scope to enhance the biofumigation effects of brassicas. In this study we compiled the results of 26 experiments conducted in southern Australia in the past 10 years to evaluate the break-crop benefit of canola and compare it with Indian mustard. Linola (Linum usitatissimum) was included in 17 experiments as a non-Brassica break crop.

The average yield of wheat growing after canola was 19% greater than wheat growing after wheat, and the yields after Indian mustard and Linola were respectively 20 and 19% greater. The results therefore do not support the hypothesis of a greater break-crop benefit of brassicas compared with non-brassicas nor a consistent difference in the break-crop benefit of different Brassica species.

The average N uptake of wheat after wheat was 95 kg/ha, compared to 112, 113 and 120 kg/ha for wheat after canola, Indian mustard and Linola respectively. The 2-crop N uptake by canola-wheat and mustard-wheat were 17 and 14% respectively greater than by wheat-wheat. Linola-wheat took up less N than the brassica-wheat sequences, because of low N uptake by Linola.

Based on recent grain prices and growing costs, the 2-year gross margin for canola-wheat was $147/ha, or 27% more than for wheat-wheat. Of the additional amount, 27% was due to canola and 73% to the following wheat. Gross margins of the other oilseed-wheat sequences were less than for canola-wheat because of low oilseed yields. The gross margin for canola-wheat would remain more than for wheat-wheat, provided that the canola price was at least 60% more than the wheat price.

INTRODUCTION

In southern Australia, there is widespread infection of wheat with root diseases that cause large yield reductions (Brennan and Murray 1988). These diseases are easily controlled when wheat is grown after a break crop such as canola, and the yield benefit to the following wheat crop has been an important factor contributing to the growth of the canola industry. The magnitude and reliability of the break-crop benefit of canola and the related Brassica, Indian mustard, have not been comprehensively assessed. Indian mustard is currently a minor crop in Australia, but offers promise for semi-arid regions where canola is not adapted.

Control of root disease by brassicas is believed to be due to biofumigation, the term given to inhibition of pathogens by compounds released by root residues. Isothiocyanates (ITCs) are the major group of biologically active compounds that cause biofumigation. They are formed from the hydrolysis of glucosinolates (GSLs) when the plant is injured or matures. Brassicas differ in the composition and concentration of GSLs contained in different tissues (Sang et al. 1984) and the ITCs formed from them have differing toxicities (Angus et al. 1994). Fungi differ in their sensitivity to ITCs; for example, the take-all fungus is more sensitive to ITCs than fungal pathogens of wheat, and 2-phenylethyl ITC is a generally effective inhibitor (Sarwar and Kirkegaard 1998).

The amount and concentration of GSLs in Brassica roots and foliage are under genetic control. Canola and Indian mustard varieties differ in their profiles of GSLs (Kirkegaard and Sarwar 1999), so it should be possible to select or breed for genotypes with roots and/or foliage that inhibit pathogens, while preserving low seed GSLs. However, the in vitro research on biofumigation has been on single compounds, so it is not known whether one combination of ITCs in Brassica tissue would produce better disease control than another combination. It is costly and time-consuming to directly test the hypothesis that breeding will lead to more effective biofumigation. A less costly, immediate but indirect approach is to compare the effect of existing lines of canola and Indian mustard on subsequent cereal yield. The aim of this study was to evaluate the magnitude and reliability of the break-crop benefit of canola and Indian mustard in the range of seasons, soils and pathogens represented in 26 experiments conducted in southern Australia over the past decade. Linola was also assessed as a break crop that did not control root disease by biofumigation.

MATERIALS AND METHODS

We compiled data from 26 published and unpublished field experiments in southern Australia in which wheat was grown after wheat or break crops in the previous year (Table 1). To avoid bias, we included data for all experiments that we had access to. Where several fertiliser treatments were included in an experiment, we selected the most profitable one.

Table 1. Experimental details

The experiments were mostly conducted on farms. In all cases, canola and Indian mustard were grown as well as wheat. Linola was included as a non-Brassica control in 17 experiments. Data on N uptake were available for 15 of the experiments, in most cases only from the seed N content. In order to calculate N uptake, we assumed that 75% of the above-ground N was contained in seed. Gross margins were calculated from equation (1):

Gross Margin = Yield x Price – Variable Costs (1)

The assumed farm-gate prices were $A150/t for wheat and $A330/t for oilseeds. A grain-protein premium and discount of $15/%/tonne was assumed for wheat protein, above and below a 10% benchmark. Variable costs were $280/ha for wheat, $313/ha for brassicas and $250/ha for Linola.

RESULTS

The yields of the break crops and subsequent wheat crops for the 26 experiments are shown in Table 2. The wheat-yield benefit of previous brassicas varied from a doubling of yield at Harden, Lock and Pinaroo in 1996, to yield reductions at Junee Reefs in 1991 and Condobolin in 1992. Of the 26 comparisons, 14 showed a yield benefit of at least 0.5 t/ha for wheat after brassicas, and 10 of the 17 comparisons involving Linola showed a similar benefit.

Table 2. Yield (t/ha) of break crops and subsequent wheat crops.

Table 3 shows that the mean yields of wheat after the three oilseeds were 19.5-22.0% greater than wheat after wheat. The mean N uptakes by first wheat crops, canola and Indian mustard were similar, but Linola took up less N. Wheat growing after the oilseeds took up 18-26% more N than wheat after wheat, and the 2-year total uptake by the Brassica-wheat sequences was 14-17% greater than by wheat-wheat. The N uptake by the Linola-wheat sequence was less than by the wheat-wheat sequence and it is likely that some of wheat-yield benefit of Linola was from residual mineral N.

The gross margin for the canola-wheat sequence was $147/ha or 27% more than the wheat-wheat sequence and had similar variability, even though the yield variability for canola was greater than for wheat. The other oilseed-wheat sequences gave lower gross margins because of the relatively low productivity of these oilseeds, rather than lack of a break-crop benefit.

Table 3. Mean yield, N uptake and gross margins. Coefficients of variation are shown in brackets.

^*Values adjusted where the Linola experiments were unrepresentative of the full data set.

DISCUSSION

This compilation confirms earlier results showing increased yield of wheat growing after Brassica break crops in south-eastern Australia (Angus et al. 1991; Kirkegaard et al. 1994 and 1997). The reason for the wheat-yield advantage appeared to be control of wheat root diseases, particularly when the second wheat crop was grown in a season with high rainfall. In low rainfall environments, wheat after the break crops showed no yield advantage or even reduced yield, probably because the break crop depleted soil water and mineral N, leading to ‘haying off’. With careful management of break crops and selection of the environments in which they are grown, it should be possible to show even greater mean benefits.

While some of the experiments showed significant advantages of wheat after Indian mustard over wheat after canola, there was no average difference over the 26 experiments in the break-crop benefit between the oilseeds. It is possible that the biofumigation effects of the Indian mustard cultivars tested in these studies may be more effective than canola for inhibiting some root diseases of wheat. The lack of a consistent yield benefit to wheat after brassicas compared to wheat after Linola supports the conclusion of Gardner et al. (1998) that, in an annual cropping system, absence of a host is as effective as the active process of biofumigation in reducing wheat root diseases. Brassica break crops appear to control wheat root disease more quickly than non-Brassica broadleaf crops, but in an annual cropping system they are no more effective. The exception may be legumes where there is additional advantage from residual nitrogen. To minimise the complexity of additional N, we chose Linola as the non-Brassica control. However, the break-crop benefit of Linola roots may be due to some disease-inhibition because they contain cyanogenic glycosides, which hydrolyse to form cyanide (Oomah et al. 1992).

The increased N uptake by wheat after the oilseeds also confirms earlier studies, but shows no advantage of Indian mustard over canola. There was also more N uptake by wheat after Linola, reflecting the low N uptake by Linola. The most surprising result in the study was the large N uptake in the 2-year Brassica-wheat sequences. One possible reason is more N mineralisation from decomposition of soil microorganisms (Severin and Förster 1988). This process may be an additional aspect of biofumigation. Another reason is more complete extraction of soil N by healthy cereal roots, as shown by Angus et al. (1991) and Kirkegaard et al. (1994).

While yield benefits of break crops have been reported in other regions, it is unknown whether the magnitude is as large or reliable as those reported here. It may be that the environment of southern Australia is particularly conducive to soil-borne diseases of wheat crops because the soil water and temperature during autumn and spring are often optimal for infection, and the generally dry soil in summer allows survival of inoculum.

For the yields presented in Table 2 and the assumed prices and costs, the average 2-year gross margin of the canola-wheat sequence was $147/ha greater than the wheat-wheat sequence (Table 3). Of that amount, $39/ha came from the canola and $108/ha from the subsequent wheat. In order to test the sensitivity of the 2-year gross margin to the assumptions, we varied the canola and wheat prices in equation (1). The results indicate that the canola-wheat sequence has the higher gross margin provided the canola price is at least 60% higher than the wheat price.

ACKNOWLEDGEMENTS

We are grateful to Rex Oram for continued encouragement and support during the course of these studies, and to the landholders who gave us access to their paddocks. Funding support was provided by the Grains Research and Development Corporation.

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