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Crop rotation for the control of wild oats in wheat

C.E. Jones

Department of Botany, University of New England, Armidale NSW 2351

Summary. The germination, growth and seed production of wild oats can be selectively stimulated in the second and subsequent years of continuous wheat monoculture by the presence of small amounts of stubble and root residues from a previous wheat crop. This effect would not be manifested if wheat were grown in rotation with a crop species such as sorghum, which has an inhibitory effect on wild oats. The presence of a sorghum crop over summer can delay and reduce wild oat emergence in the following winter phase. Rotational cropping of wheat with sorghum offers a viable strategy for wild oat control which has economic as well as weed control benefits.

Introduction

Wild oats are probably the most prevalent and serious weeds of wheat in Australia (2,5,17). Avena sterilis ssp. ludoviciana (sometimes still referred to as A. ludoviciana Dur.) is the predominant wild oat species infesting cereal crops in northern NSW and southern Queensland, whereas A. fatua predominates in the southern and western areas of the continent (4,8,16).

The published results of competition and herbicide trials reveal that wheat grain yield losses in Australia due to the presence of wild oats range from nil to 75% (19) depending on the level of infestation and environmental conditions (3). In commercial practice, grain yield losses of 100% are incurred when farmers plough heavily infested crops back in, or convert them to green feed, hay or silage. Wheatgrowers in the northern cereal-belt of NSW rank wild oats as by far their worst weed problem, and lose an estimated $3.6 million annually in reduced wheat yields despite the $5-10 million spent on wild oat herbicides in this small region alone (6). In addition, the wild oat population can increase exponentially and become uncontrollable after five years of continuous cereal monoculture (6).

The pattern of an exponential increase in wild oat density with continuous winter wheat cropping has been documented by others (7,20), with the use of herbicides serving only to slow the rate of population change, not to reverse it (7,18). The selective pre- and post-emergent herbicides available in Australia provide some degree of yield salvage in the year of use by reducing the competitive ability of wild oats, but none have long term benefits as seed production cannot be totally prevented and reinfestation occurs in subsequent years.

In a field experiment designed to assess the effects of retained crop stubble on weed growth, the post-emergent growth of wild oats was found to be increased by a factor of 10 and seed production by a factor of 42, in the presence of wheat crop residues (12). This raised the possibility that an allelopathic mechanism may be contributing to the success of wild oats in continuous wheat systems, and in subsequent glasshouse tests the amendment of soil with water leachates from wheat stubble (i.e. no physical presence of stubble) significantly stimulated the rate and the percentage of wild oat germination and the rate of post-emergent growth (11). When wheat and wild oats were grown together, the addition of 0.5% w/w finely milled wheat stubble to the soil increased the seed production of the wild oats by 66%, but had no effect on the seed production of wheat (9).

The experiments reported in this paper were undertaken as part of an investigation to determine whether any crop species were potentially antagonistic to wild oats.

Methods

Glasshouse

Stubbles from mature harvested crops of sorghum, Sorghum bicolor (L.) Moench cv. Goldrush, sunflower, Helianthus annuus L. cv. Hysun 31, canola, Brassica napus L. cv. Marnoo, wheat, Triticum aestivum L. cv. Kite and field pea, Pisum sativum L. cv. Full-pod were dried at 80C in a forced-draught oven for 1 h to facilitate milling, and ground to pass a 3 mm mesh in a laboratory mill. Soil of a type classified as Australian Great Soil Group Chocolate, sub-group Normal (14), with organic matter 4.5%, total soil nitrogen content 0.23%, total soil phosphorus level 0.18%, and pH 6.0 (1:5, soil:water), was collected from the University of New England Field Station at Laureldale, near Armidale, NSW. Four replicate 7 g subsamples of each of the milled stubbles were blended with 1400 g sieved dry field soil (the rate of 0.5% w/w is equivalent to 5 Oa stubble incorporated to a depth of 10 cm in the field), and 25 even-sized primary seeds of A. sterilis with grey lemmas (Munsell Colour 10 Y/R 3/3) were sown equidistantly at a depth of 2.5 cm in each 15 cm pot. The pots were arranged in a randomised block design and initial and subsequent water additions made to field capacity (42% w/w). The number of wild oats emerged was recorded daily up to day 16, when seedlings were thinned to five per pot. Leaf and tiller number were recorded twice per week from day 16 through to harvest on day 44, when plants were washed from the pots and separated into tops and roots. The weight of these components was determined after drying for 48 h at 80C.

Field

The field trial was undertaken at the Laureldale Field Station on chocolate basalt soil as used for the glasshouse experiment and with the same species and biotype of wild oats. The seed was collected from plants grown in a field nursery in 1986. Seed-lots of 200 wild oats were sown 3 cm deep in early January 1987 in 50x50 cm permanently marked quadrats within field plots measuring 5x 1.5 m. For the clean fallow treatment, plots were either hand cultivated (cultivated treatment) or sprayed with glyphosate (no-till treatment) at monthly intervals from the commencement of the experiment. For the sorghum treatment, the crop was established at a density of 25 plants/m2 in mid-January and the grain harvested in mid-May. Post-harvest stubble was either chopped and soil incorporated (cultivated treatment) or remained standing (no-till treatment). All treatments were replicated four times. Wild oat seedlings were counted and recorded in all quadrats every month.

Results and discussion

Glasshouse

Wild oats emerged more rapidly and produced significantly more above and below ground dry matter in the presence of milled incorporated wheat stubble than in the absence of stubble (Table 1). Field pea and sunflower stubbles also had a stimulatory effect, although not of the same magnitude as that recorded for wheat. Canola stubble tended to be inhibitory and significant reductions in final emergence percentage arid dry matter production were recorded in the presence of sorghum stubble (Table 1).

Table 1. Emergence percentage (Emg. %), leaf number and dry weight of wild oats grown in pots containing field soil amended with finely milled crop stubbles (0.5% w/w soil).

Allelopathy could be useful for wild oat management if crops identified as being inhibitory were rotated with those crops in which the weed was being stimulated. The results of this experiment were suggestive of a selective allelopathic effect. The stubbles had been finely milled and blended throughout the soil and could not be visually detected, making it unlikely that any physical factors were involved.

Although wild oats are subjected to competition for light, water and nutrients in an actively growing wheat crop, the available evidence suggests that their growth and seed production can be selectively stimulated in successive cereal crops due to the presence of plant remains from the previous year. That is, wild oat seed production can be greater than one would expect for a given wild oat population (1). Thurston (15) recorded much higher tiller, panicle and spikelet numbers per plant of wild oats growing in spring barley in England after two successive winter cereal crops than after two years of fallow. In particular, panicle number per plant of wild oats was up to 10 times greater in soil previously cropped to a winter cereal than in previously fallowed soil. In Australia, exponential increases in wild oat populations have been recorded when wheat is grown without rotation, despite the use of in-crop herbicides (6,18,20).

The indications from the experiment reported here were that the residues from sorghum cropping would have the opposite effect to those of wheat. The presence of a small quantity of sorghum stubble inhibited wild oat emergence at day 16 and the top weight of individual plants, by 59% and 52% respectively, in comparison with the no stubble treatment. Leaf number and root weight per plant were also significantly reduced. The response of wild oats to sorghum was therefore tested under field conditions.

Field

There was a tendency for wild oat germination to be stimulated by soil cultivation in the early part of the emergence period in the clean fallow plots but this trend was reversed towards the end of winter and in general the differences between cultivated and non-cultivated treatments were relatively small and unlikely to be of any significance for wild oat control (Table 2). The presence of prior sorghum, whether cultivated or no tillage, substantially delayed wild oat germination and approximately halved total germination in comparison with the clean fallow treatment (Table 2).

Table 2. Emergence pattern of wild oats in soil either maintained as a clean fallow or cropped to sorghum in the preceding summer period. Data presented are the emergence sums of four replicates each of 200 seeds. Rainfall was recorded at Laureldale Meteoro- logical Station.

Wild oats which emerge early in fallows have the potential to produce very large well-tillered plants each bearing several thousand seeds. To prevent seed production it is vital that not only these plants, but also those which emerge subsequently, be cultivated or sprayed while they are in the early vegetative stages. The emergence pattern of wild oats in bare fallows often necessitates the repetition of control measures several times during the six-month period April to September (Table 2). Conversely, the synchronised late-winter emergence of wild oats in soil previously cropped to sorghum would enable them to be relatively easily controlled, with usually only one cultivation or spray treatment required in late winter. This could be undertaken as part of the pre-sowing operations for the next sorghum crop. The inhibitory allelopathic effects of sorghum root exudates on buried wild oat seeds would be expected to vary from year to year and from site to site due to differences in soil moisture, temperature, pH, texture, mineral and humic acid content, oxygen concentration and the number and type of microorganisms present (13). Wild oat biotypes may also differ in their response. However, an alteration in their emergence pattern from a cool-season continuum to a delayed, compressed peak in late winter, may help explain the economic advantages which have been attributed to the practice of rotating sorghum with wheat for wild oat control. Furthermore, it is likely that the presence of an established wheat crop in the winter phase would significantly reduce the already delayed wild oat germination peak. Gross margin analyses undertaken by Wilson (19) showed that wild oats could be effectively controlled without the use of herbicides, and at no cost, in wheat-sorghum rotations in northern NSW and southern Queensland. The wheat-sorghum rotation was more profitable than continuous wheat, in which annual herbicide applications were necessary to maintain wheat yield in the presence of wild oats.

The soil seed bank of wild oats was found to be depleted after two successive sorghum crops in the experiment reported here, with 27% of the original seed bank emerging late in the first year, 16% in the second year and 0.1% in the third (10). The remainder of the seed presumably germinated but did not emerge due to a loss of geotropic response (1). It is feasible that the root exudates of several of the summer-growing grain legume species commercially available in Australia would also prove phytotoxic to buried wild oat seeds, and that sustainable legume-based rotation strategies could be devised to both biochemically inhibit wild oats and improve soil nitrogen status (10). The implementation of these or other appropriate crop rotations could result in a significant decline in the importance of weedy Avena spp. in cereal cropping systems in Australia.

Acknowledgements

The financial support of the Australian Special Rural Research Fund is gratefully acknowledged. Valuable field and laboratory assistance was provided by Mrs Bronwen Clark and Mr Ross Darnell, and this manuscript was carefully prepared by Mrs Barbara Blenman.

References

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