Old straw and weathered hay have long been used by horticulturists to mulch around their fruit trees to conserve moisture and to control the growth of weeds. Now, with the advent of minimum tillage farming, we find that the seedlings trying to grow through the remains of the previous crop, or pasture, or mat of weeds are often weak and stunted and the resultant crop or pasture is less productive than we would expect. So what is causing this effect? What is present in the residues of crops or pastures which delays or inhibits the growth of germinating seedlings and reduces the productivity of the mature plant?

Research in many laboratories around the world, studying the effects of residues from many different species of pasture, crop and weed plants, has implicated three different factors:

allelopathy or the leaching of compounds from plant residues which either inhibit the germination of seed or stunt the growth of seedling roots hence impairing the vigour and growth of the mature plant;

attack by fungal pathogens which are favoured by the moist conditions which are common when plant residues are present on the soil surface;

after periods of rain when the residues become waterlogged, decomposition of the plant material will commence and under anaerobic conditions this results in the production of toxic intermediate breakdown products.

In this paper we have chosen to give examples of the three effects of litter mentioned above from studies conducted at the Charles Sturt University, Wagga Wagga, and at the CSIRO Division of Plant Industry, Canberra. From these examples you will note that the phenomena are widespread and have major consequences for farming today especially in the context of minimum tillage, grazing management of pastures, and in cereal to pasture rotations.

Effects of the litter of silvergrass on crops and pastures

Silvergrass (Vulpia spp), an annual grass accidentally introduced into Australia, is one of the major weed species in crops and pastures of southern Australia and has been the subject of study by Dr Jim Pratley and his colleagues over the past six years (Pratley, 1989; Pratley and Ingrey, 1990; An et al., 1993).

They have been able to convincingly show that the presence of silvergrass residues can cause large reductions in wheat production due to the release of toxic substances (allelochemicals) which readily leach out of the litter. In a simple pot trial (Pratley, 1989) varying amounts (equivalent to 0.5, 1.0 and 1.5 t/ha) of litter at three stages of degradation (fresh collected in December immediately following senescence; partially decomposed collected in May following a dry summer-autumn period; and weathered collected in August) were placed evenly on pots presown to wheat (cv. Vulcan). The pots were lightly and frequently watered to maintain the residues in a moist condition. The effects of the different residue burdens at different degradation states on dry matter yields are shown in Figure 1. Residues collected in December had little or no effect. From about the 3-leaf stage seedlings growing in the pots treated with May residues showed signs of stress with a proportion dying within the next few weeks. Plants which survived, recovered and grew normally. Residue burdens up to 1 t/ha most retarded growth.

Figure 1. Effect of different residue burdens of Vulpia at different degradation states on wheat growth. (After Pratley, 1989).

In a subsequent trial (Pratley and Ingrey, 1990), examination of silvergrass residues was extended to examine the effects of ultraviolet light and/or moisture on vulpia litter breakdown on the germination and growth of the crops, wheat (cv. Vulcan) and canola (cv. Jumbuck) and on two pasture species, namely phalaris (cv. Sirolan) and lucerne (cv. Aurora). Pots were filled with soil and residues of two vulpia species were chopped and placed on the soil surface at a rate equivalent to 2 t/ha. For the next ten weeks the following treatments were imposed:

nil residue

residue without further treatment

residue kept dry but exposed to ultra-violet light daily for 12 hours

residue watered lightly and exposed to ultraviolet light.

The pots were then sown following this pre-treatment period and watered regularly thereafter for the next 5 weeks after which the dry matter yields of tops and roots were determined.

Germination was reduced in all species except for canola by vulpia residues, the decline being accentuated by UV light and UV light plus moisture (Figure 2). Relative to the control treatment germination declined by up to 33% for wheat, 49% for phalaris and 38% for lucerne. Foliage dry matter production was also significantly reduced by vulpia litter, especially that litter exposed to UV light and UV light plus moisture (Figure 2b).

The active form of the allelochemicals appeared to result from ultraviolet breakdown to an easily leached form.

Figure 2. Effect of vulpia residues on the germination (2a) and shoot weight (2b) of wheat, canola, phalaris and lucerne. (After Pratley and Ingrey, 1990).

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In the third series of trials residues of vulpia collected in December were mixed in a flask with increasing amounts of a fine sandy loam soil and enough water added to saturate the mixture. All flasks were incubated in the dark at 20oC for 6 days after which the liquid of each treatment was filtered and centrifuged. The supernatant was used for the bioassays in which seeds of wheat were placed on absorbant filter paper moistened with each extract, the control treatment receiving distilled water. The petri dishes were kept in a dark incubator for 48 hours after which the number of seeds which had germinated were counted, measurements made of the longest seminal root and of the length of the coleoptile. The phytotoxic effects on germination and coleoptile length of wheat exposed to silvergrass residues with and without the addition of soil are shown in Figures 3a and 3b. Aqueous extracts from the treatments of residue alone, 1:1 and 1:5 residue:soil ratio, depressed the germination and coleoptile length of wheat. Extracts from the residue alone reduced germination by up to 70%, radicle elongation by 72% and coleoptile length by 41%. These deleterious effects were reduced by the addition of soil, the effect being proportional to the amount of soil added.

Figure 3. Phytotoxicity of vulpia residues on germination (3a) and seedling length (3b) of wheat as affected by the addition of soil. (After An et al., 1993).

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Effects of pasture, crop or weeds litters on the growth of legumes

At the CSIRO in Canberra, our studies have primarily been focused on factors relating to the growth of legumes in first year pastures after cropping and on the questions relating to legume content and clover decline of older pastures, particularly those based on the perennial grass phalaris and subterranean clover (Leigh et al., 1994; Halsall et al., 1994).

One facet of this work has been confined to studying allelopathy alone, separating it from the confounding effects of fungal pathogens and products of decomposition by using a cold water extract of the residue being tested and sterilising it by filtration. We tested this extract for its effect on germination and radicle elongation of seeds and seedlings and on the nodulation and root growth of young legume plants grown under sterile conditions.

Using this technique, we examined the effects of phalaris and wheat straw residues on a range of pasture legumes using three dilutions of each extract (Figure 4). As you can see, the phalaris extract reduced germination particularly at the higher two concentrations with only red clover and Namoi vetch showing any degree of tolerance to the extract. In cases where the seed did germinate the developing radicle was stunted in growth with the tip being swollen and dark in colour.

Nodulation of the legumes was also affected so that plants which survived had reduced nodule numbers. Root growth in these young plants was also reduced so that the plants overall were at a disadvantage compared with the control plants grown in the absence of the extract. Similar results were obtained with the wheat straw extracts but the results were not as severe.

This work was extended to cover extracts from 10 grass species common to the Southern Tablelands each of which were tested on Karridale subclover, Grasslands Huia white clover and Jemalong barrel medic. The results showed that allelopathic compounds were present in most grasses although it is not known if the same chemical compound is involved in the different species.

Weed and crop species were also examined and found to be allelopathic at varying levels to the same three legume species.

In complementary trials carried out in nursery flats, Karridale subclover seed was sown under phalaris mulches which had been subjected to a number of different pre-treatments. These treatments included exposure to sunlight for 14 and 28 days under dry conditions and under wet conditions, and comparable treatments maintained in the dark at the same temperature. Control flats were set up without mulch and with polystyrene beads in place of the mulch to give a similar moist environment at the soil surface.

Figure 4. Allelopathic effects of sterile extracts of phalaris residues on germination (4a), radicle elongation (4b), nodulation (4c) and root elongation (4d) of a range of pasture legumes.

The results obtained showed the interactions between the allelopathic effects from the phalaris mulch and the development of fungal pathogens in various treatments. The control flats without mulch and those with polystyrene beads showed a healthy growth of clover with most seeds germinated and growing (Figure 5a). Only 67% of seeds germinated under the untreated mulch and, of these, several were

small and chlorotic with some dying. When the mulch was kept dry and either dark or exposed to sunlight, there was little variation from the untreated mulch.

However, when the mulch was moistened and kept in the dark, the percentage germination decreased to 63-65%, fewer of the plants which had germinated survived, and the surviving plants were smaller and less thrifty than those growing in the untreated mulch. The dry matter production from the clover growing in these flats was substantially less (50%) than that produced on the control flats. It is suggested that this reduced growth is caused by the partial breakdown of the wet residues under anaerobic conditions with the release of acetic and propionic acids both of which have been shown to be phytotoxic (Lynch, 1977; 1978). In the sunlit treatments there were fewer symptoms of phytotoxicity which was probably due to a more aerobic environment favouring the breakdown of the phytotoxic acids.

Figure 5. Effects of sunlight and moisture on the phytotoxicity of phalaris mulch on the germination and survival (5a) and dry matter production (5b) of subclover.

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Contrasting these results obtained using phalaris litter with those at Wagga Wagga where vulpia litter was used highlights two significant points. Firstly, given reasonable temperatures litter degradation in Canberra was rapid. Secondly, UV exposure was not detrimental, and moist litter, covered and probably partially anaerobic, produced phytotoxins detrimental to the germination and production of the clover plants. The differences between the research conducted in Canberra and Wagga Wagga highlights the complexity and differences in time scales likely to be encountered when seedlings, be they crop or pasture, come in contact with the products released from fresh and partially decomposed litter derived from a wide variety of plant sources.

Lest you believe that the results I have reported to you are a laboratory-induced phenomenon, let me hasten to add that field trials conducted at

the CSIRO Ginninderra Research Station near Canberra have shown that where phalaris litter was removed from between the phalaris' tussocks, subterranean clover establishment and production substantially increased. In other treatments where either phalaris litter or wheaten litter were added, clover establishment and production were reduced (Figures 6a and 6b). In a nursery flat trial it was shown that the deleterious effects observed with phalaris and wheaten mulches were not evident when the residues were incorporated into the upper soil layer.

Figure 6. Effects of the removal of phalaris litter or the addition of phalaris or wheaten mulches on the germination (6a) and dry matter production (6b) of subclover growing in a phalaris dominant pasture.

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In another field trial at Ginninderra it was shown that wheat litter and standing wheat stubble could have a large effect on subterranean clover previously undersown with the last crop. By manipulating the amount of litter left on the soil surface after harvest, it was possible to significantly increase the clover content of the first year pasture phase. Where the walker straw was left on the ground, clover yield was only 58% of the control. Where the natural litter was removed clover yield increased to 170% that of control, and where the stubble was also cut low and all litter removed, clover yield increased to 197% that of control (Figure 7).

Figure 7. Effect of wheat stubble treatment post-harvest on the subsequent growth of subclover.

Conclusions

It is clear from the diverse studies conducted both at Wagga Wagga and Canberra that plant litters are often present in large amounts on the surface soil. Furthermore, litter is ubiquitous and it varies in allelochemical potency depending both on its primary source and also on its stage of breakdown which, in turn, varies according to the rainfall conditions which prevailed. As such, and to answer the question raised by the title to this paper, allelopathy can indeed be a potent natural herbicide with far reaching effects on crop and pasture plants growing in its realm of influence. Results from both centres highlight the need to reduce the phytotoxic effects from many residues on crops and pastures, be they silvergrass, cereal residues such as wheat, or pasture residues such as phalaris. Management practices can reduce residues on the soil surface and decrease the impact of phytotoxin concentrations from the residues in the soil. They include such practices in cropping situations as trash reduction by incorporation into the soil or burning, and, in pastures, winter cleaning, heavy stocking especially in late summer before the autumn break, burning, and hay making and forage havesting of pastures which have grown rank in late spring and summer.

Acknowledgements

We wish to thank Mr J.D. Holgate, Ms S.E. Gollasch and Ms S. de Silva for technical assistance.

References

1. An, M., Pratley, J.E. and Haig, T. (1993). The effect of soil on the phytotoxicity of residues of Vulpia myuros. Proceedings 7th Australian Agronomy Conference, Adelaide, pp.162-164.

2. Halsall, D.M., Leigh, J.H., Gollasch, S.E. and Holgate, M.D. (1994). The role of allelopathy in legume decline in pastures. 2. Comparative effects of pasture, crop and weed residues on germination, nodulation and root growth. Australian Journal of Agricultural Research (in press).

3. Leigh, J.H., Halsall, D.M. and Holgate, D.M. (1994). The role of allelopathy in legume decline in pastures. 1. The effects of pasture and crop residues on the germination, survival and production of subterranean clover in the field and nursery. Australian Journal of Agricultural Research (in press).

4. Lynch, J.M. (1977). Phytotoxicity of acetic acid produced in the anaerobic decomposition of wheat straw. Journal of Applied Bacteriology 42:81-87.

5. Lynch, J.M. (1978). Production and phytotoxicity of acetic acid in anaerobic soils containing plant residues. Soil Biology & Biochemistry 10:131-135.

6. Pratley, J.E. (1989). Silvergrass residue effects on wheat. Proceedings 5th Australian Agronomy Conference, Perth, p.472.

7. Pratley, J.E. and Ingrey, J.D. (1990). Silvergrass allelopathy on crop and pasture species. Proceedings of the 9th Weed Conference, Adelaide, pp.436-439.