Chlorsulfuron herbicide was found to reduce wheat yields at some sites.
Phosphorus (P) responses were closely correlated with soil P levels.
No significant responses to zinc (Zn) fertiliser were recorded.
P and Zn nutrition was not found to interact with chlorsulfuron herbicide damage in wheat.

BACKGROUND

Applications of sulfonylurea (SU) herbicides such as chlorsulfuron (eg Glean ) and metsulfuron (eg Ally ) are known to result in reduced grain yield when used on some wheat varieties in some seasons (Lockley and Littlewood, 2000, 2001, 2002; Mullen et al. 2003). However, the positive effects of early weed control on grain yield from SU herbicides often compensates for any risk of crop damage and the cost effectiveness of these herbicides makes them a popular choice.

Some farm advisers in central west (CW) NSW have suggested that there is an interaction between crop damage caused by SU herbicides and crop nutrition, particularly zinc (Zn). It has been suggested, that the negative effects of SU herbicides on grain yield are exacerbated by zinc deficiency in wheat crops. Following on from this, it is claimed that the application of SU herbicides on wheat yields can induce Zn deficiencies.

Zn is important in the activity of a number of enzymes in plants and also in the production of hormones which control growth processes such as stem and root elongation and leaf expansion. It is also involved in the plants ability to use nitrogen (Weir et al. 1983). The interaction between herbicides and Zn nutrition of wheat has been studied in a number of experiments in South Australia and Western Australia.

Glass house pot trials in Western Australia by Robson and Snowball (1989) demonstrated soil incorporated diclofop-methyl (eg Hoegrass®) could induce Zn deficiency in wheat. The experiments used a virgin brown sandy soil with extremely low Zn levels of 0.05 ppm (DTPA). Using the same methodology and medium, Robson and Snowball (1990) reported chlorsulfuron applications induced symptoms of Zn deficiency and reduced plant dry matter. They concluded the losses in production were most likely the effect of depressed root function and that these herbicides could induce Zn deficiency where the supply of Zn for the crop is marginal. Pedersdon et al. (1994) noted the changes to root morphology and reductions in the length of lateral roots of barley as a direct consequence of metsulfuron herbicide applications. These root impacts can inhibit the uptake of water and immobile nutrients such as Zn, thereby intensifying deficiencies, and reducing yields on soils deficient in trace elements.

Field trial work in South Australia by O'Keefe and Wilhelm (1993) studied the effects of SU herbicide on grain yield of wheat growing on a sandy clay loam soil (pHwater 8.3), deficient in trace elements. Grain yield was reduced when wheat was treated with chlorsulfuron, but the yield in plots with adequate trace element nutrition was not impaired. This effect was only observed in one experiment in a total of 46 conducted on the Eyre Peninsula over three years.

Some agronomic advisers have used this information as a basis for management recommendations to farmers on the nutrition of their wheat crops. This has resulted in the increased use of Zn fertiliser products (i.e. seed treatments, fertilisers and foliar sprays) on wheat crops treated with SU herbicides. It is estimated that Zn starter fertilisers are applied at sowing to approximately 20% of the wheat crops in the Forbes district. However, the applicability of the SA and WA trials on which these Zn recommendations are based, has been questioned for CW NSW. It has been suggested that the information has come from glass house trials and on very sandy soils in WA and SA, and it is not relevant to CW NSW.

No trial evidence exists in CW NSW indicating wheat responses to Zn fertiliser, even on the high pH clay soils. Nor has the interaction between Zn fertilisers and SU herbicides been established. In response to these concerns, a series of trials were conducted across CW NSW in 2001, 2002 and 2003 to provide objective data on the potential for SU and Zn interactions in wheat.

METHODS

A series of 15 wheat trials were sown across CW NSW in 2001, 2002 and 2003 (Table 1). Most of the trial sites were sown in conjunction with NSW Agriculture wheat variety trials and therefore site selection did not take into account the likely Zn status of the soil.

Link to Table 1. Trial site details

Treatments on the trials included:

two wheat varieties (Janz and Wollaroi);
application of two SU herbicide treatments (Nil chlorsulfuron and + chlorsulfuron); and
two Zn fertiliser treatments (Nil Zn and + Zn).

All treatments were all sown with phosphorus (P) at 20 kg/ha (except at Quandialla in 2001 where 30 kg P/ha was sown). Two additional nil (P) fertiliser treatments with +/- chlorsulfuron were also included to investigate the P response at each site and the interaction of P with chlorsulfuron. In 2002 and 2003, two additional high P treatments at 40 kg P/ha were included to provide improved information of the P responsiveness of the trials sites. The trials were arranged in a randomised block design with 3 replicates.

Di-ammonium phosphate (DAP) (18N:20P:0Zn) was used as the starter fertiliser. DAP Zinc-Cote (17N:19P:5Zn) was used for the + Zn treatments. Fertiliser treatments where balanced with urea to ensure even rates of nitrogen (N) and P were applied in the DAP and DAP Zinc-Cote starter fertilisers. A basal application of N as urea was applied to the nil P fertiliser treatments at the same rate of N as the DAP treatments. Glean (chlorsulfuron 750g/kg) was used as the chlorsulfuron product. Glean was applied to the appropriate plots at the early post emergent stage as indicated by the dates in Table 1. As required, other herbicides were applied across the trial sites to minimise weed effects (Table 1).

Dry matter cuts were taken mid season. Dry conditions in 2002 and 2003 meant dry matter cuts on some sites were not taken. Plant tissue samples (youngest emerged blade) were analysed for a full range of nutrients to ensure other nutrients were not deficient, and to provide further information on the Zn and P status of the different treatments. Grain yield, protein, screenings and test weights were recorded. The results were statistically analysed using analysis of variance and spatial analysis.

RESULTS AND DISCUSSIONS

Chlorsulfuron effect (Table 2)

Link to Table 2

Link to Table 3

Phosphorus fertiliser effect

Significant yield responses to 20 kg/ha of P fertiliser were recorded at 9 out of the 15 sites over the 3 years of the trials (Table 3). The largest yield response to 20 kg P/ha was a 68% yield improvement at Tottenham in 2001. 40 kg P/ha did not give significant yield benefits over 20 kg P/ha at any of the sites in any year.

All of the significant P responses were recorded on sites with soil P (Colwell) levels of less than 35ppm (Graph 1). Yield responses to 20 kg P/ha were closely related to leaf tissue P levels in 2001 (Graph 2). Reuter and Robinson (1997) report critical wheat tissue P levels at Z 31 to be approximately 0.3%, which is strongly supported by these trials.

Chlorsulfuron did not significantly influence the level of P responsiveness at any of the P responsive sites (data not shown). The level of P response was the same regardless of wether chlorsulfuron had been applied or not. Correspondingly, P fertiliser did not reduce the negative chlorsulfuron effects observed at Gunning Gap, Tottenham and Euabalong in 2001.

Zinc fertiliser effect

A significant grain yield response to Zn fertiliser was recorded at only one site in one year. No significant dry matter or grain yield responses to Zn fertiliser (with or without chlorsulfuron) were recorded at any others sites. This is despite soil Zn levels at 10 of the 15 sites being below the indicative critical level of 0.8 ppm (DTPA) set for wheat (Peverill et al. 1999). Leaf tissue test results in 2001 showed Zn levels from 19.1 ppm (Wirrinya) to 34.6 ppm (Tottenham), being above the critical limit of 14 ppm at late tillering (Reuter and Robinson, 1997).

The evidence supporting Zn responses in wheat appears isolated to anecdotal evidence and unpublished trial results. There is no single reliable soil test for Zn in Australia, although the DTPA test is the most widely used method in Australia (Armour and Brennan, 1999). The Zn / chlorsulfuron interactions demonstrated in pot trials by Robson and Snowball (1989, 1990) and in field trial by O'Keefe and Wilhelm (1993) was with soils that had zinc levels of 0.05 ppm and 0.20 ppm respectively (DTPA extractable zinc) and were proven to be Zn responsive. By comparison, none of the trial sites in this trial series within CW NSW may be considered deficient enough to respond to Zn and exhibit interactions with chlorsulfuron.

The potential for Zn responses on soils with a pH (CaCl2) < 7 in CW NSW is low. Based on these trials, the suggested critical Zn soil level of 0.8 ppm (DTPA) (Peverill et al. 1999) does not appear to be applicable to soils with pH (CaCl2) < 7.

These trials raise the question as to why so many farmers in CW NSW are using Zn fertiliser when the potential for yield responses appear negligible.

CONCLUSION

These trials confirm the potential risk that chlorsulfuron may reduce wheat yields in some instances. However, these trials demonstrated that chlorsulfuron does not always have a negative effect on wheat yields. Pre-emergent applications and wet winters appear to increase the potential for chlorsulfuron damage in wheat. The addition of Zn or P fertiliser did not reduce the yield penalty associated with chlorsulfuron.

Ideally these trials should be run over further years, with the hope of having wet years to maximise the potential for chlorsulfuron damage. However, other higher research priorities will mean these trials will not be continued at this stage.

REFERENCES

Armour, J.D. and Brennan, R.F. (1999). Zinc. In "Soil Analysis: an Interpretation Manual". Ed. K.I. Peverill, L.A. Sparrow, and D.J. Reuter. CSIRO Publishing Collingwood.

Lockley, P. and Littlewood, W. (2000). 2000 Results Cultivar X Herbicide Screening Wagga Wagga. NSW Agriculture.

Lockley, P and Littlewood, W. (2001). 2001 Results Cultivar X Herbicide Screening Wagga Wagga. NSW Agriculture.

Lockley, P and Littlewood, W. (2002). 2002 Results Cultivar X Herbicide Screening Wagga Wagga. NSW Agriculture.

McRae, F.J., McCaffery, D.W. and Carpenter, D.J. (2004). Winter Crop Variety Sowing Guide 2004. NSW Agriculture.

Motley, K., Harbison, D. and Rice, A. (2001). Canola and wheat responses to phosphorus in the Forbes District 2000. Winter Crop Agronomy Forbes Trial results. NSW Agriculture.

Mullen, C.L., Francis, R.J., Dellow, J.J. and McRae, F.J. (2003). Weed Control in Winter Crops 2003. NSW Agriculture.

O'Keefe, P. and Wilhelm, N. (1993). The hidden costs of sulphonylurea herbicide on micronutrient-poor soils. Proc. 7th Aust. Agron. Conf. Adelaide, S.A.

Pederson, R.N., Black, I.D., Dyson, C.D and Hannam, R.J.(1994). Effects of the herbicide metsulfuron-methyl on root length, nutrient levels, grain protein, and yield of barley. Aust. J. Exp. Agric. 34, 499-504

Peverill, K.I., Sparrow, L.A. and Reuter, D.J. (1999). Soil Analysis: an interpretation manual. CSIRO. Australia.

Powell, C. (2000). Winter Crop Variety Experiments 1999. NSW Agriculture.

Reuter, D.J. and Robinson, J.B. (1997). Plant analysis: an interpretation manual. CSIRO. Australia.

Robson, A.D. and Snowball, K. (1989). The effect of 2-(4-2',4'-Dichlorophenoxy-phenoxyl)-methyl propanoate on the Uptake and Utilization of Zinc by Wheat. Aust. J. Agric. Res. 40, 981-90.

Robson, A.D. and Snowball, K. (1990). The effect of chlorsulfuron on the uptake and utilization of copper and zinc in wheat. Aust. J. Agric. Res. 41, 19-27.

Weir, R.G., Holland, J.F. and Doyle A.D. (1983). Zinc deficiency and it's treatment in northern NSW. Agdex 102/531. NSW Agriculture.

ACKNOWLEDGMENTS

Thanks go to trial cooperators for hosting the trials. Greg Gibson, Tim McNee and Sharon Taylor provided technical assistance. John Francis supervised the trials in the Condobolin district in 2001. Arthur Gilmore, Sharon Taylor, Helen Nicol and Chris Dyson carried out statistical analysis of the trial data. Catherine Evans for critical comments on drafts of this report.

The Central West Farming Systems Group, Grain Growers Association,

Hi Fert Fertilisers and GRDC funded these trials.