Microwave technology for weed management

Take home message

Microwave heating, using a suitable device to project the microwave energy onto plants and the soil, can:

Kill weeds and their seeds in the field, without limitations of wind or moisture;
Kill dormant seeds in the soil;
Significantly reduce bacterial numbers in the top layer of the soil; however these populations recover to become significantly higher than their initial values within a month of treatment;
Not significantly affect fungi or protozoa in the soil; and
Significantly increase grain yields in a number of crops, including canola and wheat.

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Herbicide resistance is becoming an important problem in Australia no-till farming systems. Many weed species have developed multiple resistance to different herbicide groups. Several strategies have been suggested to address this issue, including tillage, flaming, and steam treatment. Many of these strategies are not compatible with no-till strategies.

A sustained research programme has demonstrated that microwave heating, using a suitable device to project the microwave energy onto plants and the soil, can kill weed plants and their seeds. Microwave treatment is not affected by incumbent weather conditions such as wind or rain.

The following species have been tested with good success: Ryegrasses – annual and perennial; barnyard grass; barley grass; bellyache bush; brome grass; clover; feathertop Rhodes grass; fleabane; hemlock; mimosa pigra; parthinium; rubber vine; wild oats; and wild radish. The microwave energy density required to kill plants varies according to the species.

Microwave treatment significantly reduces bacterial numbers in the top layer of the soil; however their numbers rebound to a significantly higher population after one month (Table 1). Microwave treatment has no measurable effect on fungi or protozoa in the soil.

Microwave soil treatment also significantly increases the yield and maturation rate of subsequent crops grown in the treated soil (Table 2).

Table 1. Effect of microwave treatment and soil depth on total viable bacteria

Soil depth (cm)	Time from microwave treatment (days)	Estimated microwave treatment (J cm^-2)
Soil depth (cm)	Time from microwave treatment (days)	0	150	300	600
0	1	6.20^d	5.57^d	4.73^d	1.78^d
0	31	18.90^c	38.48^a	38.25^a	19.67^c
5	1	3.78^d	4.71^d	4.23^d	1.18^d
5	31	18.73^c	24.28^bc	29.95^b	28.22^b
10	1	4.06^d	2.93^d	3.87^d	1.74^d
10	31	16.93^c	26.13^bc	28.90^b	18.00^c
LSD (P = 0.05)					7.30

Note: entries with different superscripts are significantly different from one another

Table 2. Response of subsequent crops to growing in microwave treated soil

Microwave treatment (J cm^-2)	0	Hand weeded	168	384	576	LSD (P = 0.05)	Change from control
Canola pod yield (g pot^-1)	0.27^a	0.56^a	0.36^a	1.25^b	1.95^c	0.55	550 %
Days to flowering - canola	71.4^a	67.6^ab	70.2^a	63.2^b	61^b	7.1	14.6%
Wheat grain yield (g pot^-1)	0.66^a	0.67^a	0.68^a	0.75^a	1.25^b	0.30	90 %
Rice grain yield (g pot^-1)	40.00^a	41.3^a	43.25^a	59.00^ab	64.00^b	18.90	60 %
Tomato dry shoot yield (g pot^-1)	4.13^a	4.25^a	6.05^a	14.44^b	14.46^b	3.10	250 %

Note: entries with different superscripts are significantly different from one another

In conclusion, microwave treatment kills weeds and their seeds in the top layer of soil. Microwave treatment reduces bacterial populations in the top layers of soil, but has no effect of fungi or protozoa. Bacterial numbers recover within one month of treatment. Microwave soil treatment also enhances crop growth and yield.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support.

Contact details

Graham Brodie
The University of Melbourne
Dookie Campus, Nalinga Rd., Dookie, Victoria, 3647
Ph: 03 5833 9273
Fx: 03 5833 9201
Email: grahamb@unimelb.edu.au

GRDC Project Code: UM00053,