PROTECTING GLYPHOSATE IN SUMMER FALLOWS - MIXES WITH PRE-EMERGENT HERBICIDES AND DOUBLE KNOCKING TRIALS

GRDC code

DAQ00136 and Cotton CRC 1.1.34

The importance of the fallow

The fallow phase of the cropping rotation has the greatest potential to make or break weed management systems. Reasons for this include:
• No crop present, emerging weeds are more likely to grow and produce lots of seed
• Weed management costs are aimed to be kept as low as possible and is based on glyphosate with few or no other herbicide groups or other control methods used, particularly for grasses

This reliance on glyphosate creates potential problems. Using any one method of weed control continuously, whether it be glyphosate or cultivation, will lead to a weed species shift to plants naturally adapted to that method. In addition, reliance on one herbicide or herbicide mode of action group will also lead to herbicide resistance.

More on species shift

Weed species shift is the natural ecological succession of species that are already adapted to the predominate pressures imposed on a population. In our fallows, where glyphosate is often the main form of weed control, there are a number of species that are not controlled or have variable control with glyphosate. Continuous use of glyphosate is likely to result in an increase in these species. Fleabane is the obvious one, but other species such as the vines (peachvine, Australian bindweed) and legumes (rhyncosia, annual verbine and maloga bean) are some of the others to watch out for.

Herbicide resistance

There are two main drivers that lead to herbicide resistance, these are:
• The initial resistance gene frequency which is the chances of finding a weed with a resistant gene in a population. The more weeds, in the field, the greater the chance for resistance to develop.
• The selection pressure or herbicide use pattern in the field. One herbicide used continuously on a population imposes a high selection pressure for that herbicide.

Therefore if a population has no resistant genes present for a herbicide, there is no risk of resistance in the population when that herbicide is used. However, we don’t know that, so we need to assume that a resistance gene is present. Furthermore, if a high selection pressure is placed on that population the risk of resistance occurring is high. Poor control due to application, climatic conditions does not directly lead to resistance and often leaves both resistant and susceptible plants alive. Poor control results in weedy paddocks which as mentioned earlier, increases the chances of resistant plants being present.

Some species to watch out for

The DPI&F conducted a weed survey in 2001 on dryland farms in order to determine what weed species were present and to identify weed management issues. Recently (2008), these farms were revisited to see what changes had occurred since then (Table 1). The major change was the increase in the presence of fleabane which didn’t make the top 12 in 2001 and now is equal second in prevalence. Rhyncosia is now at number 10.

In addition, an herbicide resistance risk assessment was conducted on the biological characteristics of species surveyed (Table 2). Four species present in our summer fallows already have populations resistant to glyphosate. Note that there are a number of species that are high on both lists.

Tips for protecting glyphosate

The only way to protect glyphosate is through Integrated Weed Management. Below are some things to consider:
• Don’t rely on glyphosate all the time, rotate chemistry and use other control methods
• Control survivors of glyphosate application in order to prevent seed set, this is a must in glyphosate tolerant cotton systems, and is also of great importance in fallows – don’t use glyphosate to control these survivors.
• Monitor your fields regularly to see if there are survivors, even after double knock applications. If this is done effectively the risks are greatly reduced
• Keep good up-to-date field records. Know what has been sprayed in each field, what worked, what didn’t and why. If glyphosate has been relied on for a number of years, it’s time to change.

Table 1. Top 11 species present in summer crops and fallows in SQ and NNSW.

Table 2. Weed species in the NGR at risk of developing herbicide resistance (rating out of 10).

 

Double knock and residuals

An effective option to rotate chemistry and control survivors is “double knock” as has been presented at previous updates. A more recent advance for weedy and high risk situations is to add residual herbicides to double knock to minimise subsequent emergences. Table 3 contains some preliminary results from 2 field trials conducted near Dalby on barnyard grass, liverseed grass and feathertop rhodes grass. Some points to note are:
• Roundup®CT followed by paraquat gave above 99.5% control on all species in most cases. This is a great result, but there was still room for the odd survivor which requires remedial action.
• RoundupCT mixed with Nutrazine® resulted in poor control of barnyardgrass
• Feathertop appears less susceptible to paraquat than the other species
• The addition of residual herbicides greatly reduced the subsequent emergence of new plants.
• Dual Gold® (metolachlor) appears to have performed the best, followed by Flame® (imazapic)
• Paraquat mixed with Nutrazine was not successful at preventing emergences in barnyard grass and feathertop rhodes grass

Using residuals in fallow increases weed management costs. However if conditions are favourable, they could minimise future germinations and save one or two broadacre sprays with just some scouting and remedial action required. Applying double knock with the aim to prevent seed set, particularly in problem fields will have long term benefits and prolong the effectiveness of glyphosate. The costs to dryland and even irrigated farming if glyphosate resistance occurs will be much greater than taking preventative measures now.

Contact details

Jeff Werth
Leslie Research Centre
Department of Primary Industries and Fisheries
13 Holberton St, Toowoomba, Qld 4350
Ph: 07 4639 8851
Fx: 07 4639 8800
Email: jeff.werth@dpi.qld.gov.au

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Table 3. Effectiveness of double knock and residual herbicides on summer fallow grasses (Initial populations: barnyard grass – 670 plants/m2; liverseed grass – 204 plants/m2; feathertop rhodes grass – 645 plants/m2).

					Barnyard grass		Liverseed grassA		Feathertop RhodesB
Knock 1		Knock 2		Cost ($/ha)	% Control	New emergences (plants/m2)	% Control	New emergences (plants/m2)	% Control	New emergences (plants/m2)
Herbicide(s)	Rate (L/ha)	Herbicide(s)	Rate (L/ha)
		(7 DAK1)*			(14 DAK1)	(50 DAK1)	(7 DAK1)	(50 DAK1)	(7 DAK1)	(50 DAK1)
Untreated	-	RoundupCT	1.6	14	52	74	99	-	80	-
RoundupCT®	1.6	Untreated	-	14	99	26	98	-	99	-
RoundupCT	1.6	Paraquat	1.6	26	99	27	99	14	99	35
RoundupCT	1.6	Paraquat	2.4	32	99	50	99	9	99	52
RoundupCT + Nutrazine 900® + AMS	1.6 + 2.5 + 2.0	Untreated	-	45	63	17	99	6	94	29
RoundupCT + Flame®	1.6 + 0.2	Untreated	-	46	99	16	99	3	99	6
Paraquat + Nutrazine 900	1.6 + 2.5	Untreated	-	42	99	33	99	2	90	49
Paraquat + Flame	1.6 + 0.2	Untreated	-	45	97	16	99	1	87	5
RoundupCT	1.6	Paraquat + Nutrazine 900	1.6 + 2.5	55	99	7	99	19	99	18
RoundupCT	1.6	Paraquat + Dual Gold	1.6 + 2.0	87	99	1	100	1	99	2
RoundupCT	1.6	Paraquat + Flame	1.6 + 0.2	59	99	8	99	2	99	14
RoundupCT	1.6	Paraquat + Primextra Gold	1.6 + 3.2	85	99	3	99	5	99	9
RoundupCT + Nutrazine 900 + AMS	1.6 + 2.5 + 2.0	Paraquat	1.6	58	92	11	99	4	97	24
RoundupCT + Dual Gold®	1.6 + 2.0	Paraquat	1.6	87	99	1	100	7	100	3
RoundupCT + Flame	1.6 + 0.2	Paraquat	1.6	59	99	10	100	1	99	3
RoundupCT + Primextra Gold®	1.6 + 3.2	Paraquat	1.6	85	95	0	99	4	98	3

* Days After Knock 1
A Nutrazine is not registered on Liverseed grass
B Dual Gold, Primextra Gold, Nutrazine and Flame are not registered on Feathertop Rhodes grass