Summary

The key to minimising the development of resistance and maintaining highly productive crop and pasture rotations in southern Australia is to rotate both the crops and the herbicides used and to restrict the length of the crop rotation. The use of non-chemical techniques of weed control and non-selective herbicides (glyphosate and gramoxone) in pasture, where possible, is also recommended. All possible opportunities should be taken in the crop rotation to use alternate herbicides (e.g. Trifluralin, Glean and Logran) to the AOPP group which includes the herbicides Hoegrass, Fusilade, Verdict and Assure.

Introduction

Minimising the development of herbicide resistant weeds in southern New South Wales will require changes to the chemicals used, possibly the crops grown, and the entire rotation on some farms. As we have relatively few reported cases of resistance in the area, it is now time to develop good habits that minimise resistance developing rather than waiting for resistance to develop and then adopt policies to overcome it. Most farmers are in the fortunate position of being able to take relatively simple steps to avoid, as far as possible, resistance developing and therefore should never have to confront the problem of herbicide resistance.

This paper discusses:

(i) resistance in general in southern NSW;

(ii) the strategy to minimise the development of Herbicide Resistant Ryegrass;

(iii) comments upon commonly used rotations.

In writing it, I have relied upon the best information available. However, in the light of research and on-farm experience over the next few years, some changes in the strategies may be necessary.

(i) Resistance in southern NSW an overview

Weeds involved

While the possibility exists that every weed currently sprayed can develop resistance, the major weeds to concentrate on in southern NSW are ryegrass and wild oats. Because it was the first weed to be found to have resistance, management strategies have only been developed for ryegrass. Wild oats strategies are currently being researched prior to development over the next few years. The general principles of minimising resistance as outlined for ryegrass will be relevant to other weeds but specific details will change.

Areas where resistance is likely to be a problem

Herbicide resistant ryegrass or other weeds should not be a major problem in the Tablelands where cropping does not take place often, or in the driest parts of the cropping belt, where herbicide use is minimal. The major area where herbicide resistant ryegrass is going to be a problem will be in the wetter areas of the currently defined wheat belt or irrigation areas. In southern NSW most land west of the 450 mm rainfall line is most likely to develop a problem. It is in those areas that the maximum number of existing cases of herbicide resistant ryegrass have been reported and where there is the potential to disrupt the development of high yielding rotations.

Resistance and rotation

The key to minimising the problems of weed resistance is to ensure that in each paddock crops and chemicals are rotated in a correct way. Care should also be taken to ensure that the other purposes of rotation, such as obtaining high yields, safeguarding the environment and providing an acceptable marketable produce, are met at the same time as herbicide resistance is minimised.

(ii) The strategy for ryegrass

The strategy for ryegrass divides into three parts:

• a strategy for weed control use in pasture to minimise the chance of selecting resistant ryegrass;

• a strategy for crop rotation and herbicide use for farmers without resistant ryegrass;

• a strategy for farmers with ryegrass resistant to herbicides.

Pasture strategy to minimise resistance

Table 1 shows the basis of the pasture strategy. The pasture phase provides the best opportunity to use non-chemical, non-selective methods of controlling ryegrass. Research shows that populations of annual ryegrass decrease during the pasture phase in response to increasing competition from other species such as barley grass, brome grass and Vulpia. Set stocking at moderate to high grazing pressures can dramatically reduce the proportion of annual ryegrass in the sward. Spraytopping and cutting for hay or silage can also be used to reduce seed banks prior to cropping.

For pasture cleaning where complete control of weeds is not necessary (e.g. early years of the pasture phase), products such as Simazine, glyphosate and paraquat should be used in conjunction with grazing and agronomic management practices that favour clover dominance. These techniques, along with spray topping, are recommended in pastures where broad-leaved crops start the cropping phase.

Total pasture cleaning. A developing technology is the use of herbicides to overcome the root disease problems in wheat associated with annual grasses in pasture. In NSW recent experiments (Table 2) and many demonstrations have revealed the importance of pasture cleaning to subsequent wheat crops. However, the use of AOPP herbicides in pastures can increase the risk of selecting herbicide resistant ryegrass.

To minimise this, it is recommended to restrict the use of AOPP until the year prior to cropping. Also, the use of AOPP products in pasture should be accompanied by chemical topping or hay cutting in the same year to ensure that any resistant ryegrass does not seed. However, pasture cleaning should be used only when there are other ways of opening the rotation. Always consider other alternatives which could be:

(i) the use of long fallows which must be done in August to be effective. Also, unless fallows are commenced with chemicals the risk of soil structure deterioration later in the rotation will increase;

(ii) alternate crops such as canola, lupins and field peas to start the rotation. The problem with canola is that, while it is high yielding, it is incompatible in the areas where the weed, wild radish, is present in large quantities. Lupins and field peas have a low return as opposed to high yielding wheat and therefore are not recommended at this stage of the rotation. Their place is as rotation extenders as a second or third crop. It is also possible that the use of an alternate crop to open the rotation will not decrease the selection pressure, particularly if AOPP herbicides are used for weed control in this crop.

Table 1. Suggested chemical strategies for cleaning pasture that will not increase the risk of Herbicide Resistant Ryegrass being selected.

Situation	Preferred option	Other options or comments
1. Permanent annual or perennial pasture areas on the tableland (cropping unlikely)	Either Simazine or Simazine Gramoxone (1) or Simazine plus suitable grass herbicides (2)	May be combined with Spraytopping (with Roundup or Gramoxone). No real restriction on chemicals as development of resistance is unlikely.
2. Continuous pasture in areas where future cropping is likely (e.g. early in a pasture phase on a slopes farm where crop rotation is regularly practised).	Simazine + Gramoxone (1) or Simazine Gramoxone (3)	May have to accept a less than perfect result. Spraytopping desirable for improved performance.
3. Pasture to break crop (e.g. Canola, lupins, field peas).	Spraytop (only). Simazine Gramoxone (1). Simazine. Roundup for fallow commencement (4).
4. Pasture to oats	Spraytop (then) Simazine Gramoxone (1). Gramoxone (3) as a winter clean. Roundup (4).	Must clean Out Vulpia and barley grass prior to sowing oats if oats is to act as a disease break.
5. Pasture to wheat	Simazine + suitable grass herbicide (2) (May-July) Simazine (May) + grass herbicide (2) (August). Grass herbicide (if no Vulpia present) and spraytop.	Spraytopping Simazine Gramoxone (1) (May). If resistant ryegrass present, use Gramoxone (3) as a winter clean

(1) Simazine (1.0-1.5 L/ha) + Gramoxone (100-200 ml/ha).

(2) Suitable grass herbicides. In this case is most likely to be Fusilade, Verdict or Sertin, given that Verdict is registered for pasture. So far most demonstration and experiment work is with Fusilade (which was probably the best and most cost-competitive barley grass herbicide in 1989) with excellent results, but limited work suggests Verdict also works. Assure may also be added to the list but Hoegrass and Grasp are unlikely to be used.

(3) Gramoxone (1.5 L/ha). May severely retard pasture growth. A choice for pasture cleaning in resistant populations.

N.B.

(1) This table is a suggestion and companies would need to obtain registration or permits of the products and, if necessary, the mixes before the use to be recommended.

(2) This table only includes chemicals for pasture cleaning, cultural fallowing, chemical fallowing and grazing management; haymaking and mechanical topping may also play an important role.

Table 2. Effect of pasture cleaning on the subsequent yield of wheat at Corowa, 1988/89.

Treatment	% Weed Control 1988		Wheat Yield 1989 T/Ha
	Vulpia	Total grass
Control	0	0	1.12
Simazine	95	85	1.90
Simazine + Fusilade	95	95	3.73
l.s.d. (5%)			0.84

Crop strategy to minimise resistance

The general strategy in a crop rotation is to rotate both the crops and the herbicides to give a high yielding and high returning sequence and a variety of herbicides from a variety of chemical groups. Table 3 illustrates the strategy and provides preference for crop choice and herbicide use.

The assumptions on which the strategy is based are:

(1) farmers have not seen resistance on the area to be treated;

(2) while resistance has been shown to all products used for ryegrass control, the Trifluralin products Glean, Logran and Triazine groups have so far demonstrated the least problems. Therefore, these are used to obtain specific ryegrass control where possible with the AOPP products such as Hoegrass, Fusilade, Verdict and Assure, being selected where it is impossible to select another low risk herbicide; and

(3) the herbicides Sertin and Grasp are in a different chemical group to the AOPPs but resistance to these products usually occurs at the same time or shortly after AOPP resistance. They are therefore included with the AOPPs when considering strategies for resistance.

For southern NSW the strategy involves the use of canola as an opening crop where possible. If this is not possible, wheat on long fallows or total pasture cleaning are the preferred option depending on where long fallows are normally recommended.

As the first crop in the rotation is generally conventionally cultivated, at least in a modified sense, it is an opportunity to use Trifluralin or Yield for ryegrass control. If the rotation is opened with an alternative crop such as canola, a double rate of Trifluralin can be used.

Table 3. A strategy to minimise the development of ryegrass resistant to herbicide in crops in southern New South Wales

Situation	Preferred Strategy	Other Strategy or Comment
1. Year prior to cropping	Spraytop Fallow in August	Winter clean. See Table 1.
2. First crop	Use as alternative to wheat. Preference Canola then lupins or field peas. Use minimum tillage. Herbicide choice Trifluralin	Wheat after pasture cleaning or fallowing. Oats following cleaning or fallow.
3. Second crop	Direct drilled or minimum tillage wheat. Herbicide choice Hoegrass or Glean. Nitrogen if necessary.	Lupins (see point 4).
4. (a) Third crop Finish the rotation	Undersown direct drilled cereal and no herbicide
(b) Third crop in a continuing rotation (1)	Minimum tilled (lupins) Herbicide choice Simazine or Simazine + Trifluralin
5. Fifth crop Continue from 4 (b) 2.	Direct drilled wheat. Hoegrass or Glean

(1) The use of wheat or other cereals and Hoegrass or Glean as a third crop in a continuing rotation is not recommended.

(2) Steps 4 (b) and 5 can be repeated to extend the rotation. However, the further the crop rotation is extended the greater the risk of selecting resistant ryegrass. Five to seven years should be considered as a maximum rotation before pasture.

(3) Glean should be rotated with Hoegrass if ryegrass is the only problem.

Crops should then be rotated with wheat being followed by lupins or an alternate legume. The advantage to lupins is that Simazine or Simazine Trifluralin mixes can be used which may provide ryegrass control and help control resistant ryegrass. Because of the current economic condition, farmers may be trying to use two wheat or cereal crops between legumes. This is not ideal because it increases the use of AOPP herbicides in the rotation.

Other important factors in avoiding resistance are to restrict both the number of cultivations and the length of the crop rotation.The general rule for crop rotation is "the shorter the better”. As a guideline, 5-7 years should be a maximum. Unfortunately many farmers using lupins/wheat rotations and direct drilling are reporting better yields as the rotation proceeds and they will have to be made aware of the risk of continuing this practice. When assessing the length of the crop rotation, pasture cleaning involving AOPP herbicides, particularly without spraytopping, should be regarded as a year of crop.

Strategy for farmers with resistance

The strategy for those with a resistance problem (Table 4) is to first confirm the problem by observation, discussion and testing and then to return the area involved as quickly as possible to pasture. If an opportunity can be taken to destroy seed of the resistant grass, so much the better.

The area should then be managed in pasture for many years to avoid, as far as possible, flowering and seeding of ryegrass using heavy spring grazing, mowing and chemical topping. After this period it may be appropraite to return the area to crop for a short rotation after fallowing. In this crop rotation use should be made of the chemicals, if any, that the ryegrass was shown to be susceptible to by testing.

Table 4. Strategy to minimise the effect of Herbicide Resistant Ryegrass on farms in southern New South Wales

Situation	Preferred strategy	Other strategy or comment
Herbicide resistant ryegrass suspected	Confirm this by testing
Herbicide resistant ryegrass confirmed	Return the area to pasture as soon as possible without a crop	Undersow a subsequent crop
Once the area is in pasture	+ Keep in pasture 4-5 years. + Heavily graze the pasture area each year to prevent seed set.
Two years before re-cropping	Spraytopping to stop ryegrass seeding
Year before crop	Spraytop or winter clean (Gramoxone)	Cultivate on the break to encourage germination
Crop rotation	Short rotation 2-3 years. Crop preference - Canola + Trifluralin.Wheat + Glean or Trifluralin. Lupins + Simazine	Sertin is still useful.

(iii) Specific Rotations

Wheat-Wheat-Barley

A traditional rotation now losing favour because of low yields and the introduction of new crops. It can be high yielding particularly when combined with pasture cleaning. The recommended herbicides for the first wheat crop would be Trifluralin, Yield, Glean or Logran if wild oats is not a problem.

The second crop should be direct drilled with Glean, Logran, Hoegrass or Grasp used for control. Herbicide use should be avoided in the barley crop.

Canola-Wheat-Lupins-Wheat

The preferred rotation from both a yield and resistance point of view. The opportunity to use a double rate of Trifluralin for ryegrass control and Glean or Logran in the wheat with AOPPs in the lupins should minimise resistance problems. No herbicides should be used in the undersown crop if possible.

Cleaned pasture-Wheat-Lupins-Wheat-undersown cereal

In this rotation the AOPPs use should be reserved for the pasture and the lupin crop if

Simazone is ineffective. The later crops in the rotation should be direct drilled.

Any crop rotation over 7-8 years

These are problem rotations particularly if direct drilling, using the AOPP products such as Hoegrass, Fusilade,Verdict or Assure, has been widely used. Paddocks which have been regularly sprayed in this way should be rotated back to pasture in case resistant ryegrass is developing.

Wheat-Triticale-Barley or Oats undersown

Similar comments apply to this rotation as to the Wheat-Wheat-Barley rotation.

Lupins-Wheat-Lupins-Wheat

An excellent rotation particularly if Simazine and Trifluralin are used for ryegrass control where appropriate. In the past this has been continued for many years. This may now be questioned and the total length of the rotation should be no more than seven years. It may also not be the most economic rotation.