Introduction - Understanding The Problem

Weed build - up in ley pastures is inevitable, especially when the ley period is longer than about 2 years. Soil nitrogen accumulation reduces the clover’s competitive ability, allowing weeds, especially the grass weeds, to build up. Early weed build - up is frequently due to poor initial clover establishment. Factors such as excessive sowing depth and herbicide injury lead to patchy clover stands - an open invitation for early weed problems.

Selective grazing of stock exacerbates the problem. Barley grass, for example, ceases to be palatable at around ear emergence/flowering. With both sheep and cattle avoiding it from then on, it is able to set abundant seed.

Weedy pastures cause many problems including wool and skin damage in sheep, poisoning of stock and, of most significance to grain growing, crop disease build - up. It is also probable that weeds in the ley pasture will reduce the nitrogen available for crops.

Ley Pasture Weed Control

Before considering ley pasture weed control, some thought must be given to the prophylactic measures available:

(a) Weed control in the crop phase is a prerequisite for weed control in the pasture phase.

(b) First class establishment techniques will ensure dense, highly competitive subclover leys. This will generally prevent short - term weed build - up. Common problems include excessive sowing depth of clover seed. Frequently this can be corrected with inexpensive adjustments to machinery.

Herbicide injury in the “undersown” stage is a frequent cause of poor seed set and subsequent weak first year stands. Producers need to select herbicide carefully and, if necessary, choose the more expensive products.

(c) Given good establishment, short term leys of two years or less interrupt weed build - up.

(d) For long term leys, the introduction of useful grass (annual ryegrass) at sowing time will keep the weed grasses at bay.

A. Non - chemical weed control

Of course, there are non - chemical methods available for ley pasture weed control. Haycutting, for example, will significantly reduce the population of most common annual grasses. Clearly the value of the hay will be closely related to the grasses involved. Useful silage can be made from a wide range of weeds, effectively reducing seed set.

Grazing stock can be managed to reduce weed populations. It is worth considering also that the removal of stock at problem times (i.e. grass seed maturity) swings the balance in favour of not only the grasses but also broadleaf species such as Paterson’s curse and wireweed.

As with the chemical weed control methods discussed later, care must be taken not to swap one problem for another. For example, cutting ryegrass for hay will reduce or almost eliminate the ryegrass. If the paddock is left for pasture the following year, watch that the absence of ryegrass is not capitalised on by weeds like silvergrass! This happens quite frequently.

Burning of clover leys prior to the break can reduce the level of viable grass seed. Autumn burns are generally not thorough enough to burn all the seed. As in stubble situations, burning grass residue has no effect on take - all. Burning can, however, be very useful in preparing paddocks for operations like “winter cleaning” since heavy dry grass residue will prevent good spray coverage.

My brief for this paper is to discuss herbicide manipulation of ley pastures. Rather than go into much detail about herbicide use in the “cover crop” or establishment stages, I would simply urge producers to carefully choose their chemicals. Cheap, in - crop herbicides such as MOPA + diuron often have a disastrous effect on clover seed set.

Once established, pastures can be treated in a variety of ways depending on the weeds and what is planned for the next season.

B. Chemical weed control

(a) Selective herbicides

Both grasses and broadleaf weeds can be sprayed selectively in ley pastures.

(i) Broadleaf herbicide examples are bromoxynil and 2,4 - DB

(ii) Grass selective herbicide examples are Fusilade 212(R), Sertin(R) and Verdict(R).

These chemicals all need to be carefully chosen. Light infestations of tolerant or immune weeds amongst susceptible weeds rapidly fill the spaces left by dead weeds. A perfect example of this is silvergrass (Vulpia spp2. It shows high tolerance or immunity to all current selective grass herbicides. The removal of barleygraSs (Hordeum spp.) and brome grass (Bromus spp.) quickly leads tosilvergrass dominance.

(b) “Spray-grazing”

Broadleaf weeds such as Paterson’s curse and capeweed can be removed using

a combination of hormone sprays at sub - lethal rates and heavy grazing

with sheep. An example recommendation would be to treat the pasture with

500 g/L MCPA at 1.0 L/ha. One week later, stock the paddock at 4 - 5 times

the normal rate until all weeds have been eaten out.

A problem with this method is the potential for stock poisoning, especially if high levels of Paterson’s curse are involved. The recommended class of sheep for spray - grazing is old wethers. Because the method is carried out about 6 - 8 weeks after. the “break”, it has little or no impact on grass weeds. (It can, however, be readily combined with winter cleaning or spray - topping).

(c) Spray-topping/pasture topping

This is a particularly useful method of weed control, providing it is done as part of a planned weed control/disease programme. Weeds are sprayed at about flowering time with a view to sterilising the seed. The method is effective against grasses and broadleaf weeds such as capeweed (when in peak flowering).

The herbicides glyphosate and paraquat are used and both cost approximately $6.00/ha. The two herbicides have slightly differing roles due to their mode of action:

(i) Paraquat is a contact herbicide. In the topping process the seed heads must have fully emerged from the boot. For example, late emerging barley grass heads escape topping. Topping with paraquat is effective up until the milky dough stage of the seed.

(ii) Glyphosate is a translocated herbicide. In the topping process, the seed heads can be effectively sterilised while still in the boot. The cut - off for late glyphosate applications is slightly earlier than for paraquat.

(d) Herbicide fallow and “hay freezing”

An interesting alternative to simply sterilising the seeds is to apply enough herbicide to kill the plants altogether. Such an application can be made earlier and will cover a range or species much better. This approach has the added bonus of commencing a fallow as well as leaving the herbage in a highly palatable state.

(e) Winter “cleaning”

An old, but less - than - popular method of grass control is winter “cleaning In this method, weeds are sprayed around the end of June. This coincides with maximum weed seedling emergence and subclover plants have at least 6 true leaves. Spray.Seed (R) or paraquat can be used depending on the weeds present.

Excellent control of a wide range of grasses is possible. Grass seed - free conditions ate provided for the spring and summer. As well, cereal diseases such as take - all are controlled.

The main disadvantage of winter cleaning is that up to 8 weeks grazing are lost post - spraying. While this is probably the most common objection expressed by farmers, I believe it is offset by problem - free grazing through spring and summer and the advantages of weed/disease control when re - cropping.

Pasture Weeds And Crop Disease

I believe that in the ley farming areas, take - all disease causes annual losses of about 10% and more in epidemic years. In epidemic years, the situation is plain enough. However, disease can account for up to 15% of yield without expressingany of the normal symptoms. I ran a take - all fungicide trial at Collingullie where the difference between the “nil” and best treatment was 10% - a significant loss but there were no white heads or stunted plants. This highlighted the insidious nature of take - all.

A second important crop disease is Sclerotinia. This causes regular yield loss in rapeseed, lupins and peas. The etiology (growth pattern) of Sclerotinia is far less clearly known than take - all. There is no evidence that a lack of a host one year will reduce the disease next year. In fact, it seems that the disease is present most of the time and is promoted by seasonal conditions (moist) and crop density (dense). However, like most diseases, the presence of a host one year greatly increases the risk of problems the following year. Sclerotinia affects a wide range of broadleaf plants including weeds such as thistles and capeweed.

Spray-Topping Or Winter Cleaning?

Since the grassy weeds affect both grazing and cropping enterprises it is worth expanding on the chemical means available for controlling them. The two methods most commonly suitable are winter cleaning and spray - topping (Table 1).

Table 1. Comparison between winter cleaning and spray - topping for grass weed control.

	WINTER CLEANING	SPRAY-TOPPING
Paddock Selection	- paddock needed for hay. - paddock needed for grass seed - free spring feed.	- grassy paddock to be re cropped with oats, rapeseedor linseed.
	- full range of crop options required for next season. - paddock must have high level of healthy clover.	- best blocks to treat will be predominantly one weed species.
	- method satisfactory on mixed grass swards.	- where mixed weed populations prevent good spray-top application, herbicide fallow or hay freezing may be better treatment.
Paddock Preparation	- paddock should be closely grazed until all grasses germinated and clover has 6 true leaves.	- paddock grazed during spring flush until grasses begin to head
	- stock should be removed several days prior to spraying so that grasses recover to about 1 - 2 cm in height.	- stock are removed to promote uniform heading of grasses.
	- (burning off heavy dry grass residue prior to the “break” will prevent application problems)
Spray Timing	- usually between mid - June and through July. Later sprayings (August) may get good grass control but clover recovery will be poorer.	- usually about October but depends on seasonal conditions and species involved. Herbicide choice can influence timing.
		Glyphosate can be applied between the boot stage and milky dough stage.
		- Paraquat can be applied from when the heads are fully emerged through to the milky dough stage.
		- to reduce the effect on clover seed set, spray - top after the clover has flowered.

	WINTER CLEANING	SPRAY-TOPPING
Spray Grazing	Spray grazing for broadleaf weed control can be slotted in to either programme. Spray grazing prior to winter cleaning Is ideal because it leaves grasses at optimum height.
Herbicide Choice	Paraquat (Gramoxone(R) or Shirquat (R)) Spray.Seed (R)	- Glyphosate (Roundup CT, Roundup(R), Nufarin Glyphosate 360 and Nufarin Glyphosate 450). Paraquat, Spray.Seed (R).
		- Where there is a single weed species present, either herbicide will do a good job providing timing is correct.
		- When a range of species is present, glyphosate could be the best choice since it is effective over a longer period than paraquat.
		- If glyphosate is chosen, timing is more critical with ryegrass and silvergrass than it is with barley grass and broine grass.
		- When paraquat is used, timing is more critical with barley grass than it is with ryegrass, silvergrass or brome grass.
		- Because of paraquat’s shorter timing period, target dominant species if more than one grass present.
Herbicide Label	NOTE: Refer to labels for rates and application details especially those relative to extra wetting agent and most suitable water rate. Boom equipment must be in first - class working order.
Cost of Chemicals	- depends on rates chosen but average rates cost about $15.00/ha plus plus $4.50 for contract application. Red - legged earthmite control costs about $1.30 extra. Total - $20.80.		- depends on rates chosen but average rates cost about $6.00/ha $4.50 for contract application. Total - $10.50. Herbicide fallow or hay freeze rates are higher.

	WINTER CLEANING	SPRAY-TOPPING
Post Spraying Management	- Healthy clover will recover quite quickly but it may take several weeks to recover enough for re - stocking. The actual time will depend on clover variety and spring conditions.	- Providing there are no potential grass seed problems, topped paddocks can be re - stocked after 24 hours.
	Salt blocks may need to be supplied for wethers and rams, because such high clover content grazing can cause urinary stone problems.	Where seeds appear to be a problem, either delay stocking or use some mechanical means to flog the seed out.
Results to Expect	- Some escapes are inevitable.	Results will depend on grasses.
	Grass which is covered by wool or manure will escape. Poor coverage due to nozzle problems will cause escapes especially of silvergrass. Also expect some very late germinations, especially barley grass.	- Main problems arise from tillers which mature late, escaping the “topping” process and where a range of grass species is involved. - Under abnormally cool, moist, spring conditions it is likely that grasses will recover by re - tillering producing seed burdens. To counter this, either delay topping or increase the herbicide rate to fallow or hay - freeze levels.
Re - cropping	Method leaves paddock free of trash problems. The full range of cereal crops can be grown without risk of serious take - all problems.	- Sheep eat spray - topped grass residue completely. Paddocks are left in ideal physical condition for re - cropping. Method does not control take - all. Ideal crops to grow are rapeseed or oats.
Continued Pasture	Situations will arise where either method could be used to ensure reduced grass problems in the following pasture year. This is feasible but the ecological forces which produced the grass problem in the first place will rapidly cause them to come back. Where spray - topping is used for this purpose, ensure there is an adequate level of hard seed available or that topping is delayed so that clover seed set is not seriously reduced. As well, the thorough grazing typical of topping treatment could reduce burr levels over summer.

Summary

Satisfactory ley pasture performance is clearly vital for optimum farm income. Weed problems, especially from grass weeds, are the cause of large losses in the livestock and cropping enterprises. The adoption of a whole farm management programme to reduce grass weed problems is needed by all producers in a continual process involving crops and pastures. Herbicide manipulation has a vital part to play in the overall programme.

For Extra Information

The following relevant publications are available from the NSW Department of Agriculture:

- Herbicide for Weed Contrcd in Lucerne Pastures 1987

- Agfact P2.2.4 Pasture Establishment

- Agfact P2.2.6 Nine Steps to Successful Pasture Establishment

- Agfact P3.AB. 10 Take - all of Wheat

Chemical companies also have information in coloured booklet form:

- ICI Chemicals in Agriculture - “Spray.Seed Handbook for Upgrading Pastures and Lucerne”.

Monsanto. Roundup CT broadacre herbicide weed management series “Pasture Renovation”, “Pasture Topping” and “Fallow Commencement

The Approximate Retail Price Of Chemicals Used On Lucerne And Pasture

Chemical	Registered	Product	Commonly used rate	Cost
	Name	Price litre or kg $	Per ha	per ha $
Ainitrole + ammonium thiocyanate	Weedazol TL Plus	6.53 L	3.0 L	19.59
Asulain	Asulox	11.65 L	12.0 L	139.80
Atrazine 80	Vectal	8.95 kg	0.7 kg	6.27
Atrazine flowable 50	Various trade names	5.90 L	LI L	6.49
Benefin	Balan	9.75 L	7.0 L	68.25
Bromoxynil	Brominil, Barrage, Buctril 20, Broinicide 200		2.1 L	23.73
Broinoxynil + MCPA	Buctril MA, Broininil M, Barrage M, Broinicide M	12.20 L	1.4 L	17.08
Carbetainide	Carbetamex	30.60	2.5 kg	76.50
Dicainba	Banvel	13.50 L	0.7 L	9.45
Dicainba + 2,4 Dainine	Pastureinaster	10.35 L	1.0 L	10.35
Dicainba + MCPA	Banvel M	8.40 L	2.8 L	23.52
Diclofop methyl	Hoegrass	18.50 L	1.0 L	18.50
Diquat	Reglone, Diquat 200	12.45 L	0.7 L	8.72
Diuron 800	Various trade names	10.10 kg	2.2 kg	22.22
Diuron flowable 500	Diurex 50 Flocol	6.85 L	3.5 L	23.97
Fluazifop 212 g/L	Fusilade 212	70.00 L	0.5 L	35.00
Glyphosate 360 g/L	Roundup/Nufarin Glyphosate 360	17.50 L	1.5 L	26.25
Glyphosate 4450 g/L	Roundup CT Nufarin Glyphosate 450	18.75 L	1.2 L	25.08
Methabenzthiazuron	Tribunil	26.70 kg	0.85 kg	22.70
Metribuzin	Lexone, Sencor 70	98.00 kg	0.4 kg	39.20
MCPA 250	Various trade names	3.10 kg	3.0 L	9.30
MCPA 500	Various trade names	5.30 L	1.5 L	7.95
MCPB	Tropotox, MCPB 400,
	Metacide	7.50 L	2.8 L	21.00
Paraquat	Grainoxone W, Shirquat 1	11.20 L	1.5 L	16.80
Paraquat + diquat	Spray.Seed	9.00 L	1.5 L	13.50
Piclorain + 2,4 - D	Tordon 50 D	17.80 L	0.47 L	8.37
Proinetryne	Gaesagard	22.90 kg	0.85 kg	19.46
Sethoxydin	Sertin	34.50 L	1.0 L	34.50
	Verdict (R)	24.00 L	750 ml	18.00
Tetrapion	Frenock	26.70 L	2.0 L	53.40
Trifluralin	Treflan	8.10 L	1.4 L	11.34
2,4 - DB	Embutox 40	7.0 L	2.8 L	19.88
	Selectone, Leguinex,Buticide
2,2 - DPA	Various trade names	5.30 kg	3.5 kg	18.55
2,4 - D amine	Various trade names	4.50 L	1.4 L	6.30
1,4 - D ester	Various trade names	8.90 L	0.7 L	6.23
DC - Tron spray oil	DC - Tron	1.65 L

Prices are suggested retail. They may vary according to area, quantity used and are only a guide. (This was taken from ‘Weed Control in Winter Lucerne Pastures, 1987’).