Coping with New Challenges in Cereal Weed Control (Section I)

Jim O’Mahony and Bryan Mitchell, Teagasc, Crops Research Centre, Oak Park

SUMMARY

Successful weed control is a key requirement of modern cereal production systems. However, achieving good weed control continues to pose difficulties. Effective and economic weed control will be achieved when an appropriate rate of a suitable herbicide is applied to a susceptible weed at the correct growth stage. This requires that weeds are properly identified and their likely impact on yield is assessed. Suitable herbicides must then be selected and a rate appropriate for the weed growth stage and prevailing environmental conditions is chosen. Each of these steps requires a considerable knowledge input. The cost of weed control is mainly dictated by the weeds present. It can vary from €7.5/ha to over €125/ha. Successful weed control will involve an integrated approach to rotations, cultivations and herbicide use in each field.

New challenges for weed control include the reduction of the competitive ability of crops as a result of using reduced seed rates. Earlier sowing of winter wheat is leading to increased problems. Problems are associated with the use of reduced tillage systems, particularly with grass weeds. New weed species are also becoming prevalent, including weed beet and sterile brome. The emergence of herbicide resistance may also pose increased difficulties for weed control in the future.

The trend in winter cereals is for early post-emergence herbicide application, tank-mixed with an aphicide at the 2-3 leaf stage of the crop. However, a two-spray programme is suggested for wheat sown in September/early October. Timing is critical for success. Spring cereals will generally be sprayed at the 3-5-leaf stage. Again, timing is crucial for successful weed control. Wild oats, brome, ryegrasses, canary grass and cleavers will need specific strategies.

Research at Teagasc Oak Park indicates that herbicide rates can be reduced by up to 50% where the weed spectrum is susceptible and the herbicides are applied under very good conditions to actively growing young weeds in crops with good dense crop structures.

Introduction

Poor weed control in cereals can lead to considerable yield losses, make harvesting difficult and is visually undesirable. In addition, poor control in one season can lead to problems with increased weed populations in subsequent crops. However, despite the availability of numerous herbicides, problems with weed control in cereals persist. There are a number of contributing factors. Changing husbandry practices, such as early sowing, eco-tillage and low seed rates as well as complacency following the introduction of very successful herbicides in the 1980s are resulting in more costly weed control in crop rotations. The switch to winter cereals and the increased area of monoculture winter wheat in particular is changing the weed spectrum in many fields. The constant use of the same herbicides is further selecting for weeds not well controlled by a particular herbicide, e.g. fumitory which is not well controlled by Ally or Cougar. The situation is being aggravated by more stringent EU registration requirements, resulting in loss of some useful herbicides and much fewer new active substances being commercialised due to the high costs involved.

With these problems in mind, it is timely to examine the problems being experienced with weed control in cereals and to outline strategies which will allow these problems to be avoided in the future.

In this paper I will:

outline the fundamentals of weed control
outline new challenges for weed control
examine trends in herbicide availability and performance
suggest cereal weed control strategies for the future

FUNDAMENTALS OF Weed CONTROL

Effective and economic weed control will be achieved when an appropriate rate of a suitable herbicide is applied to a susceptible weed at the correct growth stage. This requires that:

weeds are properly identified and their likely impact on yield is assessed
suitable herbicides are selected
rate appropriate for the weed growth stage and prevailing environmental conditions is chosen

Weed Identification and Impact on Yield

Identifying the weeds present in a crop is the first key requirement to obtaining good weed control. This can be difficult, particularly when the weeds are at the cotyledon stage, but failure to correctly identify the weeds can lead to the application of an unsuitable herbicide which will lead to poor control. A knowledge of what weeds were present in particular fields in the past will give a good indication of what weeds are likely to appear in a field and weed mapping is useful in this regard.

Knowing which weeds are competitive and how they propagate is also important. Some weeds are much more competitive than others. The competitiveness of an individual weed species will vary according to crop type. Generally, weeds that share the same canopy characteristics and growth patterns as the crop are relatively more competitive in that crop type than weeds that have canopy characteristics which are different from that of the crop. For example, wild oats which has a similar canopy and rooting structure to cereals is highly competitive towards cereals. Knowing the competitive abilities of weeds helps to identify priority weeds for control. Table 1 gives the relative competitive abilities of a range of common weeds in winter wheat from a number of years of UK research. It can be seen that cleavers and wild oats are the most competitive weeds and the biggest yield robbers, and therefore control programmes must aim to achieve a high level of control of these. Other weeds such as groundsel and fumitory, are much less competitive and low levels of these in a particular crop are not likely to cause economic yield loss. In addition, crop species and even varieties of a particular species vary in their competitive ability towards weeds. For example, oats is more competitive than barley which in turn is more competitive than wheat towards weeds.

The timing of weed emergence affects weeds competitiveness and is also important from a control point of view. Weed plants that emerge at the same time as the crop are more competitive than those individuals from the same species emerging later. There is also variation on a seasonal scale between weed species with regard to their germination patterns. Some weeds germinate mainly in the spring, e.g. black bindweed, knotgrass and orache and are very unlikely to be a problem in winter cereals. On the other hand, many weeds germinate over a very protracted period, e.g. meadow grasses, chickweed, cleavers, fumitory, groundsel etc. This has obvious implications for herbicide timing and choice of residual versus contact types etc.

The time of the start of competition that will result in a crop yield loss varies according to the relative growth characteristics of the crop and weed. The late start of the period of rapid growth in cleavers is the reason why it can be controlled at a later growth stage (in wheat) without crop yield loss than for other broad-leaved weeds where rapid growth starts earlier. Grass weeds including meadow grasses and wild oats will need to be controlled early, as they can compete with the crop from an early stage, and in the case of meadow grasses existing products give poor control once the grass is past the 3-leaf stage. In general, most weeds do not compete strongly with the winter cereal crop until the spring (around gs 31), unless they are at very high populations. However, by that stage they may be too advanced to achieve acceptable weed control, particularly where winter wheat crops are sown early.

Table 1: **Relative competitiveness of weeds in winter wheat**
Weed (common name)	Competitive index (% yield loss/weed)	No. of weeds/m² to cause a 5% yield loss
Cleavers	3.0	1.7
Wild oats	1.0	5.0
Mayweed	0.4	12.5
Charlock/mustard	0.4	12.5
Thistle	0.3	16.7
Black bindweed	0.3	16.7
Field bindweed	0.3	16.7
Fat-hen	0.2	25.0
Chickweed	0.2	25.0
Annual meadow-grass	0.1	50.0
Knotgrass	0.1	50.0
Fumitory	0.08	62.5
Speedwell	0.08	62.5
Groundsel	0.06	83.3
Sow thistle	0.03	166.7

Source: IACR/HGCA UK

Herbicide Selection

Selecting the most appropriate herbicide is key to successful weed control. A number of factors must be considered. The crop to which it is applied and the weeds present are obviously key factors in herbicide selection. The chosen herbicide must kill the weeds present without damaging the crop. This will require information on weed species and growth stage as well as crop growth stage. Most herbicides have restrictions as to the crops to which they can be applied and for any crop there are normally a limited range of growth stages at which it can be applied. Site factors, such as soil type, pH and organic matter content, also need to be taken into account for some herbicides. Many herbicides are applied with other pesticides and therefore tank-mix restrictions must be taken into account when selecting the herbicide. Effects on succeeding crops will also be important as many herbicides can persist in the soil and affect establishment of the succeeding crop. Avoiding the build up of resistant weeds should also be considered.

Selecting an Appropriate Rate

Herbicides used at full rates will generally give the highest and most consistent levels for weed control in cereals. However, falling cereal margins are forcing growers to reduce costs while maintaining output and reducing herbicide input is one option to be considered. Farmers' experiences with reduced rates in appropriate circumstances have been generally positive. Extensive research (1993-'96) by B. Mitchell, Teagasc, Oak Park on the effectiveness of reduced rate herbicide in winter barley, winter wheat and spring barley confirms that rates can be reduced by up to 50% in certain circumstances without loss of yield or quality, although weed control can be slightly lower than with full rates. Reductions below 50% rate can give unacceptable levels of weed control. Herbicides used at 25 % of the recommended rates give moderate levels of weed control but tend to give lower grain yield.

Table 2: Effect of reduced herbicide on weed control in winter barley 1993-'96
Product	Control (%)	Yield (t/ha)
Cougar 1.5 l/ha	95	7.4
Cougar 0.75 l/ha	80	7.5
Cougar 0.375 l/ha	70	7.4
Untreated	-	5.6

Table 3: Effect of reduced herbicide on weed control and yield of late-sown winter wheat, 1993-'96 (Site, Co. Wexford)
Product	Control (%)	Yield (t/ha)
Ally 30 g/ha + Starane 0.5 l/ha	95	11.8
Ally 15 g/ha + Starane 0.5 l/ha	85	11.7
Ally 7.5 g/ha + Starane 0.5 l/ha	71	11.4
Untreated	-	9.5

However, when using reduced rates it is essential that the correct product is applied under good conditions when weeds are at their most susceptible. The levels of weed control obtained with reduced rates of herbicides varied with weed species and the growth stage of weeds at the time of spraying. Thus, selection of herbicides and rates of herbicides should be field specific. Because of higher crop competition reduced herbicide rates tends to give more consistent results in barley than in winter wheat.

Trials at Oak Park indicate that rates down to 50% of suitable herbicide applied pre-emergence or early post-emergence can give optimum results in a high proportion of crops where grass weeds are not a problem. If inadequate weed control is obtained in winter cereals with a reduced autumn herbicide, a follow-up spray of reduced rate herbicide in the spring is necessary.

Trends in herbicide availability and performance

There will be over 35 different cereal herbicide formulations on the Irish market in 2004. Therefore, there will generally be a number of herbicide options for any particular weed problem and selection of the most appropriate option is vital to success.

The 35 different herbicide formulations tabulated according to their major use/activity are presented in Tables 4-7 as follows:

Table 4: Herbicides for broad-leaved weeds in cereals

Table 5: Herbicides for wild oats and grass weeds in cereals

Table 6: Herbicides for grass and broad-leaved weeds - winter wheat and barley

Table 7: Herbicides for undersown spring cereals

These tables provide an overview of the products available, the crops to which they can be applied, and the correct growth stages for application as well as comments as to the efficacy of each product in particular situations.

Table 4

Table 5: **Herbicides for wild oats and grass weeds in cereals**
Herbicide	Safe crops	Crop growth stage	Wild oat growth stage	Rate/ha (Euro/ha, ex. vat)	Comments
Puma Extra (Fenoxaprop - P - Ethyl 69 g/l) (FOP)	Wheat	Emerg. to flag leaf, gs 39	Emerg to mid stem elongation g.s. 33	0.8-1.0 L Water 200 l/ha	Controls R.S. meadowgrass and bent. Tank mixes:-Wheat- Do not mix hormone herbicides or SUAs except Eagle. Leave 7 days interval between application of these and Puma. Include oil on wheat
	Barley All varieties except Opal	up to gs. 31		(50-63)	Barley- Only fungicides and aphicides allowed Not oil/adjuvants on barley
Cheetah Super (Fenoxaprop - P - Ethyl 55 g/l) Croplink Fathom (FOP)	Wheat	Emerg. to flag leaf	Emerg to flag leaf gs 39	1.0-1.5 l Water 200 l/ha (35-52)	Controls R.S. meadowgrass, bent and awned canary grass. Tank mixes:-Wheat- Do not mix hormone herbicides or SUAs except Eagle. Leave 7 days interval between application of these and Cheetah Super
Grasp (Tralkoxydim 250g/l) (DIM)	Barley Wheat	2 leaf to flag leaf gs. 12-39	2 leaf to 1st node	1.0-1.4 l (33-47)	Controls ryegrass and bent. Reduced rate for early use. Must add output @ 0.375% spray vol Do not mix hormone or SUA herbicides. Must leave 15 days interval between application of hormones or SUAs and Grasp or 5 days after Grasp application. Compatible with fungicides, insecticides and PGRs (except Meteor) and Starane.
Pelikan (Flamprop - M-isoprophyl 200g/l) (FOP)	Barley Wheat	Leaf sheath Erect to flag leaf Gs. 30-47	Before heading	3.0-3.5 l (62-73)	Controls onion couch and black bent. See label for tank mixes. Only Starane, CCC, Mn chelate and fungicides may be mixed.
Topik (Clodinafop-propargyl 80g/l Cloquintocet-Mexyl 20g/l (FOP)	Wheat	gs. 11-39	gs. 11-39	0.19-0.375 l (25-50) Add Actipron at reduced rate<	Good control of RSMG and moderate control of Italian ryegrass, onion couch, black and loose silky bent at gs. 11-23. Tank mixes-ok except hormone herbicides.

Table 6: **Herbicides for grass and broad-leaved weeds - winter wheat and barley**
Name	Chemical	Crops	Rate/ha	Latest timing	Strengths	Weaknesses
Arelon Guideline Tolkan SIPTU	Isoproturon 500g/l	Wheat Barley	3.0-5.0 l	Depends on Weeds	Good on Wild Oats Flexible timing, useful in tank mixes	Limited Weed Spectrum
Cougar	Isoproturon 500g/l Diflufenican 100g/l	W. wheat W.barley	1.5-2.0 l	End February	Good on wide range of weeds	Weak on Fumitory, Poppy
Bacara*	Flurtamone 250g/l Diflufenican 100g/l	W. wheat W. barley W. oats	0.5-0.75 l	End of December gs 32	Can be used on oats for AM-Grass	Limited weed spectrum
Stomp Trump	Pendimethalin 400g/L Pendimethalin 236 g/l Isoproturon 236 g/l	W. wheat W. barley	2.5-5.0 l 4.0-5.5 l	gs 23 gs 31	Useful in tank mixes for Fumitory and Meadow-grasses Good weed spectrum including Fumitory, Poppy, Meadow Grasses	Limited Weed spectrum Weak on Groundsel
Affinity	Carfentrazone-ethyl 7.5 g/kg IPU 500 g/kg	W. wheat W. barley	2.25 kg	gs 29	Good on Grass, Cleavers, Speedwells. Can use at low temp.	Not very ??? residue
Monitor	Sulfosuluron 80%	W. wheat	2.5 g	From Feb. 1 to gs 39	Good on brome, scutch, rsmg, cleavers, mayweed, charlock, shepherd's purse, fool parsley	Do not use in tank mix or in sequence with others Sus. Must add adjuvant.

Table 7: Herbicides for undersown spring cereals Herbicides
Trade name	Active ingredient	Stage of growth
Trade name	Active ingredient	Cereals No. leaves	Clover No. trifoliate leaves	Grasses No. leaves
Alistel	2,4 DB MCPA Linuron	W.B.O. – 5	1+	2	3.5 l
Acumen	Bentazone MCPB MCPA	W.B.O. – 2	White 1+ Red 1-3 only	2	5 - 6.5 l
Legumex DB	2,4 DB MCPA	W – 3 B.O. – 1	1+	2	7 l
MCPA Do not exceed 1.5L/ha	MCPA	W.B.O. – 4	White 3-4 Red 1	2	1.5 l
Nintex	2, 4 DB CMPP	W – 4 B.O. – 2	1	-	7 l

The EU has harmonised pesticide legislation across the member states with Council Directive 91/414/EEC. This directive requires that registered active substances have acceptable risk to human health and the environment, and that products have acceptable efficacy. The EU is in the final stage of reviewing all active substances that were notified so that they comply with the demanding standards set out in Directive 91/414/EEC. As a result of this process a number of herbicides have been withdrawn from the market in 2003 (Table 8).

Table 8: **Cereal herbicides withdrawn in 2003**
Product Name	Active substance	Main use
Avenge 2	Difenzoquat	Wild oat control
2,4DP	Dichloroprop	Broad-leaved weed control
Fortrol	Cyanazine	Grass and broad-leaved weed control
Legumex Extra	Benazolin	Broad-leaved weed control in undersown cereals

The registration for Commando (Flamprop-M) expired on 31 December 2003 and Pelikan (Flamprop-M) is due to expire on 31 March 2004.

The launch of new cereal herbicide active ingredients in Ireland is now a rarity. This means that in the future herbicide options may become more limited as old products are removed and no new products come to market to replace them. However, there are some new herbicides in the pipeline.

Hussar, containing the SU iodosulfuron-methyl-sodium 5% ww, has been launched on the Irish market by Bayer in the last year. Target weeds include annual meadow-grass, ryegrass and broad-leaved weeds, e.g. cleavers, chickweed, mayweed etc. Crops include winter and spring wheat, spring barley and winter triticale.

There will be a number of new generic wild oat herbicides available in 2004, e.g. Croplink Fathom containing fenoxaprop-P-ethyl. Croplink is awaiting PCS clearance for Avena which contains tralkoxydim the active ingredient in Grasp.