Marta Monjardino^A, David J. Pannell^B, Stephen Powles^A

Abstract

Most cropping farms in Western Australia must deal with the management of herbicide-resistant populations of weeds such as annual ryegrass (Lolium rigidum) and wild radish (Raphanus raphanistrum). Farmers are approaching the problem of herbicide resistance by adopting integrated weed management (IWM) systems, which allow weed control with a range of different techniques. One important question in the design of such systems is whether and when the benefits of including pasture in rotation with crops exceed the costs. In this paper, the Multi-species RIM (Resistance and Integrated Management) model was used to investigate the value of including pasture phases in the crop rotation. For this farming system, the most promising of the systems examined appears to be so-called "phase farming", involving occasional three year phases of pasture rather than shorter, more frequent and regular pasture phases. This approach was competitive with the best continuous cropping rotation in a number of scenarios, particularly where herbicide resistance was at high levels.

Herbicide resistance has become a major problem in Western Australian dryland cropping, following the widespread and persistent use of herbicides for weed control. Contributing factors include the adoption of minimum and no-tillage systems, and reduced areas of pasture in favour of intensive cropping rotations. Persistent application of herbicides without the traditional weed control provided by cultivation and grazing has led to a high herbicide selection pressure on weed species such as annual ryegrass (Lolium rigidum) and wild radish (Raphanus raphanistrum) (Llewellyn and Powles, 2001; Walsh et al., 2001). For several reasons, including resistance, these two species are the most widespread and economically damaging weeds in that cropping region.

Farmers are managing herbicide-resistant weed populations by adopting weed management (IWM) systems, which allow weed control with a range of non-herbicide techniques along with changed herbicide usage (Powles et al., 1997). Non-herbicide methods include the use of pasture, which can help in effective weed control by allowing a combination of grazing and other methods such as application of non-selective herbicides and cutting for hay or silage before infesting weeds can produce seed.

This study uses a recently created bio-economic simulation model (Monjardino et al., 2002) to evaluate the value of pasture within a cropping-based system involving the integrated management of ryegrass and wild radish. The focus is on two specific questions:

1. How large do the seed banks of annual ryegrass and wild radish need to be before it is worth introducing a pasture phase?
2. How do key biological and economic variables affect these break-even weed densities?

In the Multi-species RIM model, both weed seed production and expected crop yield after competition with the other species are calculated through the following equation:

(1)

Where,

Y = Weed seed production or proportion of grain yield after competition

P₀= Reference density of the crop at standard seeding rate

P₁= Density of species 1 (eg. crop)

P₂ = Density of species 2 (eg. ryegrass)

P₃ = Density of species 3 (eg. wild radish)

k_2.1 = Competition factor of species 2 on species 1

k_3.1 = Competition factor of weed species 3 on species 1

a = Background competition factor (plant density at which yield loss is half the maximum yield loss, i.e. density at which: 1 – PGY = M/2)

M = Maximum proportion of grain yield lost at very high weed densities

The parameter values for equation 1 are shown in the Appendix together with biological key factors that drive the pattern of weed population change over time.

Multi-species RIM comprises a selection of seven different enterprises, including four crops (wheat, barley, canola and lupins), as well as three types of pasture for grazing by sheep (sub-clover, cadiz serradella and volunteer pasture). The sequence or rotation of crops and pasture over time is specified by the user. When any of these enterprises is chosen, production of grain, hay/silage or livestock occurs. However, crop yield can be significantly reduced by weed competition. In addition, short rotations may affect potential crop yield (due to disease) as may some control methods, for example by delaying crop sowing or through phytotoxic damage by herbicides applied in-crop. Yield benefits provided by rotation with legume crops or pasture (due to nitrogen fixation) are also accounted for (Monjardino et al., 2002; Pannell et al., 2001).

In the Multi-species RIM model there are 50 herbicide and non-herbicide control options available (for more details on each method, see Monjardino et al., 2002):

• 27 crop-selective herbicide options for grass and broadleaved weeds (herbicides of high and moderate selection pressure);
• 6 non-selective herbicides (herbicides of low resistance risk);
• 17 non-herbicide methods, including cultivation, delayed sowing, seed catching and stubble burning. Grazing during a pasture phase is another important non-herbicide option. Heavily weed-infested crops or pasture can be cut for hay/silage or used for green manuring.

The effect of each control strategy on weed mortality and seed set is specified for each enterprise (Monjardino et al., 2002). Gill and Holmes 1997, Gorddard et al. 1996, Matthews 1996, Powles et al. 1997 and Schmidt and Pannell 1996 suggest that no one method available provides the optimal weed management strategy. Instead, a combination of a range of weed control methods can achieve very effective and sustainable weed control (integrated weed management, IWM). Because control methods are conducted at different times, their combined impacts are considered to be multiplicative rather than additive (Pannell et al., 2001).

The Multi-species RIM model allows the user to specify the initial herbicide resistance status of ryegrass and wild radish with respect to each of nine herbicide groups (modes of action), including herbicide mixtures.

The model calculates costs, revenues, profit and net present value. It also includes complexities such as tax and long-term trends on prices and yields. Costs associated with cropping, pasture and various weed control options have been specified in detail. They account for costs of input purchasing; costs of machinery operating, maintenance and repayment; costs of contracting of labour for hay and silage making; and costs of crop insurance. There are also costs of crop yield penalty due to practices such as green manuring and delayed sowing or due to crop grain contamination with wild radish seeds. Resource degradation costs associated with some non-herbicide methods such as cultivation and burning are also represented in the model. Economic returns from crops are based on grain and hay yields and sale prices. Net returns from sheep are specified as a long-term trend of gross margin per dry sheep equivalent (DSE), combining returns from wool and meat.

Because the model is run over 20 years (T), annual net profit must be discounted to make them comparable at the start of the modelled period. A real discount rate ( r) of 5 percent per year is used for this purpose. The sum of discounted net profits or net present value (NPV) is the main economic criterion used to compare weed management strategies. In results presented later it is expressed in an annualized form on a per hectare basis.

The value of pasture in the rotation was investigated for the following enterprise sequences:

1. A pasture-pasture-wheat-wheat rotation (PPWW), with self-regenerating sub-clover pasture.
2. A barley-pasture-wheat-wheat rotation (BPWW), with volunteer pasture.
3. A barley-pasture-pasture-wheat-wheat rotation (BPPWW), with self-regenerating sub-clover pasture.
4. A barley-pasture-pasture-pasture-wheat-wheat rotation (BPPPWW), with self-regenerating sub-clover pasture.
5. A barley-lupin-wheat-wheat rotation (BLWW).
6. A barley-lupin-wheat-wheat rotation with a single three-year Cadiz serradella pasture phase in years 9-11 (BLWW+ PPP).
7. A lupin-wheat rotation with a single three-year Cadiz serradella pasture phase in years 9-11 (LW+ PPP).
8. A lupin-wheat-wheat rotation with a single three-year Cadiz serradella pasture phase in years 9-11 (LWW+ PPP).

Thus the sequences examined included pasture proportions in the rotation varying from 25 to 50 percent (a, b, c and d), or occasional three-year pasture phases in long-term crop rotations (f, g and h).

All cropping and pasture rotations were investigated for a scenario of herbicide use where a maximum of two herbicide applications of Groups A and B (each) were available before complete herbicide resistance evolved in both ryegrass (Groups A and B) and wild radish (Group B). It was assumed that there were up to 10 applications available of each moderate-risk herbicide group (C, D, F and G) and up to 15 applications available of each low-risk herbicide group (I, L and M). These assumptions about herbicide resistance status are varied later in the paper.

Complementing the strategies of pasture and herbicide application, many combinations of other control methods were investigated in order to find the best management systems. The range of IWM methods employed across the eight rotational strategies is listed in Table 1.

Table 1. Number of uses of each weed management method across all rotations.

The weed management strategy remained relatively constant across all rotations. A decrease in the practice of delayed seeding was observed when more lupins were grown, due to the advantages of early sowing of this crop.

Table 2 shows the average annual returns over 20 years (annuities) and the final weed plant densities across all systems considered in this study. The best rotation in financial terms was the continuous cropping sequence, BLWW ($117/ha/yr), closely followed by the same BLWW rotation and by the LWW rotation, both with a three-year pasture phase once during the 20-year simulation ($113 and $111/ha/yr, respectively). Overall, the shorter, more regular cereal-pasture rotations proved less profitable ($73-84/ha/yr) than the cropping sequence and the phase farming systems. Final weed numbers were low in all scenarios.

Table 2 Annuities and final weed densities across all rotations.

Even though all pasture rotations had similarly low finishing weed numbers (Table 2), weed population patterns varied over the 20-year period. As Figs. 1 and 2 illustrate, wild radish density was kept very low across most years and rotations, but the density of ryegrass had peaks of 10 and 12 plants/m² under PPWW. This is because it was not economical to use any Group A herbicide in that particular scenario. Years of higher ryegrass density (years 8 and 16) corresponded to a second consecutive wheat year. Those ryegrass peaks were then brought under control by effective control tools employed in the following pasture phases. Inclusion of pasture phases as long as three years led to excellent ryegrass control over the whole period. This was due to more frequent and intense grazing and a sequence of 10 simazine (Group C) applications in those rotations.

Fig. 1 Weed densities over 20 years for a pasture-pasture-wheat-wheat rotation.

Fig. 2 Weed densities over 20 years for a barley-pasture-wheat-wheat rotation.

Table 3 Net value ($/ha/yr) of different pasture phases across a range of initial weed seed densities.

In comparison with the default scenario (I), the value of pasture initially decreased (by $8 ha/yr in scenario II) and then increased considerably (by $22 ha/yr in scenarios III and IV) as herbicide use was reduced. The initial reduction in pasture value is likely to be a direct consequence of less Group C herbicides being available, for many of the original ten simazine applications were used in the pasture phase. However, a further decrease in herbicide use brings out pasture as a crucial tool of weed control due to other methods specifically employed in this phase (e.g. grazing, spray-topping, burning). The lower values obtained for pasture in the last two scenarios (V and VI) result from a general low level of returns for both rotations, as weed control becomes very difficult to achieve with hardly any herbicides available. These results show that a pasture phase may make a significant contribution to long-run profitability in situations of low herbicide availability due to herbicide resistance.

Break-even values. An indication of the potential for pasture to become a profitable inclusion in the land-use sequence is the increase in pasture profitability needed for the value of the pasture rotations to equal the value of the cropping rotation (BLWW, $117/ha/yr). Such increases might occur either by increasing the number of sheep (dry sheep equivalent or DSE) carried per hectare (currently 1 to 6 DSE/ha, depending on the type and year of the pasture phase and the grazing intensity) or by increasing the gross margin per head (currently $11/DSE). The increases required to match the net returns of the continuous cropping sequence are shown in Table 5.

Table 5 Break-even values of sheep gross margin for each pasture rotation.

The Multi-species RIM model was used to evaluate the value of including pasture in the rotation. For this farming system, the most promising of the strategies examined appears to be so-called "phase farming", involving occasional three year phases of pasture rather than shorter, more frequent and regular pasture phases. This approach was competitive with the best continuous cropping rotation in a number of scenarios, particularly where herbicide resistance was at high levels.

Break-even analyses showed that for short rotations, the increase in livestock returns or stocking rates required to make pasture phases economically attractive were very substantial. On the other hand, three-year phases of pasture performed relatively well financially, and in situations of high herbicide resistance they performed particularly well.

It is important to appreciate that the model used, while detailed in many respects, excludes some issues that would tend to favour inclusion of some pasture. For example, RIM is limited to a paddock or field scale but there are some relevant issues that only come into play at the whole farm level (e.g. related to management of livestock feed and crop machinery). For farmers giving weight to these issues, the RIM model still provides useful information about the ranking of alternative pasture strategies.

The authors acknowledge the contribution of A. Draper, M. Ewing, R. Llewellyn and M. Walsh in reviewing this paper. The Grains Research and Development Corporation and the Cooperative Research Center for Weed Management Systems provided financial support for this research.

Combellack, J. H., and Friesen, G. (1992). Summary of outcomes and recommendations from the First International Weed Control Congress. Weed Technology 6, 1043-1058.

Gill, G.S. and Holmes, J.E. (1997) Efficacy of cultural control methods for combating herbicide-resistant Lolium rigidum. Pesticide Science. 51, 352-358.

Gorddard, R.J., Pannell, D.J. and Hertzler, G. (1996) Economic evaluation of strategies for management of herbicide resistance. Agricultural Systems. 51, 281-298.

Hoyle, F. (1999). "Green manuring a new focus for cropping,". Department of Agriculture of Western Australia, South Perth.

Llewellyn, R. S., and Powles, S. B. (2001). High levels of herbicide resistance in rigid ryegrass (Lolium rigidum) in the wheatbelt of Western Australia. Weed Technology 15, 242-248.

Matthews, J.M. (1996). Cultural management of annual ryegrass. Plant Protection Quartely. 11 (1), 198-199.

Monjardino, M., Pannell, D. J., and Powles, S. B. (2002). A multi-species bio-economic model for management of Lolium rigidum and Raphanus raphanistrum in Australian cropping systems. Weed Science (submitted).

Pannell, D. J., Stewart, V., Bennett, A., Monjardino, M., Schmidt, C. and Powles, S. (2001). RIM, Ryegrass Integrated Management: a decision tool for the management of annual ryegrass, University of Western Australia, Crawley, Perth

Schmidt, C. and Pannell, D.J. (1996) Economic issues in management of herbicide-resistant weeds. Review of Marketing and Agricultural Economics. 64 (3), 301-308.

Walsh, M. J., Duane, R. D., and Powles, S. B. (2001). High frequencey of chlorsulfuron resistant wild radish (Raphanus raphanistrum L.) populations across the Western Australian wheatbelt. Weed Technology 15, 199-203.

Table A1. Parameters used in the multi-species yield-density equations.

Table A2. RIM parameters associated with population dynamics of annual ryegrass and wild radish.

* Germination refers to % of total initial seed bank.

Table A3. Seed production indices representing seed production by different cohorts of ryegrass (RG) and wild radish (WR), relative to seed produced by healthy (early germinating) weed plants.