Pesticides Research No. 114 2007 – Effects of Mechanical Weed Control in Spring Cereals – Flora, Fauna and Economy

Effects of Mechanical Weed Control in Spring Cereals – Flora, Fauna and Economy

1 Introduction

1.1 Background
1.2 Aim and conditions

1.1 Background

In the context of protecting biodiversity in modern intensive agriculture, pesticide use is a focus area which in Denmark has been treated by the Bichel Commission (1999). This Commision recommended several approaches to reducing the use of pesticides and since that time a number of research projects have been carried out, mainly financed by The Danish Ministry of Environment. Organic farmland was already known to hold a higher diversity of plants, insects and birds than conventionally farmed areas (Braae et al. 1988, Hald & Reddersen 1990), and accordingly the Bichel Commission recommended that a higher proportion of Danish farmland should be farmed organically, because a promotion of this practice may act as one of the approaches towards the general goal of reducing the use of pesticides.

Within research, particular attention was also paid to organic farming and its methods. However, there was very little knowledge about the immediate effects of a switch of farming practice in contrast to the much broader information about the status after five or more years of organic practice.

Weeds are major constraints to crop production (Marshall et al. 2002), and as a consequence weed control is an almost universal farm operation. On one side, weed control through herbicide use accounts for a major part of the Danish pesticide consumption, and on the other side, mechanical weed control is well established in organic farming (Rasmussen et al. 1997). Mechanical weed control therefore called for particular interest. Furthermore, an earlier project on effects of reduced of pesticide use, accomplished through a reduction of dosage per hectare, partly included mechanical weed control, which proved fairly efficient but did also leave a slight question on possible side effects on ground dwelling predators (Esbjerg & Petersen 2002). A follow-up project dealing with the conversion phase from conventional to organic farming (Navntoft et al 2003) supported the need for further investigation of effects of intensive mechanical weeding on non-target organisms. In that project it was found that arthropods living above the ground, mostly on plants, were unaffected or increased after the conversion to organic practice while the abundance of arthropods on the soil surface was reduced. Thus there were positive biodiversity effects already the first year after conversion, but also rather clear indications of negative effects on a part of the fauna on the soil surface.

Other studies have demonstrated that mechanical weed control has direct, negative effects on beneficial, epigaeical arthropods. Mechanical crop treatments generally reduce the numbers of polyphagous predators directly, e.g. through mechanical damage, and indirectly through habitat disturbance (Thorbæk & Bilde 2004). Specifically for weed harrowing, Thorbæk & Bilde (2004) found a significant direct negative effect on spiders but no significant effects on carabids or staphylinids, neither directly nor indirectly.

Efficient weed and pest control, whether mechanical or chemical, affects populations of farmland birds negatively by reducing the amount of available food, such as weed seeds and arthropods (e.g. Potts 1986, Boatman et al. 2004). Furthermore, it is widely believed that mechanical weed control destroys sizable numbers of nests of bird species such as Skylark Alauda arvensis and Lapwing Vanellus vanellus that breed in the fields. The evidence of an effect on Skylark nests was until recently largely anecdotal, but a recent project (Odderskær et al. 2006) demonstrated that between 50 and 100 % of the nests that were exposed to weed harrowing were destroyed. However, the impact on the Skylark population was negligible because very few nests were active at the time when weed harrowing is normally carried out. By contrast, the negative effects of farming operations on Lapwing breeding success are well documented (e.g. Galbraith 1988, Shrubb 1990, Berg et al. 1992) and may have serious effects. The Danish population of Lapwing was reduced by approximately two thirds from 1976 to 2004 (Heldbjerg 2005), and Lapwing populations all over Europe have plummeted since 1990, to such an extent that the species is now considered Vulnerable according to IUCN Red List criteria (BirdLife International 2004). The available demographic data indicate that the main reason for this decline is an insufficient production of fledglings (Petersen submitted).

Following up on the work of the Bichel Commission, Ørum (2004a) found that a so-called Skylark-friendly farming practice might be implemented at a cost of 10 to 25 DKK per ha. This farming practice did not include mechanical weed control. Ørum (2004) further demonstrated that a general reduction of pesticide use, accomplished through taxes or quotas, would not by itself lead to a more Skylark-friendly practice; implementation of such practices would always require special attention.

Farmers are basically looking for weed control strategies that effectively control the weeds and cause as little damage to the crop as possible. Mechanical weed control may be one efficient strategy to consider. According to the Danish Agricultural Advisory Service (Petersen 2002) weeds can effectively be controlled by using weed harrowing in spring barley, but the mechanical weed control is in general more expensive (around 0.5 hkg per ha) than the most efficient low-dose herbicide strategies.

The present project aims at quantifying the effects – positive or negative – of weed harrowing on selected flora and fauna elements and also at elucidating the efficiency issue. Naturally, the focus is on organic farms where mechanical weed control is the sole option and is often used very systematically, sometimes at high frequency, i.e. up to 4-5 annual treatments (or even more in row crops such as maize). However, the efficiency (profitability and weed control effect) of supplementing or substituting chemical weed control by weed harrowing in conventional farming is also considered, mainly with the purpose of estimating the costs of a potential adoption of more biodiversity-friendly weed control strategies. This aspect is necessary if further suggestions for protection of biodiversity shall be considered.

1.2 Aim and conditions

The present project has two main aims:

1) to investigate and quantify the effects of mechanical weed control on flora and fauna in order to elucidate the significance of this deweeding methodology when it interacts with other IPM components and the focus is promotion of natural elements through diminished use of pesticides.

2) to calculate/ model the costs of particular practices necessary to obtrain particular improvements of flora and fauna.

These two aims are interconnected to several hypotheses and some presumptions.

Hypotheses

That intensive mechanical weed control:

1) Causes floral damages at a level which raises questions about the balance between flora damage and agricultural benefit.

2) Causes density reduction of the epigaeic insects which are both agriculturally benefial and important as bird food.

Presumptions

a) The anticipated biological results will be so well related to the results of preceding projects (Esbjerg & Petersen, 2002, Navntoft et al. 2003) that the costs at farm level of improved natural elements (farmers expense of a certain number plus one extra of wild flowers, insects and Skylarks on a particular area) can be calculated.

b) That the results can be used in a broader context – the results on effects of mechanical weed control can be extrapolated to other types of agricultural practice.