Bleeker, P.O.

Kropff, M.J.

Kurstjens, D.A.G.

Perdok, U.D.

Name not available

Seven ways to improve the selective ability of mechanical intra-row weeders

Wageningen Yield 

Seven ways to improve the selective ability 
of mechanical intra-row weeders 
 
D.A.G. Kurstjens, P.O. Bleeker, M.J. Kropff & U.D. Perdok 
Soil Technology Group, Wageningen University, P.O. Box 43, NL-6700 AA Wageningen, 
The Netherlands, Dirk.Kurstjens@user.aenf.wau.nl 
 
 
Introduction 
In most farming systems, particularly the selective weed control methods (that control weeds 
but spare the crop when applied to both) fulfil a crucial role. Although many alternative 
preventive (e.g. cultivation, competitive (inter)crops, green mulch, weed seed harvesting) and 
curative (e.g. mechanical, thermal, biological) methods are available, farmers still rely heavily 
on the selective action of herbicides. As the available number of selective compounds will 
decrease (Chapman, 2001), the reliability and cost-effectiveness of other methods should be 
improved to facilitate their adoption. Possibilities for successful non-chemical weed control 
particularly influence the adoption of organic farming practises. Here, a main problem is 
controlling intra-row weeds in early growth stages of slowly growing, weakly competitive 
crops that are sensitive to mechanical damage. Basically, this problem could be tackled by: 
1. Decreasing the number of weeds requiring control in this sensitive period (e.g. by false 
seedbeds, delayed sowing, pre-emergence flaming, competitive (inter)crops), 
2. Improving crop resistance against damaging relative to the weeds (e.g. by altered seedbed 
structure, row-applied nutrients, crop cultivar selection), 
3. Improving the selective ability of mechanical weeding operations. 
This paper focuses on the last option and presents seven possibilities that emerged from field 
and laboratory experiments. 
 
Material and methods 
Laboratory harrowing experiments with three species (Lolium perenne L., Lepidium sativum 
L. and Chenopodium quinoa Willd.) in early growth stages were performed on sandy soil to 
explore the mechanism by which plants are covered (Kurstjens & Perdok, 2000) and uprooted 
(Kurstjens et al., 2000). Mechanical damage and plant response (mortality and fresh weight 
reduction after six days) of individual plants was assessed at various working depths, working 
speeds and soil moisture contents (Kurstjens & Kropff, 2001). 
From 1999 until 2001 field experiments in sugar beets, seeded onions and maize on sandy and 
clay soil have compared several tools for mechanical intra-row weed control (Bleeker et al., 
2000; Kurstjens & Bleeker, 2000; Bleeker, 2002). Torsion weeders, finger weeders, ground-
driven brush weeders and spring tine harrows were used at various crop growth stages and 
aggressiveness of adjustment. Intra-row weed control and crop damage was assessed at 
cultivation (percentage uprooted), before the next cultivation (weed counts) and at harvest 
(crop yield), occasionally supplemented by recording the time required for hand weeding. 
 
Results and discussion 
Although the experimental evidence is still limited and cannot be fully presented in this paper, 
several possibilities for improving the effectiveness of selective mechanical intra-row weeders 
could be inferred: 
1. Improving the handling of machines by making them quickly and easily (preferably 
centrally) adjustable and by training farmers in using implement adjustments and in 
estimating the allowable crop damage directly after cultivation. 
2. Maintaining an accurate and constant working depth is necessary to prevent uprooting of 
crop plants while maintaining the ability to control emerging weeds. Increasing laboratory 
harrowing depth from 10 to 30 mm increased seedling uprooting from 8 to 31%, whereas 
uprooting of emerging plants increased from 39 to 65%. Thus, uneven soil surface and 
spatially variable seedbed conditions may locally cause high crop plants losses. 
3. By precision steering of implements, spatial patterns of soil disturbance and soil flow 
around tines that cause spatially heterogeneous patterns of uprooting and soil-covering 
could be better exploited. In laboratory harrowing experiments, particularly plants near 
and on the tine paths were uprooted, whereas the selectivity between emerging plants and 
seedlings of the same species was highest in the zone between two tine paths. In addition, 
there were zones were much uprooting and little covering occurred and zones showing the 
opposite. With precise steering, this phenomenon could open the following possibility: 
4. Exploiting the specific strengths of the crop and weaknesses of the weeds, regarding their 
resistance to being uprooted and/or soil-covered. Implement adjustments such as working 
depth, working speed and tool-crop row spacing allow the mode of action to be 
predominantly uprooting or covering. The covering selectivity between small firm plants 
and tall flaccid plants could be also changed by manipulating the covering mechanism 
(soil surface level upheaval and plant downward bending). 
5. Preserving the intra-row soil in a workable condition and improve trafficability of wheel 
tracks, to increase the workable time. Even when using light tractors, the trafficability 
under moist soil conditions often appeared to be more critical for timely weed control than 
topsoil workability for the implement. In the laboratory, soil moisture content affected the 
selectivity of uprooting and covering damage. Ridge tillage and controlled traffic might 
contribute to improved timeliness and selectivity of mechanical weed control. 
6. Developing mechanical weed control strategies for specific weather conditions. For 
instance, in dry sunny conditions: killing weeds mechanically by uprooting; in wet, cloudy 
conditions: smothering weed growth by soil-covering. Although effects of weather after 
cultivation have not been explicitly studied in our experiments, results confirm those of 
previous studies. In the laboratory, drier soil at harrowing (5% as compared to 16% w/w) 
increased mortality of uprooted plants after six days without irrigation from 36 to 91%. 
Compared to 12 dry days after harrowing, applying 6.4 mm of water directly after 
aggressively harrowing L. perenne (50% covered, 34% uprooted) increased total mortality 
from 35 to 43% and increased the fraction of plants remaining buried from 12 to 33% 
(unpublished laboratory results). In the field, the weed control effect in a wet period was 
sufficient to reduce hand weeding by 30-66%. Control of covered weeds was not 
markedly influenced by rain after cultivation. Further systematic research of plant 
recovery from mechanical damage is needed to confirm these occasional observations. 
7. Decreasing the variability of sensitivity within the crop and weed populations. As 
predominantly the smallest plants within the population were damaged, the acceptable 
loss of small crop plants and the size of the largest weed plants limit the aggressiveness of 
cultivation and weeding effectiveness. 
Some of the above options (1-3) require technical innovations before their importance can be 
evaluated. Others (4-7) require more research before purposive innovations can be initiated. If 
farmers could improve intra-row weed control using versatile, simple and cheap mechanical 
weeders, the effectiveness of selective herbicides could be preserved. 
 
References 
Bleeker, P.O. et al., 2000. PAV Bulletin Vollegrondsgroenteteelt 4: 12, 29-31. 
Bleeker, P.O., 2002. Projectrapport 1236335, PPO- Lelystad, The Netherlands. 
Chapman, P.J., 2001. Proceedings BCPC conference – Weeds 2001, 803-810. 
Kurstjens, D.A.G. & P.O. Bleeker, 2000. EWRS Workshop Physical Weed Control, Elspeet, 
The Netherlands. www.ewrs.org/physical-control/meeting.htm. 
Kurstjens, D.A.G. & U.D. Perdok, 2000. Soil & Tillage Res. 55, 193-206. 
Kurstjens, D.A.G. et al., 2000. Weed Res. 40, 431-447. 
Kurstjens, D.A.G. & M.J. Kropff, 2001. Weed Res. 41, 211-228. 


Seven ways to improve the selective ability of mechanical intra-row weeders

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