ABSTRACT

Weeds infestations of oilseed rape crops by species of the Brassicaceae family is a growing problem in France due to short rotations cropping systems and lack of efficient herbicides.

Most of the critical infestations are due to Sinapis arvensis and Raphanus raphanistrum. Sisymbrium officinale, rapistrum rugosum, Calepina irregularis, Barbarea intermedia are species of growing importance.

These weeds are early competitors of the crop, during its establishment period, and may cause severe troubles.

Studies have been achieved to determine the most appropriate herbicides against the species and to precise the conditions of both efficacy and selectivity. Some suitable solutions have been identified. The main limits of these solutions lie in weather interactions on both efficacy and selectivity, and the need to optimize efficacy and selectivity on species of the same botanical family, according to weeds and oilseed rape development stages.

INTRODUCTION

Brassicaceae weeds infestations in oilseed rape crops have been a growing problem in France for about 20 years, due to short rotations and low or nil efficacies of current herbicides. The more current problematical species regarding their effects on OSR crops are: sinapis arvensis, sisymbrium officinale, barbarea intermedia, calepina irregularis, lepidium campestre. Herbicides efficient on these brassicaceae and presenting acceptable selectivity to oilseed rape are rare. Brassicaceae infestations are problematical for two main reasons (Perny et al), more or less important for the different species:

- direct competition to the crop, which may range by 50% of yield. These species are generally annual species and germinate in summer or autumn in OSR crops. They are quite fast growing, of rather big size, and very often develop more rapidly than OSR. Competition generally occurs early in the season, which explains these high competitive effects.

- presence of impurities (depending on specie) in OSR harvest. These impurities are often difficult to sort, and contain high glucosinolates levels, which may affect harvest quality. Furthermore impurities as seeds are problematical for sowing seeds production.

MATERIALS AND METHODS

Specific herbicides experiments were achieved by CETIOM in different regions in order to identify herbicides presenting acceptable levels for both efficacy and selectivity.

All experiments were managed according to the recommendations of the French Commission des Essais Biologiques, following the OEPP standards.

Classical experimentations designs for practical efficacies and visual selectivity evaluation are made of 2 or 3 complete blocs, with adjoining non treated test plots. Most of these experiments were not harvested. Other experiments were specifically designed on non infested fields for selectivity evaluation of the most promising active ingredients, and harvested for yields comparisons.

Efficacies for each specie were evaluated by counting on the test plots and visual estimation of the efficacy or counting on treated plots.

RESULTS AND DISCUSSION

CETIOM references on the main pre-sowing and pre-emergence herbicides (table2) show that some of them are efficient on certain brassicaceae species. Capsella bursa pastoris is controlled by tebutam + clomazone or by metazachlore, cardaria draba by metazachlore, diplotaxis erucoides and sisymbrium officinale by tebutam + clomazone (confirmed by other informations), cardamina hirsuta seems to be controlled by metazachlore.

On the other side, two main weeds species, sinapis arvensis and raphanus raphanistrum, are not controlled by these current pre-emergence herbicides, as well as less frequent species, like barbarea intermedia, calepina irregularis, lepidium campestre, myagrum perfoliatum and rapistrum rugosum (observations in farm conditions).

Experiments were achieved to identify post emergence solutions and the conditions of both efficacy and selectivity.

The synthesis of table 3 shows the interest of isoxaben and diflufenicanil + isoproturon on sinapis arvensis and raphanus raphanistrum. Associations carbetamide+dimefuron alone or added with pyclorame + pyridate are not convenient against weeds.

The use of isoxaben has been specially studied, with success, to find a compromise between selectivity and efficacy. The selectivity results are less satisfying for diflufenicanil + isoproturon.

Isoxaben

Concerning isoxaben, selectivity to OSR depends on the formation of a waxy cuticle on the leaves, which keeps isoxaben and forbid it to penetrate in the plant. Rapeseed naturally gets this cuticle when temperature decreases and after first frosts, generally after 4 to 6 leaves stage. Sinapis a. and raphanus r. do not, and are sensitive to isoxaben during leaves and rosette stages. The isoxaben efficacy on Sinapis a. and raphanus r. - at the limited doses permitted by selectivity requirements - decreases rapidly as these weeds develop, specially after stem elongation, which is quite frequent in French conditions at autumn (see chart 1). Isoxaben was authorized in France in 1991 for post-emergence applications on rapeseed, at a maximum rate of 50g/ha. Higher doses or sooner applications cause direct selectivity problems and can induce higher sensibility to phoma (leptosphaeria maculans), one of the major diseases of OSR in France.

Diflufenicanil + isoproturon

In cold regions, frost generally destroys elongated sinapis and raphanus, but real infestations and competition problems are encountered in regions where frost is limited and/or weeds growth quite rapid (Atlantic coast, Center and South of France): in these regions, sinapis and raphanus may elongate before OSR reaches 4 to 5 leaves, i.e. before the selectivity stage.

Other solutions were studied, with diflufenicanil + isoproturon association. Results are less depending on weeds stages (chart1), but visual selectivity is problematical; final selectivity may be considered as acceptable on well implanted crops in regions where frost is limited (the regions with the heaviest sinapis and raphanus problems), but insufficient in cold regions (the major part of the production area). Selectivity symptoms at basic rate (37.5g:ha dff + 300g/ha IPU) are heavy discolorations (white stains and OSR leaves/ up till note 5 on scale from 1: no symptoms to 9: total foliage necrosis) and growth slow down and reduction (vigor reduction from 10 to 30% of non treated test plots; up till note 3 on scale from 1: no symptoms to 9: vigor reduction > 66% of non treated test). Prohibitive symptoms are observed with double rate applications. These results show that early applications at 4 leaves stage are risky whatever the application rate, specially in cold regions. Symptoms are less important on healthy OSR of 6 leaves or more. Limited leaves deformations were observed in few situations. Effects on OSR plant populations and yields (measured on non infested experiments) are generally not significant, except in one situation with heavy plant losses.

Dff + IPU applications could be considered as a last resort solution in case of very heavy infestations, and limited to healthy OSR crops in tempered regions with cool winter, regions with frequent frosts in winter must be avoided. Due to these very restrictive limits, the authorization of this herbicide on OSR have not been requested from the official services.

CONCLUSION

Brassicaceae weeds control in OSR crops remains a consequent problem. Available pre-sowing and pre-emergence herbicides are not efficient on sinapis a. and raphanus r. which are the most problematical species. Post emergence solutions are a difficult compromise between efficacy and selectivity. Practical results remain irregular.

ACKNOWLEDGMENTS

This study was initiated by Y. Regnault (CETIOM). The results have been obtained on the CETIOM experimental stations of Nancy, Dijon, St Florent sur Cher, Le Magneraud, En Crambade, Béziers and the associated teams of Agen and Lyon.

REFERENCES

1. PERNY R.A, PERES A., Les principales crucifères adventices du colza d'Hiver/ CETIOM ANAMSO 1993

2. PERNY A, KROUTI M, QUINSAC A, RIBAILLIER D. Crucifères adventices du colza, incidences sur la qualité des récoltes. Phytoma La Défense des Végétaux N°426, mars 1991.

3. PERNY A, KROUTI M, QUINSAC A, RIBAILLIER D. Les crucifères adventices du colza... plantes à incovénients multiples. Bulletin FNAMS semences N°116