Paraggio medic (Medicago truncatula v. Paraggio), Snail medic (Medicago scutellata v. Kelson), grazing vetch (Vicia dasycarpa) and Faba bean (Vicia faba v. Fiord) are currently recommended for cover crop management in South African vineyards (Fourie, Louw & Agenbag, 2001).

The control of these broadleaf species by registered post-emergence herbicides, however, has not been evaluated. Therefore, the ability of three systemic herbicides, a contact herbicide and a mixture of two systemic herbicides ( see Table 1 ) to control these cover crops were determined in a field trial.

Materials and Methods

The trial was executed in a Chardonnay/99 Richter vineyard on the Nietvoorbij campus of ARC Infruitec-Nietvoorbij and trained onto a hedge trellising system. The above-mentioned species were established during the first week of April at the prescribed seeding densities, namely

25 kg/ha for Paraggio and Snail medic, 50 kg/ha for grazing vetch and 120 kg/ha for Faba bean. Eight treatments ( see Table 1 ) were replicated three times in a randomised block design for each species. Each experimental plot (replication) covered a surface area of 2,75 x 3 m.

The herbicides were applied during the flowering stage of the cover crops in 1998 (19 October) and before flowering in 1999 (6 September) by means of a Multispray knapsack sprayer with a 1,8 m boom on which five Teejet 9502 nozzles were mounted 400 mm apart (Fig. 1). The application volume for the systemic herbicides was 200 l/ha and that of the contact herbicide 400 l/ha.

Fig. 3. Effective control of Snail medic by 1080 g glyphosate (6 l/ha of Sting).

The efficacy of control was determined by evaluating the cover crops visually 35 days after the herbicides were applied. The untreated control was taken as 0% and total control as 100%. Herbicide efficacy was expressed as a percentage of the untreated control.

Results and Discussion

The scorching agent paraquat controlled all cover crops effectively (95% and more) if applied during the flowering stage (Table 1). Faba bean, however, showed some regrowth in the 500 g paraquat treatment. If applied before flowering, the cover crops were able to recover. Paraggio medic regenerated to such an extent that the above-ground growth equalled that of the untreated control 35 days after application (Fig. 2).

Fig. 1. The Multispray knapsack sprayer used for the application of herbicides.

Grazing vetch was controlled effectively at any growth stage by the glyphosate/MCPA mixture as well as both dosages of MCPA ( see Table 1 ). Faba bean seemed to be resistant to MCPA, with only the glyphosate/ MCPA mixture giving acceptable control if applied before the flowering stage. Snail medic was controlled effectively by the higher dosage of MCPA applied during the flowering stage, with some resistance occuring if applied before flowering. The glyphosate/MCPA mixture gave excellent control irrespective of the time of application. Paraggio medic was resistant to both MCPA and the glyphosate/MCPA mixture.

Fig. 2. Regrowth of Paraggio medic 35 days after the application of 1000 g paraquat (5 l/ha of Gramoxone).

Both formulations of glyphosate controlled the cover crops effectively if applied before flowering (Figure 3), with the exception of grazing vetch that showed a slight resistance towards the lower dosage (1080 g).

Conclusion

  • Paraggio medic, Snail medic and Faba bean should be controlled effectively by 1080 g/ha glyphosate, if applied before flowering. A higher dosage, however, must be used for grazing vetch.
  • If a scorching agent is to be used, it should be applied from flowering onwards to prevent regrowth.
  • The lower dosage of MCPA and the glyphosate/MCPA mixture can be used for the control of grazing vetch, irrespective of the growing stage of the cover crop.

Literature Cited

FOURIE, J C, LOUW, P J E, & AGENBAG, G A. 2001. Effect of seeding date on the performance of grasses and broadleaf species evaluated for cover crop management in two wine grape regions of South Africa. S. Afr. J. Plant & Soil 18, 118-127.

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