Peanuts rely on paraquat

Paraquat keeps weed-free in the critical periodUsing paraquat is an increasingly important part of successful peanut production. It has three key advantages: it controls a broad spectrum of weeds; it can be used selectively to keep crops weed-free during the critical early growth period; and, it is effective against herbicide resistant weeds. Peanuts are low-growing crops that do not compete well with weeds. Growers must use weed management strategies to keep their crops free from weeds until the rows meet and close the canopy if they are to achieve good yields. Unfortunately, US growers, in particular, now face the additional challenge of weeds that have become resistant to many of the herbicides at their disposal.

Weed control options

Weeds usually cause the greatest loss in yield to peanuts from competition in the first six weeks after planting.1,2 The University of Florida recommends weed control programs that include a pre-plant incorporated or pre-emergence herbicide treatment followed by two post-emergence passes. The most critical spray should be applied early post-emergence at ‘cracking’ or soon after. This is when the emerging peanut shoots break through the soil surface. At this time weeds are likely to be still small and easier to control. Paraquat has a huge advantage here because it will achieve broad-spectrum weed control from an over-the-top spray without the peanuts sustaining any long-term damage. In this case, it effectively acts as a selective herbicide

Description

A chemical product used for eliminating certain types of weeds only (ie either grasses or broadleaved weeds) and not affecting specified crops.

Authoritative On-line References and Resources

http://www.weeds.iastate.edu/ An invaluable source of contemporary information about herbicides and weeds from Iowa State University.
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The leading paraquat product Gramoxone SL 2.0 can be used to control emerged annual grass and broadleaved weeds

Description

The leaves are "broad" as opposed to the "narrow" leaves of grasses. Also called 'dicots' having two seed leaves, while grasses are 'monocots' having one seed leaf.

Authoritative On-line References and Resources

http://www.iwss.info The International Weed Science Society.
up to 15 cm (6 inches) tall. A split application of the top rate (16 fluid ounces/acre = 280 g/ha) may be made, once at ‘ground crack’ and four weeks later, to achieve the best effects. Tank-mixing bentazon will improve control of tough weeds such as smartweeds (Polygonum spp.), while adding the hormone weedkiller 2,4-DB will result in better control of morningglories (Ipomoea spp.).3

Paraquat is a contact herbicide

Paraquat does not move systemically through plantsAlthough paraquat may scorch crop leaves, it is a purely contact herbicide, which will not translocate to growing points. Any damage is, therefore, confined to the first-formed leaves, which are quickly replaced by many others. The image on the right was produced by placing drops of a paraquat solution at the center of a leaf and then detecting where the paraquat moves to. The more intense colours indicate that the herbicide is concentrated at the point of application and does not move out of the treated leaf. Peanut is an indeterminate plant, which means that vegetative growth continues while the plant is also flowering and setting pods. While first flowers do not normally form until the end of the paraquat application window, this tendency to vigorous vegetative growth undoubtedly encourages recovery in temporarily scorched plants. Researchers at the University of Georgia found that when they applied paraquat alone or mixed with bentazon to peanuts at various times after cracking up to 28 days later, there were no or only very small effects on yield.4 The addition of bentazon tends to reduce the amount of damage to peanut leaves. Such experiments are carried out in plots kept free of weeds. So, in practice, substantial increases in yields would be expected from eliminating weed competition with no negative effects from crop damage. There was a bonus in that the incidence of disease (tomato spotted wilt virus) was significantly reduced when paraquat treatments were applied. 

Herbicide resistance

Paraquat is recommended for use in programs to control all important herbicide resistant weeds in US peanut crops. North Carolina State University has reviewed the status of weed resistance in peanuts.5 Many of most prevalent weeds in peanuts are resistant to commonly used herbicides in many areas of the peanut growing states from Virginia south to Florida and west to Texas and New Mexico. Weeds posing the greatest problems are listed in Table 1. Resistance to dinitroanaline, imidazolinone and sulfonylurea chemistry herbicides is widespread amongst populations of these weeds. However, paraquat with its distinctive mode of action can be used to effectively control these resistant weeds. Other active ingredients may be mixed to improve overall weed control. For example, metolachlor will add soil residual control; and bentazon will improve foliar activity on more difficult to control weeds because, as a photosynthesis inhibitor, it slows the action of paraquat leading to a more thorough kill.  You can read more about this effect here. Table 1. Peanut weeds resistant to commonly used herbicides and the efficacy of paraquat.
Weed  Failed herbicides Paraquat effective?
Palmer amaranth (Amaranthus palmeri) and other pigweeds Dinitroanalines: ethalfluralin, pendimethalin, trifluralin Imidazolinones: imazapic, imazethapyr Sulfonylureas: chlorimuron, diclosulam Yes
Common cocklebur (Xanthium strumarium Imidazolinones: imazapic, imazethapyr Sulfonylureas: chlorimuron, diclosulam Yes
Common ragweed (Ambrosia artemisiifolia) Imidazolinones: imazapic, imazethapyr Sulfonylureas: chlorimuron, diclosulam Yes
Goosegrass (Eleusine indica) Dinitroanalines: ethalfluralin, pendimethalin, trifluralin Imidazolinones: imazapic, imazethapyr Sulfonylureas: chlorimuron, diclosulam Yes

 
Conclusions

Paraquat’s contact-only properties, broad spectrum weed control and distinctive mode of action mean that it is a critically important component in weed management in peanuts. In fields in which resistant weeds have not yet arisen, it is vital to use a variety of herbicide modes of action to keep resistance at bay.

References

  1. University of Florida: Weed Management in Peanuts 2013 
  2. Clemson University (S. Carolina): Peanut Production Guide
  3. Syngenta Crop Protection: Gramoxone SL 2.0 label
  4. Tubbs, R S, et al. (2010). Influence of paraquat on yield and tomato spotted wilt virus for Georgia-02C and Georgia-03L peanut. Peanut Science, 37, (1), 39-43
  5. North Carolina Cooperative Extension Service: Managing herbicide-resistant weeds in peanuts in the United States

Notes

The brand name of the original and leading paraquat product is Gramoxone.