Weed control in sunflower
Despite its large final size, sunflower is very sensitive to weed competition during the first growth stages of the plant. Weeds compete with plants in terms of space, access to sunlight, nutrients, and water, while they can also serve as hosts for important diseases and pests. Strong competition for a dry-land crop like sunflower can cause up to 60-90% yield loss (1). As a result, farmers must create and implement integrated management practices to control the different weed species more efficiently.
Even before sunflower sowing, farmers need to know three essential things to make informed decisions concerning the weed management strategy they will follow. Farmers need to know:
- Which weed species are present, the size of their population, and their distribution in the field. The crop rotation cycle applied in a field can affect the weed species grown and their population yearly. You can define the major weed “enemies” and direct your efforts to control them.
- The type and efficiency of the weed management techniques that were applied previously. Keep in mind that the repeated use of a single weed control measure can favor the survival and growth of specific weed species. This includes, of course, the use of herbicides. It is essential to keep a list of the active compounds used and the efficiency of the application in the different weed targets. Reduced efficiency could reveal the development of herbicide-resistant weed populations, making their control extremely difficult. You can discuss this with your local licensed agronomist.
- The number of days that the field needs to be completely weed-free so that our young seedlings will develop during the first critical stages without competition from weeds. This period is usually characterized as a “critical period“. In sunflowers, this period lasts from sowing until 6-9.5 weeks after crop germination (Golipour et al., 2009, 2).
The most important weeds of sunflower
Several different species of weeds” threaten” sunflower’s growth and yields. These could be classified as grass, broadleaf species, or obligate parasites. The species variation and abundance may vary from one region or field to another and between years. Some of the most common and destructive species: kochia (Kochia scoparia), lambsquarter (Chenopodium album), marestail (Conyza canadensis), and redroot pigweed (Amaranthus retroflexus) are commonly observed in sunflower fields. Other weeds like field bindweed (Convolvulus arvensis), Canada thistle (Cirsium arvense), wild buckwheat (Polygonum convolvulus), wild sunflower (Helianthus annuus), wild oat (Avena fatua), common ragweed (Ambrosia artemisiifolia), green foxtail (Setaria viridis), cleavers (Galium aparine) and wild mustard (Sinapis arvensis) can also be observed in a large population in sunflower fields (1).
During the past decade, the obligate parasite broomrape (Orobanche cumana, Orobanche ramosa) is becoming a significant problem in most sunflower-producing areas. The problem is even bigger in Europe and Asia, where the parasite can cause losses of up to 80% (Louarn et al., 2016). The broomrape is a non-photosynthetic, root parasite plant, that gets its nutrients and water entirely from the host, in this case, the sunflower. The control of this plant is complicated because it produces many seeds that can survive in the soil for many years and sprout when stimulants (Strigolactones) from the host root are detected.
Weed control measures – Integrated weed management in sunflower
The best results can be achieved with a combination of different control measures.
1. Cultivation practices
- Pre- and Post-emergence Tillage
Pre-planting tillage is an important measure to control one or more weed flushes, prepare the seedbed and offer a weed-clean start to the crop. The sowing can take place right after the last harrowing. The efficiency of weed control increases by applying a stale seedbed technique (irrigate the soil 7-14 days before planting to stimulate weed seed germination and then till in a shallow depth (3).
Even after crop emergence, the farmer can harrow the field 3-5 times during the plant’s four- to six-leaf stage (V4 to V6) (2). The first post-emergence harrowing can be performed after the development of the first pair of true leaves of the sunflower (Tonev et al., 2019). It is advised to use spring tooth harrows or/and rolling guard cultivator and avoid solid spike-tooth harrows, which can damage the crop. S-tine or similar row-crop cultivators are often set up for 76 cm (30 in) row spacing (4). Harrowing may be needed, especially when a prior herbicide application has failed. It can effectively control grass and broadleaf weed species. However, post-emergence harrowing may cause up to 8% sunflower plant losses. Alternatively, if the weed seeds are still in an early stage, the farmer can also use a rotary hoe. (2). It is advised to hoe during the hottest part of the day when the sunflower is wilted to reduce stem breakage. Avoid deep cultivation (maximum at 75 mm), especially when sunflowers have reached a height of 30 cm (12 in), by preferring a low speed of under 10 km (6 miles) per hour (5, 6).
After sunflower harvest and early plowing of the predecessor, additional summer and autumn tillage operations will help reduce the number of weeds and the soil seed bank.
- Soil solarization
With this technique, the farmer uses passive solar heating by applying clear polyethylene sheets to moist soil for around 58 to 61 days during the hot season. This will lead to an increase in soil (upper 5 cm) temperature by around 10oC (50oF). The measure is much more effective in regions with high temperatures after or before the sunflower cultivation period. The best results are obtained when this technique is combined with tillage that exposes the weed seeds from the deeper soil layers to the surface, sunlight, and heat. A soil temperature above 45oC (113 oF) for several days can significantly reduce the survival and germination of weed seeds, including broomrape seeds (5).
- Clean the equipment before using it or/and when moving from one field to another. This will minimize weed seed dispersion.
- Biological control & Crop rotation:
Monoculture favors the increase of weed populations specific to sunflowers. It is advised to rotate sunflower with other crops, like pea, barley, wheat, soybean, and/or corn. After sunflower is cultivated, a grass species, like corn or wheat, will allow the use of herbicides specialized in grass weeds and broadleaf weeds in consecutive years. In this case, though, be careful not to use herbicides with an extended residual activity that can harm the next crop. You can discuss this with your local licensed agronomist.
Except for the measure mentioned before, scientists have found that the presence of Arbuscular Mycorrhiza can help control the parasite by reducing the germination of the seeds of Orobanche. These widespread soil fungi live in a symbiotic relationship with the roots of most plant species offering many advantages to the plants (Louarn et al., 2012).
- Variety selection – Herbicides-resistant varieties
In recent years, the herbicide-resistant sunflower varieties (no GMOs) have gained much popularity and are usually the top choice for many farmers. The two most widely-used technologies are the Clearfield® and the ExpressSun®. More specifically, the Clearfield sunflower varieties are tolerant to imidazolinone herbicides (imazapyr, imazapic, imazethapyr, imazamox, imazamethabenz, and imazaquin).
Imidazolinone herbicides control most annual grass and broadleaf weeds and parasitic plants like Orobanche spp. (Pfenning et al. 2008; Santos et al. 2012). On the other hand, ExpressSun sunflower varieties are tolerant to sulfonylurea tribenuron, which controls many broadleaf weeds, including Cirsium arvense (Zollinger, 2004), but it is not effective against grass weeds. By cultivating such sunflower varieties, the farmer can apply nonselective herbicides or herbicides that would target and harm the crop under normal conditions.
In fields with extensive problems with broomrape, the best measure would be to choose a sunflower variety resistant to the parasite. There are several available in the market.
2. Chemical control – Use of herbicides
Herbicides are widely used in sunflower cultivation because they are a relatively cheap and efficient solution. However, you should always consider the consequences on the environment and biodiversity before using any herbicide. You can discuss this with your local licensed agronomist.
The first application of liquid or granular herbicides can be applied before sowing to offer a weed-free field for sunflower seedlings to grow during the critical period. Depending on the weed population, another post-emergence application may be needed. After discussing with the agronomist, you can choose an active compound that will allow you to effectively control the most harmful and abundant weed species. If you cultivate a herbicide-resistant sunflower variety, you have more options. You can follow the instructions on the product label to know the target plant groups, when to apply it, and at what rate. You can discuss this with your local licensed agronomist.
The efficacy of tribenuron is lower than imidazolinones on weeds like Ambrosia artemisiifolia and Galium aparine, making the Clearfield® varieties more popular. However, both imazamox and tribenuron-methyl have limited efficacy against Chenopodium album and Portulaca oleracea, while oxyfluorfen offers a great control against the broadleaf weeds like Chenopodium (Osman et al., 2014; Tonev et al., 2020). Similarly, trifluralin-based herbicides can sufficiently control Amaranthus, Chenopodium, kochia, and foxtail species (4). Sunflower is very sensitive to hormonal herbicides, and their use should be avoided. You can find more information concerning the herbicides, their mode of action, and application time here (7). Finally, it is advised not to use herbicides with the same mode of action multiple times to avoid the development of herbicide-resistant weeds.
After discussing with your local licensed agronomist, you can consider an integrated weed management strategy and apply a combination of the suggested controlled measures mentioned above.
Golipour, H., Mirshekari, B., Moghbeli, A., Hanifian, S. (2009). Critical period of weeds control in sunflower, Helianthus annus L. Journal of New Agricultural Science, 5(17). en11.
Louarn, J., Carbonne, F., Delavault, P., Becard, G., & Rochange, S. (2012). Reduced germination of Orobanche cumana seeds in the presence of arbuscular mycorrhizal fungi or their exudates. PloS one, 7(11), e49273.
Louarn, J., Boniface, M. C., Pouilly, N., Velasco, L., Pérez-Vich, B., Vincourt, P., & Muños, S. (2016). Sunflower resistance to broomrape (Orobanche cumana) is controlled by specific QTLs for different parasitism stages. Frontiers in plant science, 7, 590.
Osman, A., El-Habieb, R., Elkhawad, M. (2014). Herbicidal efficacy of oxyfluorfen (Sharoxy 24% EC) for pre-emergence weed control in sunflower. Persian Gulf Crop Protection, 3(4), 37–44.
Pfenning, M., Palfay, G., Guillet, T. (2008). The Clearfield® technology – A new broad-spectrum post-emergence weed control system for European sunflower growers. Journal of Plant Diseases and Protection, Special Issue, XXI, 647–653
Santos G., Francischini A.C., Constantin J., Oliveira R.S., Ghiglione H., Velho G.F., Neto A.M.O. (2012): Use of the new Clearfield system in sunflower culture to control dicotyledonous weeds. Planta Daninha, 30: 359–365
Tonev, T., Dimitrova, M., Kalinova, Sht., Zhalnov, I., Zhelyazkov, I., Vasilev, A., Tityanov, M., Mitkov, A., Yanev, M. (2019). Herbology. Publisher Vidinov & son. ISBN: 978-954-8319-75-1 (Textbook in Bulgarian).
Tonev, Tonyo, Shteliyana KALINOVA, Mariyan YANEV, Anyo MITKOV, and Nesho NESHEV. “Weed association dynamics in the sunflower fields.” Scientific Papers. Series A. Agronomy 63 (2020): 586-593.
Zollinger R.K. (2004): Advances in sunflower weed control in the USA. In: Proceedings 16th International Sunflower Conference, Aug 29–Sept 2, 2004, Fargo, USA: 435–439.
Weed Management in Sunflower Farming
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