Arable News

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Getting sugar beet weed control right

As growers set about drilling the new crop this month, the thought of spending money on sugar beet herbicides may not be welcome

As growers set about drilling the new crop this month, the thought of spending money on sugar beet herbicides may not be welcome, but when you get weed control right it is one of the most satisfying things to achieve, claims Pam Chambers. Treated herbicide trial plots (left) at Broom’s Barn in 2012 yielded 94.7t/ha (adj. yield) compared with untreated plots (right) that yielded 20.4t/ha (adj. yield).Last season in a United Phosphorus trial at Broom’s Barn in Suffolk, herbicide treated plots yielded 94.7t/ha (adjusted yield) compared with untreated plots that yielded 20.4t/ha (adjusted yield). In monetary terms that is a difference of 2,045/ha. However controlling weeds is not all about yield gain – the prevention of weed seed return to the soil, the removal of weeds that can be a nuisance at harvest and the management of resistant weeds are all important factors to consider.
Generally a field will have five key weed species and it is useful to match these against effective herbicide actives (see Table 1 near end). Not all expected weeds however will put in an appearance each season; knot-grass was generally conspicuous by its absence during 2012 but field pansy, ivy leaved speedwell, late germinating mayweeds and volunteer oilseed rape seemed to thrive. Although relatively easy to control, oilseed rape volunteers can cause a problem due to late emergence as they are tall and can impact significantly on yield.
Resistant black-grass is becoming an issue in sugar beet but fortunately actives such as ethofumesate, metamitron and tri-allate can be used as these have some activity against resistant black-grass and are from a different mode of action to actives used elsewhere in the rotation, and also to the graminicides used in the beet crop.
Where resistant black-grass is known to be a problem then a pre-emergence spray based around metamitron and ethofumesate should be used. However, bear in mind that costs can soon escalate to 160/ha or more if tailoring herbicide programmes to control resistant black-grass – so only use if necessary.
The trend in recent years has been for all sugar beet fields on a farm to be treated with the same programme, tailoring spray recommendations to the ‘worst’ field. This can make things easier for the spray operator. At Broom’s Barn in 2012 untreated plots on one field resulted in 19 weeds/m2 whereas on another field it was 193 weeds/m2. The cost of herbicides to achieve acceptable weed control on the first field could be 40-50/ha less and also involve less spray passes, saving time and money.   Product selection
The rates of products and spray intervals can be fine-tuned during the season, but selecting the actives and, consequently, the products that will be used is generally decided well before drilling.
In Table 1 examples of ‘building block’ products and formulated products are shown. Flexibility is a benefit where a number of ‘building block’ products are chosen and then mixed and matched according to how the season progresses. Alternatively, ready-formulated products can be selected which may be more expensive but are suitable where cost has to be measured against time available.  
It is important to note that there are differences between product labels even though the actives may be the same. The number of applications allowed, maximum individual doses and formulation types may differ. Restrictions also exist for a number of actives that are now used in sugar beet, eg ethofumesate. The BBRO produces summary charts giving details for many products.
Climatic conditions will always upset the best of plans; the rainfall pattern for the spring of 2012 compared with 2011 was very different and this had a huge impact on weed control in sugar beet.   Psychologically we think less herbicides are required for small weeds and more for larger ones, but that is not always the case. Weed control in 2011 was difficult as weeds were very small and tough due to the adverse growing conditions, and rates of herbicides being applied were often too low with many struggling to control weeds, in particular the polygonums such as black-bindweed.
In 2012 the high rainfall delayed spraying and herbicides were applied to large but relatively soft weeds, however rates were generally very high. Consequently many were surprised by the very good weed control achieved.
Plan in advance but be prepared to adjust as the season progresses. There are lots of ways to achieve the same end result but time and cost will impact on what programme and products to use. *Pam Chambers was formerly knowledge transfer manager at Broom’s Barn, Rothamsted Research and is now technical support manager (UK and Ireland) for crop protection company United Phosphorous.  Table 1: Key sugar beet actives and their properties Seasonal tip: Consider sprayer capacity and labour availability ahead of spraying
Spray operators with limited time may not be able to meet their agronomists’ expectations for weed control in sugar beet.  Discussing the acreage to be sprayed, location of fields and other time commitments beforehand can help avoid conflict during the season. It may also influence spray product choice ie formulated mixes rather than using a number of ‘building block’ products.
The comparison of FAR, conventional and delayed spraying programmes should be evaluated, as each have their strengths and weaknesses. BBRO carried out an evaluation of ‘spray systems’ looking at efficacy, cost of programmes and application costs during 2012. Results can be downloaded from www.bsonline.co.uk


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