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weeding
The back-breaking job of hand weeding

 

Herbicides accounted for more than 40% of the global market for crop and non-crop pesticides worth $64 billion in 2018 (Phillips McDougall)

Selective herbicides, such as 2,4-D and MCPA invented in the 1940s started the move away from various forms of hand and mechanical weeding, yet this still continues in many developing countries.  It is increasingly difficult to find enough labour at the right time and at affordable wages to manually control weeds.  Problems are exacerbated by the movement of people away from the countryside to work in cities.

Often it falls to women and children to do the back-breaking work of hoeing, slashing or pulling weeds.  Having cleared a field, the job has to be repeated in just a short space of time as new flushes of weeds emerge.  In many developing countries, herbicides free women, in particular, to feed, educate and care for their children.

Hand weeding one hectare of maize can take 250 person-hours of work, while it can take just two hours for one person to apply herbicide from a knapsack sprayer to the same crop area.  With weed control taking a fraction of the time needed previously, herbicides allow smallholders to supplement their income from additional employment.

From the 1960s, non-selective herbicides, which control a very broad spectrum of weeds and are deactivated on contact with soil, allowed the development of no-till farming.

No-till farming

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Non-selective herbicides (paraquat from the 1960s and glyphosate from the 1970s), enabled the adoption of no-till (direct drilling; zero tillage) and other reduced cultivation systems for crop production to be established.  If weeds are removed by herbicides before planting, then there is no need to plough to bury weeds.  No-till systems can increase yields when crops are appropriately established and have many environmental and economic benefits.

The benefits of no-till systems, which are only possible with herbicides, are summarised in the box.  Combatting climate change is especially topical.  Ploughing aerates the soil excessively, causing the oxidation of organic matter.  Not only does this destroy good soil structure (built-up after rotational pasture, for example), but it releases large amounts of carbon dioxide.  Spraying a herbicide to burn down weeds before planting in a no-till system can reduce emissions of CO2 by more than 80%.

Farming must be profitable.  CropLife America has estimated the economic benefits of herbicide use in the US.  In maize, soybeans, wheat and rice, the benefit to cost ratio was around three and in cotton it was nearly double that. 

References and further reading

CropLife America

Gianessi, L P (2013).  The increasing importance of herbicides in worldwide crop production.  Pest Management Science, 69, (10), 1099-1105

Robertson, G P et al. (2000).  Greenhouse gases in intensive agriculture: contribution of individual gases to the radiation forcing of the atmosphere.  Science, 289, 1922-1925