Regen ag weeds threaten arable

 By: Dr  Jacqueline Rowarth

In Business9 Minutes

It’s worth over $2 billion to the New Zealand economy, it underpins the $20 billion animal protein industry and it is under threat. 

It isn’t the wine industry – though wine certainly fits the points of export income and well-being support – the unsung industry is arable. It is the industry that grows grain, seed and other crops that are exported globally, it supports the continuous improvement of domestic pastures and it provides animal feed such as maize silage and grain.

It’s under threat from well-meaning farmers embracing the concept of regenerative agriculture and growing hyper-diverse (multi-species) pastures under long grazing management. 

Hyper-diverse pastures cannot be managed to control all species at peak quality. Some set seed, and the re-seeding is part of the management for creating pasture resilience and improving pasture longevity. The seed setting rejuvenates the pasture, filling in gaps and making the most of resources. 

For the arable industry the problem is that the species in diverse mixes don’t always stay in the paddocks where they were sown.

This is the same philosophy as deferred grazing in hill country where re-seeding in summer is followed by seed germination as the soils receive rain (traditionally in autumn). 

In areas where closer management of pasture is possible such as dairy farms, maintaining pasture at high quality (grazing at three leaf-stage) should mean that there are no gaps in the pasture to fill. 

Dr Kerry Harrington, Associate Professor in weed science at Massey University, has explained that it is difficult to manage the grazing of complex mixtures to keep more than just a few species present over time.  “Most of these species do not persist as long as the perennial ryegrass and white clover swards shown by good science to work best in NZ,” he says. “Components that die off are quickly replaced by weed species.”

Containment problem   

For the arable industry the problem is that the species in diverse mixes don’t always stay in the paddocks where they were sown. In particular, rapid seeders such as mustard, radish and oats are getting away from home farms, being transported in birds and then ejected on neighbouring land. The ejected seed isn’t always viable – but anything that gets away has potential to cause problems. On arable farms the non-crop seeds are classified as a weed with the potential to contaminate a crop leading to extra expense or rejection on quality grounds. 

New Zealand arable farmers have developed the seed and grain industry over the past few decades. More than 40 different grain and seed crops are grown on 180,000 hectares (Canterbury, Southland, Manawatu, Hawke’s Bay, Wairarapa and Waikato regions) and growers can have up to 20 crops on their farm each year.

It’s a specialist industry with strong management and careful advance planning to avoid seed contamination and biosecurity breaches. The growers now provide approximately 60% of the global radish seed, 50% of white clover seed and 40% of the world’s carrot seed.

The growers’ reputation also means that they are trusted to multiply seed in what is the northern hemisphere winter, allowing plant breeders and seed companies to produce new and improved cultivars of vegetables more quickly than would otherwise be possible constrained to an annual growing cycle. It is the reputation of the NZ grower and the ability to produce high yields of uncontaminated seed that has attracted interest from overseas.

Arable growers manage crops in rotation to ensure that appropriate herbicides can be used between and within crops as appropriate. Brassica seed from pastures can cause a 20-year problem. Oats will generally have a shorter longevity, but the general principle remains – a neighbour’s pasture can wreck a ten-year rotation for an arable grower because removing a brassica from a brassica crop or ‘wild’ oats from an oat crop can’t be done chemically.

Yet another issue from regenerative agriculture advocates is the suggestion that glyphosate (traded as Roundup) should be used at a low rate and ‘cut with fish and fulvates’. The addition of anything organic reduces the activity of glyphosate (and is simply an expensive way of adding a small amount of organic matter to soils which already have considerable quantities); using less than recommended rates of glyphosate encourages the development of weed resistance.

Dr Harrington has suggested that advocating unproven brews makes little sense given the amount of research that has been done to show how to use it most effectively. “Trying to keep rates very low will simply result in poor control of perennial weeds.  It can also lead to a build-up of resistance to glyphosate.” 

Weeds are a threat to efficiency. Dr Hossein Ghanizadeh and Dr Harrington published research in 2019 indicating that there are 245 plant species from 40 plant families which are troublesome weeds in New Zealand. In pastures, grazing management and using competitive pasture species can play a more important role than herbicides for weed, they say. “Integrated weed management using a combination of herbicides and good pasture management strategies leads to the most cost-effective and efficient control of pasture weeds”. 

Low greenhouse gas production 

This control, and maintenance of pasture at high quality, contributes to low greenhouse gas (GHG) production per unit of meat and milk. Professor Tony Parsons, now retired from Massey University, has shown that long-pasture grazing, which is associated with bottom death, seeding and loss of pasture quality, decreases efficiencies. This is particularly the case when impact (GHG or nitrogen loss) per unit of protein is considered.

Arable growers are also efficient in production. For example, NZ holds records for yields in both wheat and barley, and we are low producers of GHG, at least in part because of low use of chemical inputs.

Spread of non-desired seeds will increase requirements for chemical intervention and erode the competitive advantage that our growers have held in seed quality. Two billion dollars are at risk.

Although a weed is simply a misplaced plant, the birds can’t differentiate between weeds, seeds and farms. Arable growers and overseas seed buyers can.


  • Dr Jacqueline Rowarth, Adjunct Professor Lincoln University, is a farmer-elected director of DairyNZ and Ravensdown. The analysis and conclusions above are her own.