Weeds Cost the NZ Economy Millions

WEEDS COST THE NZ ECONOMY MILLIONS

August 14, 2014

There’s something worse than vampires rising up from the ground.

They’re weeds and an international soil scientist says they’re sometimes regarded as a benign problem because they can be dealt with. However Dr John Baker says some of the treatments may not be as benign as first thought.

He points out that options range from waiting until they die of natural causes or speeding the process up by burial, uprooting, burning, cutting, crushing or poisoning.

“Waiting for natural causes to take effect only applies to annual weeds because they’re designed to live for one year. The problem is that nature isn’t silly and all annuals go to seed before they die,” he says

“To control annual weeds in this way, you need to remove and dispose of the seed heads before they die and this involves careful timing.”

Instead burial or uprooting has been used for years and is the main reasons why the mouldboard plough was invented.

“But we now know that any form of cultivation progressively kills off the soil biology and this has detrimental effect on soil health and its ability to grow crops sustainably,” he comments.

“It’s about the most inorganic thing you can do to soil.”

Dr Baker explains that burning may be one step better than cultivation, but not much. Burning turns the plant material into carbon dioxide and kills seeds and pests but it pollutes the air and ensures that none of the organic matter returns to the soil to feed the soil biology.

Cutting and crushing can also be effective but only on plants that die as a result of these actions. Sadly, that does not include most perennial weeds, which have remarkable powers of recovery.

Finally, there’s poisoning. Surprisingly while it’s almost universally regarded as the least attractive and environmentally responsible option of all, it’s also usually the most effective Dr Baker comments. But it depends on which herbicides are used.

Two of the more troublesome buried weeds in NZ are wild turnip and nodding thistle because they can remain buried for 40 years or more and then spring into life when uprooted again. Many farmers have been forced to avoid cultivation altogether where those weeds are known to be buried.

Other weed seeds like annual grasses thrive on shallow burial or just being mixed with a thin layer of soil called chitting in the United Kingdom. Aggressive minimum tillage or surface disturbance tools cause the weeds to flourish and has been one of the main causes why British farmers lost faith in early no-tillage machines Dr Baker says.

It’s also a problem in Australia and North America.

The dilemma that Dr Baker, who has a MAgrSc in soil science and Ph.D in agricultural engineering from Massey University, points out is that some conservation tillage practices can be the enemy of some weeds but the friend of others.

Instead Dr Baker recommends low-disturbance no-tillage and planting cover crops that keep the soil covered with either living plants or dead crop residues. Alternating broad-leaf and narrow-leaf crops allow as wide a range of herbicides to be used as possible (with emphasis on those that are environmentally friendly) because many have modes of action specific to either broad or narrow leaf plants but not always both.

“Seeding needs to be done with low-disturbance no-tillage rather than aggressive no-tillage or minimum tillage, and certainly not full tillage,” he says. “The aim is to stimulate as little weed germination as possible and then shut the light out with a crop that forms a rapid canopy.”

The weeds are sprayed out before establishing the next cash crop, which should also be done with low-disturbance no-tillage Dr Baker says. These practices have already allowed farmers to sow cash and forage crops without weeds intruding where even minimum tillage (and certainly conventional tillage) had made this impossible in the past.

Rotations have other benefits. They keep the soil biology active which enriches the soil ready for the next major planting. And of course legumes will also harvest nitrogen from the atmosphere and add it to the soil.

The most effective rotations centre on low-disturbance no-tillage machinery, which is no surprise to John Baker. Following 30 years of university research he decided there had to be a better way of sowing seeds.

He researched and developed Cross Slot® no-tillage drills which penetrate through crop residue or vegetation on top of the ground and sow seed and fertiliser in different bands at the same time.

The Cross Slot process causes minimal disturbance to the soil and crop residues, leaving most weed seeds un-germinated, traps the humidity, preserves the micro-organisms and soil life and largely prevents carbon dioxide from escaping into the atmosphere.

Further, by leaving the stubble from the previous crop to decompose on the surface, it helps sequester new carbon into the soil.

Low-disturbance no-tillage is the equivalent of keyhole surgery as opposed to ploughing which is invasive surgery and strips the soil of its precious carbon. The result of low-disturbance no-tillage is usually increased crop yields, the near elimination of crop failure, reduced soil erosion and the gradual mitigation of soil compaction.

“The critical advantage of low-disturbance no-tillage over any other method is that it mimics nature as closely as possible Dr Baker says.

“At a time when crop weeds are costing the New Zealand economy millions through lost production, low-disturbance no-tillage is the answer.”

ENDS

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