Narrow rows, deep roots drive yields

Mr Johnson was a guest speaker at the recent SA No-Till Farmers Association annual conference at Tanunda, where he advised attendees about critical herbicide timing in no-till cropping operations.

"If your wheat crop still has weeds in it at the three-leaf stage, you have lost yield and it's irreversible," he said.

Mr Johnson said a crop could sense a weed growing next to it through the quality of light being reflected off the plant.

"The wheat crop will say to itself, 'I need to out-compete the competition beside me, I need to grow more top to get higher and intercept the sun'," he said.

"This means the crop then focuses less on its root system, which eventually means less water intake and ultimately less yield."

If croppers were looking for more yield, they needed to focus on improving their access to moisture below the soil surface, "to go deeper", and the easiest way to do this was to improve the root system of the crop.

"That is why you really need to keep the crop clean from emergence through to three-leaf stage - it's the critical period," he said.

Another weed-control suggestion was to keep row-widths narrow.

"In no-till operations, there is less tillage, meaning more soil density," he said.

"This makes the plant grow slower in the beginning, as it is tougher to grow roots when the soil is more dense."

In trials Mr Johnson had undertaken in Canada, when they went from 38-centimetre to 19cm rows there was an 8pc yield increase, while trials going from 19cm to 10cm varied from 0pc to 4pc.

"What we drew from this was that we achieved a 1pc yield increase for every 25 millimetres you narrowed your rows," he said. "But far more importantly, the weed control improved significantly as there was more crop competition."

Given that Australians grew cereals in such hot conditions, soil temperature should also be closely monitored.

"Because you plant at the tail-end of your summer, when soils are hot, tillage only exacerbates that," he said.

"Baling straw should also be outlawed, because the organic matter and residue cover is so valuable, and straw spread is critical, including the chaff.

"Residues come into play during grain fill, as it is cooler under the residue than without it - it's simple thermodynamics."

If farmers could cool the temperature under the canopy, even by only 5C degrees, it would help the plant tolerate SA's higher temperatures.

"Particularly in those days where it gets over 38C - wheat goes into temperature shock - so you really have to try and focus on keeping the soil temperatures low," Mr Johnson said.

Another way to increase yields was to make sure crops had uniformity, particularly at emergence, through even residue spread and accurate planting depth.

"You can't get a uniform stand if you can't get the seed at the same depth," he said.

"This is where croppers need to get rid of their hoe-drills - the only way you can get accurate seed placement in the soil is by sensing the depth right beside where you put the seed in the ground and a hoe-drill just can't do that.

"Disc drills may require more maintenance, can be slower and require a smarter operator, but they sense depth better and give more accurate seed placement. It makes a difference in uniformity and therefore yield - it really does have pretty big implications."

He suggested seeding at about 25mm deep (assuming there was moisture), to ensure the best balance between a deep root system and a strong emergence.

"You always have to plant at about 25mm deep to make sure you get your crown roots as deep as you can," he said.

"You don't want the crown up in the hot dry zone, but planting 75mm deep won't help you get a deeper root system either.

"The 25mm-deep plants are the ones that grow the fastest - they emerge first, and those are the plants that will have the biggest yield."

Crop rotation was another way to increase yields, mainly through reduced nitrogen use.

"Certain rotations can increase yields by up to 0.5t/ha, particularly wheat after canola, or wheat after pulses - both work quite well because of disease reduction and the break crop can result in less nitrogen needed."