Irrigated wheat - The root of the matter

Maarten Stapper

By Brad Collis

Maarten Stapper holds up a sturdy looking wheat plant. “These are the key,” he says, placing a finger against roots that are still dripping black mud. “If your plant can’t anchor itself, then everything else is for nothing … including all your calls for new varieties with more stem strength.”

Dr Stapper (left) is a scientist who likes to get quickly down to basics; talking to growers about what they can do, rather than what scientists can do.

His main area of interest as a senior researcher with CSIRO Plant Industry is maximising the yield potential of irrigated wheat, but he has a message that he believes is pertinent to all grain growers.

“The average farmer is achieving 50 percent of his or her farming system’s potential at best. The top farmers in many districts might be achieving 80 percent. So whether you are average or above average, there is still enormous potential for improvement on the farm. So we have to identify these points for improvement.”

His argument is that improved crop varieties alone are unlikely to deliver on promises and expectations if the on-farm growing conditions for the plant are not also improved – in particular the health and condition of the soil.

Dr Stapper believes much improvement still needs to be made to conventional farming systems.

He says we need to work harder on agriculture’s basic resource – its soil biological systems.

In the case of irrigated wheats, he says, calls for varieties with increased stem strength to make heavy crops stand upright and not lodge (fall over) are often missing the point.

“You need good root development and good soil conditions, and that’s something you work at on the farm, not in a laboratory,” he says.

Dr Stapper says the ideal soils for irrigated wheat have porous, organically-rich topsoil with a diverse soil biota. This improves drainage into the subsoil, and leads to improved deep water and nutrient retention, and root growth.

After the first year of a GRDC funded project looking at the management practices needed to achieve 7-8 tonne crops with maximum water-use efficiency, Dr Stapper has developed a preliminary set of principles:

Dr Stapper has found that a May sowing is important for several reasons. It minimises the imbalance between the production of grain and vegetative matter, and helps to avoid lodging.

The balance between grain and vegetative matter is important because too much vegetative matter reduces the amount of light reaching the base of the stems, and this affects stem strength.

Following pre-sowing irrigation, sowings in April with heavy tillering winter wheats are therefore not suited for high yield targets.

But while a later sowing helps to maintain a more appropriate growth pattern, there is a potential trap for the unwary: “Often we’ll get late August or early September rains that deceive us because the soil surface looks nice and wet – but that’s when the plant has reached the stage where its roots have been sucking the subsoil dry,” he says.

“So you have to check the subsoil moisture, and be ready for the first irrigation.”

The first trials under the GRDC Irrigated Wheat Evaluation Project, during the 2002 season, evaluated 140 genotypes (varieties and breeding lines) from most breeding programs in Australia, Crop and Food Research in New Zealand and CIMMYT in Mexico.

Because of the dry season, a shortage of irrigation water affected some trials, and conversely, lodging was rare because stems were generally stronger than they would be in wetter seasons. Nitrogen management was difficult without the support of follow-up rains and leaf diseases were absent or low in mid-season sowings. Leaf rust and barley yellow dwarf virus were present in the April sowing.

Nonetheless, the project achieved good results from the genotype evaluation experiments sown midseason at Griffith, Benerembah, Deniliquin and Moama, plus extra experiments at Griffith that involved:

The table below shows a summary of yield and grain testing results which were done at NSW Agriculture’s Wagga Wagga Agricultural Institute.

GRDC Irrigated Wheat Project - 2002 yield and grain test results

The main message from these first trials is that sowing early is ineffective for high yield. Most of the winter-type varieties in the April sowing lodged more than 60 percent and had low yields, with a total biomass (grain and straw) of 24 t/ha or more.

A sowing rate of 70 kg/ha gave an average plant density of about 130 plants /m2 with 15 to 26 tillers per plant for late maturity (eg Rosella) to very late maturity (eg Rudd), respectively. Shoot densities (main stems plus tillers) were around 1800/m2 for the Rosella-type winter wheat, and up to 3000 for the Rudd-type red feed wheat.

Heavy tillering caused very early canopy closure, well before the start of stem elongation, resulting in weak stems and lodging. Genotypes that lodged later and less were still not yielding high as there was too much pre-flowering growth. This resulted in a low harvest index (the proportion of grain from total biomass) and heavy stubble.

Also, early application of nitrogen tends to promote tillering and early growth in all crops, similar to early sowing, and should be avoided if aiming for high yields.

Sowing rates of 100 kg/ha in May sowings gave an average plant density of 180 plants/m2 with two (500 shoots/m2; eg H45) to six (1300 shoots/m2) tillers per plant depending on variety and available nitrogen.

Maximum yields were achieved with mid-flowering in the last week of September or first week in October. In the dry 2002 season, yields decreased by 150 kg/ha for every day’s delay in flowering from mid-October. Previous studies showed this to be 100 kg/ha/day in ‘normal’ years, associated with increasing temperatures (see graph).

Effect of flowering date

Effect of flowering date

The 2002 experiments also looked at commercial nitrogen application rates on 15 Chara paddocks across the region. Topdressing nitrogen up to early stem elongation increased yields, but crops top-dressed after flag leaf achieved only lifts in protein. Most maximum yielding test-strips had protein greater than 11 percent.

For further information:
Dr Maarten Stapper, CSIRO Plant Industry,


Region North, South, West