Friday, March 29, 2024

A maize of crop options

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Choosing the right crop after grazing a paddock with cattle could help reduce risk of nitrogen and other nutrient losses to the environment, Foundation of Arable Research delegates at the Arable Research In Action (ARIA) field day in December were told. In a graphic presentation on the dynamics of nitrogen distribution and possible loss from urine patches, Plant & Food Research’s Trish Fraser explained how work is under way to assess the potential of different crops to mop-up excess nutrients.
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As soon as grazing animals are introduced to a farm system some control of nutrient distribution is lost, she warned delegates, demonstrating how a urine patch would look if the 1200kg nitrogen/ha spot nitrogen rate in the centre were applied as urea.

On average, dairy stocking rates of about 6000 urine patches/ha/year are deposited affecting about 25% of the paddock. Depending on soil type, rain, irrigation and other factors, up to 120kg N/year is leached from those but often less than 8kg N/year is lost from the 75% of the paddock not dowsed with urine, diluting the average loss.

“It all depends on the pasture or crop on top as to how much nitrogen can be retrieved,” Fraser said.

Pasture, with its dense but relatively shallow roots, typically recovers 40-45% of urinary nitrogen with a further 20-30% assimilated into soil organic matter “for future use”.

Deeper-rooting crops such as maize might use up to 200-300kg N/ha. Depending on yield, kale uses a similar amount but an ability to take up more nitrogen than required for growth – the so-called “luxury uptake” – could mean kale sucks 700kg N/ha out of the ground. While that might prevent loss to the environment in the short term it could create livestock health issues at grazing, Fraser said.

Other more unusual crops such as sunflowers are being investigated for their ability to mop-up soil nitrogen, as are better-known options such as Italian ryegrass. Chicory-plantain-grass mixes show promise as a way of reducing animal nitrogen intake, and as a result, excretion in urine.

Plants that inhibit nitrification are also being sought and while the best-known are tropical species, some temperate species are “showing potential”, Fraser said.

Soil compaction behind low yields

Many potato crops fall well-short of their potential and soil-borne disease and compaction appear key causes, Plant & Food Research’s Craig Tregurtha, pictured, told growers at the Foundation of Arable Research’s ARIA field day late last year.

Despite increased irrigation and disease control inputs, average yields in Canterbury have stagnated at 50-60 tonnes/ha, well-short of the 90t/ha predicted by models and achieved by the best crops, he said.

A survey of 11 crops in 2012-13 found six affected by compaction, five by powdery scab and all by the soil and seed-borne disease, rhizoctonia. Yield averaged 56t/ha, only 64% of the 87t/ha yield predicted by models.

Now, a three-year Sustainable Farming Fund project is under way to measure the effect of soil-borne disease, previous cropping and pre-plant cultivation practice at sites in Pukekohe, Manawatu and Canterbury.

“Obviously as costs increase, with static or decreasing yields potatoes become less and less economic,” Tregurtha said.

He encouraged growers to dig soil profile pits to assess the effect of compaction.

“Do that at crop maturity and the roots will tell you so much about what is happening in your paddock.”

Damage to soil structure could happen quickly – for instance, in one pass with a destoner if soil at its operating depth was too wet. Remediation of such damage is “very, very slow”, Tregurtha warned.

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