Thursday, March 28, 2024

Nitrogen losses halved under fodder beet

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Scientists analysing soil solutions under fodder beet crops at Lincoln University’s Ashley Dene Farm have found nitrogen leaching losses close to half that of what’s previously been predicted. It’s taken some painstaking research in trying conditions for those collecting the data but two winters of collecting soil solution samples have shown nitrogen leaching losses in the order of 50-60kg N/ha/year. That’s almost half the nitrogen losses indicated by models such as Overseer and is also lower than numbers coming from crops such as kale using the same measurement techniques. The research was done over the 2014 and 2015 winters with the results corroborating trials that had previously been done using lysimeters. The 2014 and 2015 studies were part of Pastoral 21 (P21) trials funded by DairyNZ, Beef + Lamb New Zealand and the Ministry of Business, Innovation and Employment.
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The P21 programme includes studies at four locations – Waikato, Manawatu, Canterbury and south Otago – with the Canterbury programme focused on developing practical dairy farming systems that combine high production and profit with lower nitrate leaching.

The results from the wintering study aren’t based on modelling or predictions but actual readings from close to 4000 soil solution samples taken from directly under paddocks grazed by dairy cows under research conditions.

‘They were literally on their hands and knees in the mud twice a week.’

Lincoln University soil scientist Professor Keith Cameron said it meant four technicians going into freshly grazed areas twice a week for five months over both winters to collect soil solution samples from 100 suction cups buried 70cm underground.

“They were literally on their hands and knees in the mud twice a week,” Cameron said.

The soil solution samples allowed the scientists to determine the concentration of nitrogen and together with drainage recordings they were able to calculate the actual leaching losses over the 2014 and 2015 winters.

They also did the same study using 100 suction cups buried under a kale crop but, with only one year of data, scientists were only prepared to say losses looked to be more in line with Overseer predictions for that crop rather than give specific leaching loss numbers.

Lincoln University professor of dairy production, Grant Edwards, and DairyNZ principal scientist Dr David Chapman, who led the P21 farm systems research project in Canterbury, said the science teams were yet to understand why the losses were so low under fodder beet.

The crop has low crude protein levels and urinary nitrogen concentrations of cows grazing fodder beet are subsequently low at 2.5-4g N/litre but it had been thought the high stocking rate and length of time cows grazed the high-yielding crop would still push leaching losses up. That doesn’t appear to be the case but it’s not yet clear why.

“There are some theories but more work needs to be done to understand what’s behind these numbers,” Chapman said.

This was important because it might hold clues as to how other interventions could be used to reduce losses under other crops. The results of the trials were also important for farmers using fodder beet on their milking platform.

Edwards said farmers growing a crop on the milking platform would be increasing their total nitrogen losses compared with an all-grass system but based on the research results it appeared they wouldn’t be leaching as much as previously thought.

There could be more positive implications for those harvesting fodder beet bulbs elsewhere and bringing them on to the milking platform to replace other autumn supplements.

A technician collecting samples from suction cups after cows have grazed fodder beet.

That’s because the low crude protein levels in fodder beet used as a bought-in supplement would dilute the overall crude protein levels in the cows’ diet and reduce urinary nitrogen deposits on to the soil at the riskiest period for nitrate leaching.

Overseer general manager Dr Caroline Read said the findings from the research and data would be helpful to the Overseer team once they were fully available and the research was published.

An extension to the second phase of P21 funding finishes at the end of February, 2017 but discussions are underway to extend the successful collaborative model involving scientists from Lincoln University, DairyNZ, AgResearch and Plant & Food Research.

Chapman said proposals were currently being prepared for another five-year P21-style programme of work.

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