Friday, March 29, 2024

A fresh look at leaching

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A fresh look at a 16-year-old research trial looks set to turn recent thinking about the relationship between stocking rate and nitrate leaching on its head.
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DairyNZ principal animal scientist Dr John Roche is lead author on a paper submitted to world-leading science journal The Journal of Dairy Science, which shows nitrate leaching per hectare decreases as stocking rate increases.

There’s a few provisos around the findings – most importantly that a lift in stocking rate isn’t accompanied by an increase in bought-in supplement or additional nitrogen fertiliser inputs.

The results appear to fly in the face of recent thinking that a sure-fire way to reduce nitrate leaching losses is to cut stocking rate to enable higher per cow production.

They also appear to support New Zealand’s low-input style dairy farming system where stocking rates are set to closely match the pasture growth curve and cows are dried off early to reduce feed demand over autumn rather than fed significant amounts of purchased supplement to extend lactation.

Potentially the research has huge ramifications for NZ dairying in the current nutrient limit-setting climate as well as the low payout environment.

That’s because it implicates bought-in supplements not stocking rate as the villain in the nitrate leaching story.

The original research trial was carried out from 1998 to 2000 at what was DairyNZ’s precursor, the Dairying Research Corporation.

It was run by scientists including Dr John Penno, now Synlait chief executive, and Kevin Macdonald.

They’re co-authors on this latest paper along with AgResearch researchers Dr Stewart Ledgard, Mike Sprosen, Stuart Lindsey and Irish scientist Dr Brendan Horan from Teagasc Moorepark.

During the 1998-2000 study, AgResearch installed 180 ceramic cups and several lysimeters allowing the actual soil solution concentrations of nitrate nitrogen to be measured over time.

A range of results from the stocking rate trial have been published in journals since, including the fact the higher-stocked farmlets grew more grass of higher quality and cows utilised more of it even though fertiliser inputs were the same and condition of pastures and soils was similar at the outset.

Published results showed cows also produced more milksolids (MS) per hectare including fat, protein and lactose, but on a per cow basis, production declined because of lower feed availability per cow and shorter lactation lengths at the higher stocking rates.

But this latest paper is the first time the relationship between stocking rate, nitrate leaching losses and profit has been teased out

While the nitrate leaching results are clear cut the reasons behind them aren’t entirely understood and require more research but, as part of the scientific paper, Roche and his co-authors offer up some theories.

They’re essentially based on the fact as stocking rate increases more cows are dried off earlier on the higher stocking rate farmlets and those cows have a significantly reduced feed intake over the autumn period.

Their daily pasture consumption, once dried off, is about half that of lactating cows which means their nitrogen intake is also halved through the autumn.

It’s widely accepted that nitrogen deposited in autumn and winter has the greatest risk of leaching because plant growth rates are slowing and, as winter comes on, the risk of drainage through the soil profile increases.

Because dry cows consume half the amount of nitrogen as lactating cows do during autumn the amount of nitrogen excreted in their urine also reduces which means nitrogen deposition per cow through the autumn is much lower on the high stocking rate farm even though there are more cows per hectare.

Another possibility is that at a higher stocking rate the nitrogen in the feed is spread across more cows and therefore more urine patches, assuming the number of urinations per cow per day remain relatively constant.

The urinary nitrogen excreted per cow calculated as part of the research decreases as stocking rate increases and that may also lower the concentration of urinary nitrogen providing the number of urinations and volume of urine doesn’t decrease with reduced feed intake.

The results clearly show that pasture harvested increases as stocking rate increases and the greatest harvest of drymatter – and therefore nitrogen via crude protein levels in the pasture – was between calving in July and February.

That’s when risk of nitrogen leaching losses is lower because plants are actively growing.

An as-yet untested theory for the reduction in nitrate leaching with higher stocking rate is that an increase in surface soil compaction at higher stocking rates results in a greater horizontal spread of urine after it’s excreted onto paddocks.

Together with the lower nitrogen rates in the urine that could mean plants are better able to take up the more widely dispersed nitrogen when it gets to the root zone.

It could also mean gaseous losses are increased because the urine is near the surface for longer. Importantly however previously published results on the trial didn’t show any increase in soil compaction and, if anything, pasture growth was increased.

Back to the future

Findings that nitrate leaching levels could be lower at higher stocking rates give the dairy industry in New Zealand a chance to restore its competitive advantage and meet environmental targets.

Large-scale dairy farmer Colin Armer said over recent years the industry had lost its way with the increasing use of supplements and a production focus.

“This research shows that we can produce milk based on stocking farms to the grass curve and still minimise nitrogen losses – we can get back to our competitive advantage,” Armer said.

Several of his farms are within nitrogen-loss sensitive zones near Lake Taupo and the results of the trial showed it was too simplistic to look at cutting stocking rates as the only answer to farming in such areas, he said.

Once nutrient loss became a limiting factor, using “profit per kilogram nitrogen lost per hectare” as a metric really helped shape thinking.

The research results backed that up.

Armer has long been a champion of grass-based, low-input style dairying as has Canterbury dairy farmer Alistair Rayne.

Rayne said it was well-known that profitability was closely linked to pasture utilisation, which lifted along with stocking rate in the trial.

Matching stocking rate to total feed grown was fundamental to profitable dairying and the results showed that approach offered a fantastic win-win solution if farmers were prepared to get back to true grass-based farming.

“We can improve pasture utilisation, profitability and, as it turns out, leach less nitrogen by getting back to the fundamentals.”

It appeared many farmers had lost the confidence and core skills to lift and achieve high pasture utilisation and had become fearful of changing their feeding levels, he said.

A key aspect of the findings was that supplements weren’t bought in to keep cows milking through autumn and cows were culled or dried off early to bring feed demand down to match supply.

The comprehensive trial had shown that while it was likely per-cow production would decrease it was still possible and clearly important that cows remained healthy and fertile and met their usual condition score targets.

Paper co-author Dr John Penno said it provided results for further thinking around stocking rates and nitrogen loss, including setting the economically optimum stocking rate.

Farmers and consultants were grappling with those decisions in light of nutrient regulations and payout and the findings offered the basis for some new modelling on how farmers could increase their profit and minimise environmental footprint in what has become a very challenging set of circumstances.

The experiment

A large-scale farm systems experiment – 188 cows randomly allocated to two replicates of five different stocking rate treatments.

  • Stocking rates (cows/ha) 2.2, 2.7, 3.1, 3.7 and 4.3
  • Cows were dried off in autumn to match feed demand to seasonally declining feed supply from pasture, rather than buying in supplements to extend days in milk.
  • Cows were dried off systematically based on body condition score and time to calving but remained on the platform.
  • Milk production and milk yield components measured.
  • Feed intakes measured.
  • Nitrate leaching measured using ceramic cups in paddocks.
  • Profitability was calculated per hectare and per kg nitrogen leached per hectare.

Why profit per kg nitrogen leached is important

Financial success is dictated by operating profit relative to the primary factor limiting output, DairyNZ principal animal scientist John Roche contends.

He makes the point in a new scientific paper that reviews the results from a stocking rate trial carried out from 1999 to 2000.

Just as profit per cow is the measure used in a housed-cow system because output is limited by the number of beds or feeding space, profit per hectare is important in a system where land is the limiting factor.

When the amount of nitrogen leached becomes a limiting factor, it too should be a metric used to compare profitability.

The original analysis of the stocking rate trial found operating profit per hectare didn’t increase directly in line with higher stocking rates but showed a curvilinear response.

It rose as stocking rates increased initially but at the highest stocking rates began to decline. At low milk prices ($4.30/kg MS), this decline was much less.

When measured relative to nitrogen leached per hectare, operating profit increased linearly with stocking rate.

Importantly the relationship was primarily because while production per hectare increased with rising stocking rate, days in milk per cow were reduced because supplement was not brought into the system to maintain lactation and nitrate leaching decreased with increasing stocking rate.

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