Words by: Karen Trebilcock

AgResearch scientist Dr Ross Monaghan is hoping a linear lysimeter in one of the Southern Dairy Hub’s fodder beet paddocks will help unlock the secrets of nitrogen (N) leaching on the experimental farm.

The lysimeter, which is 10 metres long and at a depth of 40cm, was installed in November and will measure the amount of nitrogen leaching from the soils in real time when the fodder beet is grazed in winter by dairy cows and afterwards as the paddock lies bare before conditions allow it to be replanted.

The hub, near Invercargill, is looking at ways for southern farmers to reduce nitrogen losses by up to 30% and feeding winter crops is known to be “particularly leaky”, Monaghan said at the February field day on the farm.

However, research so far is showing losses are not restricted to grazing and the time before cultivation back into grass or another crop, as previously thought.

Due to last year’s dry summer and winter in Southland, urine appears to be still leaching through the soil profile although concentrations on autumn-grazed fodder beet have peaked and are now declining while winter-grazed crops were yet to peak at the end of January possibly showing there was still winter deposited urine in the soil available to be leached.

Monaghan said results so far showed autumn-grazed and lifted fodder beet paddocks appeared to leach more N than those winter-grazed and lifted, possibly due to the winter plants still growing and taking up soil mineral N.

The lysimeter’s results would be compared with the computer modelling of Overseer and could be an accurate tool for farmers to measure N losses in the future, although the cost of the equipment was high.

The hub’s lysimeter is on an annual lease.

As well as the lysimeter, hundreds of ceramic cups were placed at root level in the winter crop paddocks in January and will also measure N losses and hopefully the results would correlate, he said. The cups, which also gave last year’s results of N losses in the winter crops, are also costly as they take time to install and then sample and provide a snapshot instead of the continuous real time analysis of the lysimeter.

DairyNZ senior scientist Dr Dawn Dalley said this year’s winter crop paddocks, were tested for mineral N in October before cultivation and showed a wide range of results.

Mineral N is fixed by clovers and comes from animal urine and dung.

A two-year crop rotation has been established at the hub to minimise the number of paddocks out of the grazing rotation in early spring.

The paddocks which had been cropped once had only about a quarter of the mineral N compared to those which had been in pasture, especially those where the crop had been lifted instead of fed in the paddock.

The results of the testing were used to determine the fertiliser requirements for each of this year’s winter crop paddocks. Yield assessments on crop paddocks were starting mid-March and feed budgets would follow.

Dalley said staff were so far hesitant to place balage on to crop paddocks in the autumn as last year growth deficits meant they had to reposition balage during winter months.

“We’ll probably be looking at placing balage on weekly when the crop is getting fed and picking those frosty days to do it to minimise damage,” she said.

“We’re also finalising at the moment the order of crop grazing, and in which areas the crops will be lifted and in which they will be grazed, to streamline getting the paddocks back into new grass or crops in the spring.”

The grazing direction was also being worked out, with cows eating crop from the top of the slope down in accordance with good practice. Large un-grazed grass buffer zones have been left between crop and drains to minimise sediment and nutrient losses to waterways.

Portable water troughs and back fencing would also be used to prevent soil damage and keep cows out of the mud.

One set of results that is back is the weight measurements from calves wintered on fodder beet compared with those wintered on kale last year.

The calves were kept together until April when they were split into two mobs with one transitioned on to fodder beet while the other mob was introduced to kale the following month. Weights started showing a variance at the first weighing at the end of May and by early October the calves wintered on kale were on average 25kg heavier than those wintered on fodder beet.

“Since winter they’ve been put back together so it will be interesting to see if the weights of those wintered on fodder beet come back up,” Dalley said.

Fodder beet is traditionally not used for wintering young stock as it has a low protein, calcium and phosphorus content.