Clinging to the northern bank of the Rakaia River the last of three Canterbury catch crop trials for this season is growing on a Te Pirita dairy winter forage block that forms part of a three-year Sustainable Farming Fund project to show the benefit of catch crops to reduce nitrate leaching.
Lincoln Agritech’s Dr Peter Carey is leading five catch-crop trials that could be pivotal to the way farmers think about minimising nitrate leaching and nutrient runoff while improving farmers’ bottom lines as well.
Winter forage blocks are a known problem for nutrient loss because, though they might occupy relatively small areas of a farm, they can comprise up to 50% of a farm’s nitrate loss.
The project is a continuation from Carey’s doctoral work at Lincoln University where he studied the use of catch crops to mitigate nitrate leaching under winter forage grazing.
In autumn and winter, non-lactating dairy cows eat large quantities of feed in a relatively short time to build up body condition and deposit a lot of urine onto bare soil when plant growth is minimal. The conversion of the nitrogen in urine to soil mineral forms, such as nitrate, can lead to large nitrogen leaching losses of 80-120kg N/ha through field drainage.
Carey said success will ultimately be measured by establishing the practice of sowing winter catch crops as a normal part of winter forage management and demonstrating effective alternatives to farmers that lower nitrate leaching losses after winter forage grazing.
Sowing a crop immediately following winter forage grazing, however, can be problematic but hardy cereals such as oats can still establish in the cool conditions and once the soil warms can rapidly mop up some of the soil nitrogen, reducing the amount available for leaching.
A number of lysimeter and field trials have shown sowing a catch crop can reduce nitrate leaching losses by as much as 40% as well as improving nitrogen-use efficiency and farmers’ profitability. As ever in an agronomic trial, farmers ask how the results translate to their own soil, climate, stocking rates and budgets.
Carey is comparing results from three sites in Canterbury and two in Southland on ex-kale and ex-fodder beet paddocks covering a range of soil and climate conditions.
“The one over in Methven is a bit stonier. These ones here (Te Pirita) are a little bit better and we’ve got a bit more free-draining soil on top, probably hitting stones at about 45cm so you get a whole mix.
“The thinner Canterbury topsoils, like Balmoral soils, often cover a deep bed of stones and gravels and present the most challenge to prevent nitrate leaching.
“That’s simply because it’s more free-draining and it’s harder to hold that nitrogen up. As you can imagine, stones don’t hold much water, so if stones are occupying the greater part of the soil then water gets through quicker.”
Where the soil is deeper there is more opportunity to retain nitrate against leaching.
Drilling a catch crop here has two benefits – the crop takes up the surplus soil nitrogen but also transpires soil water, even in late winter, reducing drainage.
A contractor for the Lincoln Agritech trial, Quigley Contracting’s Andrew Gorman, said farmers often first consider the risk of establishing a crop like oats.
“From a feed marketing point of view it’s a bit of a battle to get guys to put oats in and cut them for silage or something like that because you take them through to whole crop and on a dairy farm it becomes a feed that’s neither here nor there. You can’t milk off it but it’s too good to grow to winter off.”
But their protein value is excellent if farmers can use the oats at a leafy stage, Gorman said.
“That would then allow you to follow with a second-year crop of kale or something like that if you were cutting November/December. It’s still pretty reasonably productive per hectare, I reckon.”
Carey said the Lincoln dairy farm has already shown the combined feed value as green chop silage and a following late spring-sown winter kale crop is greater than an earlier sown winter kale crop alone.
“If you were taking the crop off in November and then drilling you would get a bit less feed than if you were able to put in another crop in October. But we’re pulling off 10 tonnes of oats here. Combine that with maybe a fodder beet crop of 20t and you’re actually making more money, really, and you’re lowering your environmental footprint at the same time.”
Triticale is another whole-crop option, possibly for farmers taking crop back into pasture but, unfortunately, it matures too late for some farmers.
“There’s not enough people with patience in the world.”
Considering that, the best way to work with oats might be to take it off at the leafy stage and direct drill grass into it.
“You’ve got a good cut of quality oat silage then you can graze it, then direct drill grass. You’ll get good quality grass off it.”
Carey said that is one of the reasons that the trial is comparing an oats/ryegrass combination with oats alone.
“Once you’ve had a couple of years of winter forage crops you can cut the oats then go into the Italian ryegrass for a couple of years before maybe heading back into perennial ryegrass .”
Carey recommends farmers tread lightly when it comes to crop rotation.
“We like to think that farmers are going through a soil restorative phase after a couple of years of winter forage crops.”
Carey said the aim of the catch crop project is to upscale the applied research into working winter crop rotations in Canterbury and Southland and adapt it to the various soil and climatic conditions.
