Catch crops prove a valuable environmental tool

// Forage Cropping

Along with several other organisations, B+LNZ helped fund the catch crops for reduced nitrate leaching project. The outcomes highlighted the value of catch crops as a tool to capture and use nitrogen left in the wake of winter forage crops, provided the correct management principles were followed.

catch crops

This two-part series outlines how to make the most effective use of catch crops.

Making the best use of catch crops:

  • Sow as early as possible after grazing.
  • Select winter-active species.
  • Target minimum or no-tillage.
  • Target high plant populations.
  • Minimise weed competition.
  • Apply nitrogen fertiliser from October if required.
  • Harvest at green-chop silage (booting) for maximum yield and quality.

Intensively grazed winter forage crops are a known hot spot for nitrogen leaching, but catch crops have been found to reduce this leaching by up to 50 per cent.

Sown as soon as possible after grazing has finished, catch crops have been the subject of a Sustainable Farming Fund project led by Peter Carey from Lincoln Agritech with support from Brendon Malcolm and Shane Maley from Plant and Food Research and AgResearch.

They carried out trials at both plot and farm scale to determine how to make the most effective use of catch crops to reduce nutrient losses while generating dry-matter.

Speaking a recent workshop, Brendon Malcolm says catch crops mop up N and reduce drainage by taking up water, but timely sowing with the appropriate winter-active species was important to make the most effective use of these crops. July-sown oats had the greatest impact on reducing nitrogen losses.

At paddock scale, oat crops were capturing up to 100kg N/ha by the end of the leaching period, N that would be otherwise be lost to the environment.

Yields in catch crops grown for green-chop silage were typically between 8-10t/ha with a maximum of 12t DM/ha.

Brendon says while the oats sown in July were very slow to come away, they were still capturing significant amounts of N through the root system, despite the lack of above-ground foliage.

“They can be effective, even if they don’t look like much.”

Growing a crop at a time of year when the paddock would normally be bare, does add to bottom line and an on-farm trial looking at an ex-kale crop showed that it only took a crop of 2-3t/ha to break even.

Returns per hectare were greater in the direct drilled crop versus the crop established with tillage- $1620 versus $1520/ha, although Brendon acknowledged that tillage was sometimes necessary to provide good seed-to-soil contact if the field surface was too pugged from the previous forage crop like fodder beet, for example.

Early sowing critical

Peter Carey says early sowing is one of the most important factors with catch crops.

“The key point is sowing as early as you practically can. Even if germination is slow, it doesn’t matter too much as the cool temperatures that slow plant growth also slow the rate of nitrification and nitrate production.”

Modelling carried out in Canterbury on free-draining soils showed the earlier the sowing, the greater the amount of N captured. The quantity captured is reduced under higher rainfall, but generally speaking, winters are getting warmer and drier so the opportunity to sow these crops is increasing.

A sowing date trial comparing crops sown on 11 July, 3 August and 31 August highlighted the importance of early sowing. By harvest in November, there remained a big difference in the crops despite the three-week difference in sowing dates.

“Some years you just cannot get on the paddock – maybe one in every five years – and you just can’t do anything about that.

Species selection

Peter says they have focused on oats as they are robust, will germinate at lower temperatures and produce quality green-chop silage.

A trial comparing Italian ryegrass, with Triticale and oats showed oats (Intimidator) to be the stand-out performers in terms of both dry-matter yield and nitrogen captured.

An oat-Italian ryegrass mix is proving popular amongst some farmers, as while the yield of the oat crop is slightly compromised due to the lower sowing rate, growers benefit from Italian ryegrass coming through post-harvest to provide summer feed.

For information about catch crops go to B+LNZ’s Knowledge Hub or the Catch crops for cleaner water Facebook page.