Catch Crops

The key attributes of catch crops when following autumn or winter grazed crops are that they:

• are cold tolerant
• are winter active
• have fibrous deep root systems capable of removing nitrogen at depth.

Catch crop benefits vary depending on weather conditions, particularly during establishment, with direct-drilling or light pre-drilling cultivation being recommended methods where possible to minimise nitrogen mineralisation.

Catch crop costs

The gross margins (revenue minus costs) and cents per kg dry matter of catch crops can be used to compare different crops.

The numbers will vary depending on the yield and how the crop fits into an individual farm system, which is influenced by catch crop sowing date, the timing of feed requirements and the sowing date of the following crop or pasture.

The Forages for Reduced Nitrate Leaching (FRNL) research trials gross margin and c/kg DM varied from $173/ha to $1263/ha and 8.1 c/kg DM to 15.3 c/kg DM respectively. For more examples see the management guidelines below.

To compare gross margins and cost of different catch crops, use this spreadsheet.

Species to consider

FRNL ran trials of different crops comparing crop nitrogen uptake and residual soil nitrogen as indicators of the risk of nitrate leaching.

A variety of species or combinations can be used as catch crops. Consider the below when selecting species or combinations thereof:

• Establishment opportunities of the catch crop; what is the usual soil temperature at the time of planting and can heavy machinery normally access the paddock at that time of year?
• Main crop/regrassing planting date; consider effect on potential yield of the following crop or pasture.
• Ability to fit into the farm system, e.g. planned crop rotation, harvesting or grazing dates, end use of land planted (permanent pasture or crop), use of catch crop:
  • green manure
  • grazing
  • green chop / whole crop silage
  • grain or straw

Examples of catch crops

Oats: A low temperature germinating, fast-growing cereal that can be sown in autumn, winter and spring. Used for winter green feed, green manure or silage. More information here.

Triticale: Tall cereals developed from crossing wheat and ryecorn, some varieties can be planted in autumn and winter behind grazed crops. More information here.

Ryecorn: Used in dryland situations where it is sown in early summer for grazing through until spring. More information here.

Italian ryegrass: Establishes quickly and will grow at lower temperatures than perennial ryegrass. It can be sown by itself or in combination with a cereal crop. More information here.

Faba beans: Best planted between early March to late April following a summer crop to use as a late spring feed. This legume fixes nitrogen, which in some crop rotations can be beneficial for the next crop. More information here.

Optimising catch crops

To maximise catch crop yield and reduction in nitrate leaching it is recommended to:

• target high populations (300 plants/m²) with cereals, to minimise the time it takes for crops to reach canopy closure
• aim to sow seed at about 3 cm depth, to ensure good soil-to-seed contact, and reduce the risk of bird damage
• add phosphorus (50-100 kg P/ha as Triple Super) down the spout with the seed to aid the establishment of oats. Nitrogen will not be required early on in the growth phase, so avoid using DAP. Typically after a grazed crop, nitrogen fertiliser will not be needed at all for a catch crop.

How catch crops help the environment

A catch crop is any crop that is grown with the primary objective of catching excess nitrogen in soils that otherwise may be lost through leaching. The reduction in nitrate leaching losses results from the crop's rapid uptake of residual mineral nitrogen and the reduction of the soil water content of the soil, which reduces the risk of drainage.

Sowing a catch crop in autumn or winter can reduce nitrate leaching losses. The earlier the crop establishment, the greater the potential to reduce nitrate leaching.

DairyNZ's FRNL project ran catch crop trials in Waikato, Canterbury and Southland comparing different species for biomass at green chop (beginning of November), whole crop silage stage (end of December 2018) and plant nitrogen uptake.

The results showed that while all catch crops can reduce nitrate leaching and produce significant yield, some performed better than others. Cereals (e.g. oats, ryecorn, triticale, wheat or barley) can be used as catch crops after winter grazing and are considered more effective than grass species (e.g. Italian ryegrass). It is recommended that an agronomist is consulted to match the best species and cultivars to your farm and goals.

Reducing nitrate leaching

FRNL research trials have shown that using a catch crop can reduce nitrate leaching. The main factors to reducing nitrate leaching potential are early crop establishment, autumn/winter rainfall, establishment method and how and when nitrogen fertiliser is applied.

The earlier the crop is planted, the greater the opportunity to reduce nitrate leaching. The following table shows the modelled nitrate loss reductions (%) at different planting dates of an oat catch crop following grazed fodder beet.

The variation in rainfall, methods of sowing (i.e. direct drill vs cultivation) and fertiliser use are not shown in this table. These will also influence catch crop performance. For more information on these research results see the management guidelines at the end of this page.