Catch crops

What

A catch crop is a fast-growing crop that is grown between successive plantings of a main crop.

The purpose of a catch crop is to increase annual dry matter production, to take up soil mineral nitrogen to reduce the risk of leaching or runoff.

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

• Have fibrous deep root systems capable of removing nitrogen at depth
• Are tolerant to the cold
• Are winter active

Catch crops are particularly useful to reduce excess nutrients in soil after intensive activities, such as winter grazing. Livestock urine adds large amounts of nitrogen back to the soil which can leach into waterways and affect water quality. Traditionally, once the forage crop has been eaten, the land has been left bare until the spring crop can be sown which leaves the excess nitrogen vulnerable to loss.

Why

The reduction of nitrate leaching risks by growing catch crops varies from year to year depending on weather conditions, particularly during catch crop establishment. This is due to more leaching occurring under wetter conditions. The cultivation method is also an important consideration, with direct-drilling or light pre-drilling cultivation being recommended methods where possible to minimise nitrogen mineralisation.

To make 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.

Research by Plant and Food has shown that incorporating fast-growing catch crops – such as oats and other cereals – immediately after winter grazing can reduce nitrogen leaching by up to 50%, as well as providing cost-effective feed for livestock. The key findings of there research trials are summarised below.

Performance

The performance of catch crops depends on a number of factors. Under wetter conditions, performance is lower as more nitrate becomes mobile but reductions of 35% are still possible. Cultivating by direct drilling can also result in a lower yield but is recommended to minimise nitrogen mineralisation from the soil. A Canterbury field trial managed to achieve gross margins of more than \$2,000 per hectare.

In Southland, the research investigated new tillage technology and showed that use of a single-pass cultivator and drill to sow the catch crop, could result in returns of \$1,600 per hectare. Conventional cultivation may not be possible because of the wet soils. Cultivation method may can be limited by the amount of animal treading damage when sowing catch crops after winter grazing.

Sow early

Early sowing is one of the most important factors with catch crops. 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.

A Canterbury 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.

Species selection

Oats have been the focus of the Plant and Food Research trials 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.

High sowing rates

High sowing rates of 110-120kg/ha are needed to get good nitrogen uptake, fast canopy closure, and to produce a high yielding crop according to Shane Maley from Plant and Food. They recommend for an oat-Italian ryegrass mix, rates of 70-80kg/ha of oats and 20kg/ha of Italian ryegrass. For an oat crop, aim for a plant density of 300 plants/m^2^ to achieve canopy closure as quickly as possible. This also helps to out-compete weeds.

Nitrogen fertiliser

The majority of catch crops won’t require nitrogen – especially at sowing as this will defeat the purpose of the catch crop. Even if the crops are slow to establish, they will still be capturing N.

Some crops may require a dressing of N in October just to finish the crop off, but Plant and Food Research says 60% of catch crops won’t need any. It depends on the season and the soil type.

Harvest

Trials have shown that for maximum yield and quality (ME) the crop should be harvested at the green-chop silage (booting) stage.

Harvest as the panicles begin to emerge, if not before. Once the panicle emerges, quality declines quickly. Crude protein levels peak at 13-15% of DM at Growth Stage 39-45 before dropping off.

Things to be aware of:

• The sowing of the subsequent winter forage crop will be delayed by taking an oat crop to green-chop silage. However overall, the paddock will be growing more total feed over a 12-month period.

• If it is too wet to get on a paddock before the end of August/beginning of September, then planting a catch crop is probably less attractive.

• Catch crops can be difficult to fit into a milking platform rotation, but an oat and Italian ryegrass mix can help by growing feed into summer.

• Catch crops are not recommended in a fodder beet-to-fodder beet rotation as the harvest of the catch crop is past the optimum sowing window for fodder beet.

• In seasons where water is limited, catch crops can deplete soil moisture that would be otherwise available for subsequent main crops.

• Catch crops also provide feed at a time of the year when feed is often not required. Conserving feed adds costs and logistical challenges.

• Sourcing the appropriate seed can be difficult due to the popularity of catch crops. It is important to get good seed as vigour is important when sowing in winter.

Researchers at Plant and Food are currently developing protocols for establishing catch crops across a range of soil types and climates. They estimate that use of catch crops on half the area of land used for forage crops in Canterbury and Southland could create a net return from feed production of around \$150 million per year and reduce the environmental load of nitrogen by 6,000 tonnes. 

Learn more about catch crops from the Researchers at Plant and Food Research here or follow their Facebook page for catch crops.

References