Beef and Lamb NZ are encouraging farmers to consider planting catch crops this spring to capture and utilise residual soil nitrogen left in the wake of winter grazing.
Key learnings from the Forages for Reduced Nitrate Leaching and Catch Crops to Reduce Nitrate Leaching research projects led to the establishment of a resource called ‘Catch Crops for Reduced Nitrate Leaching’ which is a guide that provides insight into key lessons from the two research projects mentioned above.
What is a catch crop?
A catch crop refers to a cover crop that is sown for the primary goal of reducing nitrate leaching (Horrocks et al. 2021). While cover crops provide a range of benefits, a catch crop is distinct in its objective to primarily reduce nitrate leaching as the main environmental benefit of crop establishment.
What are the benefits of catch crops?
Catch crops provide the environmental benefit of taking up excess nitrogen in soils as a way to reduce nitrate leaching to the freshwater environment during high risk periods, such as after intensive winter grazing. Horrocks et al. (2021) state that catch crops sown after winter grazing can take up significant quantities of nitrogen and reduce nitrogen leaching losses by up to 50%. If soils are left fallow for several months following winter grazing, the risk of nitrate leaching is high due to urine patches deposited during grazing of winter crop (Horrocks et al. 2021). The wetter conditions of winter and early spring, combined with low evapotranspiration, results in increased soil drainage which provides the mechanism for nitrate leaching. Nutrient losses typically range from 50 – 180 kg N/ha, which creates both environmental risk and a farm expense (Horrocks et al. 2021).
Although catch crops provide environmental benefit, they also provide other benefits to a farm system through increasing annual dry matter production, reducing imported supplement in dairy systems, as well as aiding soil restoration (Horrocks et al. 2021). Other benefits of catch crops include weed suppression, ground cover, organic matter returns, and soil protection.
While nitrate leaching is a naturally occurring process, being able to reduce the amount of nitrate lose from fertiliser and urine patches is beneficial for environmental sustainability but is also beneficial for utilising nitrogen resources on farm to establish crops that benefit your farming enterprise overall (Horrocks et al. 2021). Leached nutrients are wasted nutrients so being able to utilise this excess nitrogen in your farming system can provide a win-win for environment and farm productivity.
What are the most effective catch crops to use after winter grazing?
Horrocks et al. (2021) state that 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
For this reason, cereals such as oats, ryecorn, triticale, wheat or barley make good catch crop options after winter grazing and are proven to be more effective than grass species. What cereal species are best suited to your farming system will depend on climate, soil and management factors.
Important catch crop management considerations
Horrock et al. (2021) state a range of considerations that should be considered when establishing a catch crop. These are a direct translation of the advice given on page 11 of the ‘Catch Crops for Reduced Nitrate Leaching’ Guidelines.
Timing
Catch crops should be sown as soon as practicable after forage crop grazing, regardless of the time of year.
Although they are very slow to get going in winter, the luxury uptake of N in early growth stages (high N%) means that they are still having an environmental benefit.
In addition, when soil temperatures begin to rise to more optimum levels in spring, the early sown crops are better positioned to more efficiently harness energy from the sun and convert that into biomass and correspondingly, N removal.
Ground conditions
Always aim to sow catch crops with as little prior cultivation as possible, while still achieving good soil-to-seed contact.
Excessive cultivation could further compromise soil structure and enhance N mineralisation, which can increase the risk of nitrate leaching.
Direct drilling is the preferred method of establishment, but under particularly poor ground conditions (wet/pugged), some form of cultivation may be necessary.
Winter Activity
Cereals such as oats, ryecorn, triticale and wheat all have good winter activity.
Barley can also be used but is slower to establish and less tolerant of wet and cold conditions.
Ryecorn and triticale can have marginally better environmental performance than oats and wheat in colder climates such as Southland.
Forage oats, however, is the most consistent performer in terms of yield, N uptake and quality. Sowing oats with Italian ryegrass is also a good option.
Sowing rates
Target high populations (300 plants/m2) with cereals to minimise the time it takes for crops to reach canopy closure.
Generally, aim to sow seed at about 3-4 cm depth, to ensure good soil-to-seed contact and reduce the risk of bird damage.
In particularly wet environments, shallower seed depth placement (<2cm) and a rougher seedbed is likely required to prevent severe surface capping and allow seedlings to emerge.
Nutrient needs of the catch crop
Nitrogen and other nutrients are typically present in sufficient amounts to establish a catch crop after winter forage grazing.
Depending on the season, paddock history, and the size of the previous fodder crops, signs of N deficiency can occur from mid-spring onwards. To ensure both crop quality and quantity targets are achieved, the crop should be monitored for these signs, with a judicious application of N fertiliser (40-50 kg N/ha) likely beneficial.
Nutrient needs of the next crop
Catch crops may deplete the soil of essential nutrients. Nitrate reduction is the objective, but other nutrients will be taken up by the crops.
Soil testing to determine what needs to be applied to the subsequent crop is recommended, particularly on light soils following a wet winter/spring period.
Weed control
Suppression of weeds is important to get the maximum benefit from sowing a catch crop.
Using high seed rate s(300 plants/ m2), and where necessary herbicides, are important to ensure faster canopy closure to suppress weeds.
Termination
Timing of harvest affects metabolisable energy and yield.
Green-chop stage (booting; growth stage 45-52) is when crop quantity and quality components are most balanced. Delaying harvest beyond green-chop silage maturity results in an initial decline in ME amongst other quality components (causing a temporary reduction in feed value) which then increases again as starch is stored during grain filling.
The best time for whole-crop silage harvest is when moisture content is within the range of 35-42% DM, which may occur before the completion of grain fill (growth stage 87).
Irrigation and water management
Catch crops reduce leaching by using water and reducing drainage.
In dryland areas or areas where soil moisture restrictions may arise, water availability for the following crops will need to be considered and managed carefully.
Where can I find more information on catch crops?
Beef and Lambs ‘Scene and Herd’ Podcast has a great episode titled ‘Catch Crops – a way to reduce N leaching after winter crops’. This episode is with Brendon Malcolm of Plant & Food Research, and he discusses how catch crops work, the issues to consider and how to get the best results.
You can listen to it via podcast platforms or here.
To read more of the Guidelines for Catch Crops for Reduced Nitrate Leaching, as has been referred to throughouth this blog, you can find it here.