The benefits of legume crops for pasture renewal has been a long-known but perhaps underutilised practice. Agronomist Elizabeth Grace reviews those benefits.
are a broad group of nitrogen-fixing plants including clovers, lucerne, peas and beans. The roles of legumes range from permanent pasture species through to summer cash crops.
All legumes contain bacteria known as rhizobia which colonise root nodules and convert atmospheric nitrogen into the plant-available nitrogen source, ammonia. This nitrogen can be used by the plant and is added to the soil upon the death of the legume. In many instances this reduces or mitigates the need for nitrogen fertiliser application in the rotation.
Utilising a legume crop as part of a pasture renewal scheme is advantageous as it conditions the soil well, ready for new grass seed, without artificial N inputs. Post-harvest, the legume vine – particularly pea straw, has a high nitrogen content. This renders it a quality green manure.
When considering gross margins and costs, the pros and cons of a legume crop must be considered at face value and with the secondary benefits in mind. These include improved yields on the following crop (such as new ryegrass swards or winter wheat crops) due to: improved soil structure, break crop weed and pest control, and increased soil organic matter.
Maintaining clover
Red and white clover are important species – especially in dairy grazing as they are a valuable source of protein for stock. White clover can be grouped into three size categories and it can be valuable to plant a mix of types to aid with persistence. The large varieties will contribute the largest amount of fixed nitrogen per hectare, however are less persistent and less tolerant of dry soils than the intermediate and small types.
If white clover is drilled with ryegrass seed in the same row it can often be outcompeted due to the faster growth and taller growth habit of the ryegrass.
A 50:50 mix of seed types intermediate and large can work well for dairy systems. The optimal pH for clovers sits about 6. Broadcasting clover seed can improve clover percentages in a sward compared to drilling with ryegrass. White clover is usually outcompeted for light for rotational grazing is preferable to ensure the clover is not shaded out.
Red clover has excellent summer growth but very poor winter growth and hence it is usually managed as an annual crop. It has much better growth rates than white clover and a taproot to aid in persistence. It can achieve 12-16 tonnes of drymatter/ha when grown with ryegrass. Including clover can increases the protein content of the sward by about 20%.
Peas as a restorative crop
Growing a summer pea crop (seed or process peas) is beneficial for both soil health and structure. Plant-available N from a typical yielding pea crop is not often quantified in national research however studies of similar systems in Canada have found an average contribution of 25kg N/ha from a pea crop. This does not factor in above-ground vegetative matter from the pea seed themselves which are typically harvested hence removed from the system.
A legume crop often has a shorter growing window than other crop alternatives such as maize which means the paddock can be returned to grass sooner, allowing a jump-start for new grass.
To help determine which paddocks will benefit most from a legume crop and subsequent new grass, the pasture renewal checklist from Pasture Renewal Charitable Trust’s website is a good starting point.
Collective benefits
Whichever the legume crop you decide to incorporate into your rotation some key messages remain the same:
- Optimise soils and growing conditions to favour the legume, especially when grown in a mix
- Do not apply excessive nitrogen fertiliser as legumes can lose their competitive advantage and yield potential can decrease.
- Straw or trash can be used as a high-protein forage for stock or used in a silage mix such as with oats.
- No exogenous N inputs can assist in complying with council nutrient-budgeting requirements.
- Fine, firm seed preparation and no soil pans will ensure legume roots can grow well, optimising N fixation.
- Legumes can act as an ideal break crop to control alternative weed species which can benefit paddocks prior to re-grassing.
