Clovers in sustainable agriculture
- Reduce reliance on fertiliser nitrogen (150 - 250 kgN/ha/annum)
- Enhance summer grazing productivity
- Homegrown protein source (25-30% crude protein)
- Increase forage intake and enhance nutrient supply
- Improve soil structure
- Fertility building
- Attract pollinators
Understanding white clover
White clover is a perennial herbage legume. The key to its survival and production potential is the stolon (multi-branched creeping stem), as this provides the sites for new leaves, roots and flowers. The stolon is a store for carbohydrates and proteins and provides the ability for the plant to over-winter and regenerate in the spring.
Varieties of clover vary in their leaf and stolon characteristics, and it is these often quite marked differences that help to determine agronomic performance. The latest breeding development within white clover has introduced a variety with both stoloniferous and rhizomatous root structures, to aid grazing tolerance, drought tolerance and winter hardiness.
White clover is nearly always grown with a companion grass, and most typically with ryegrass, with the type of ryegrass being dependent upon the primary use of the sward.
Developments in white clover breeding have increased the versatility of its use and the longevity of white clover within swards, with greater nitrogen tolerance being a key feature in more intensive systems. Sustainable systems incorporating white clover range from long term pastures for continuous sheep grazing (using small-medium leaf white clover varieties such as AberDance) through to medium term pastures for rotational sheep or cattle grazing (modern medium leaf varieties such as AberLasting and AberNormous).
Key benefits of white clover
Rhizobium bacteria, which exist symbiotically within protuberances or ‘nodules’ on clover roots, fix nitrogen from the air into a form that can be utilised by the plant; this process is called nitrogen fixation. This nitrogen becomes available for companion grasses and/or subsequent crops as it is released following plant decay or from the dung and urine of grazing livestock that have grazed white clover.
The average nitrogen fixation from white clover in New Zealand is 185 kg N/ha/year, but has the potential to be upwards of 250 kg N/ha/year, depending on factors such as farm system, soil pH and nutrients available for white clover growth.
The optimal white clover percentage for animal productivity benefits is 20 to 30 percent, and around 30 percent for dry matter yield performance.
Response per unit of feed intake is greater for white clover than it is for grass. This higher nutritive value is due to a lower proportion of structural fibre, higher protein content, and more of certain key minerals than grass.
Unlike ryegrass, white clover has the added advantage of retaining high digestibility throughout the season, as there is continual renewal of leaves and petioles and relatively little stem development.
White clover root systems improve soil structure and can help to overcome problems of soil compaction.
Studies at IBERS have demonstrated improved soil structure resulting from a greater white clover component in the sward:
- White clover has been shown to significantly decrease the density of soils and increase porosity
- Fertiliser recovery from soils with improved structure due to white clover was shown to increase from less than 50% to over 75%
- General movement of nutrients was improved, with the result that more grass was produced
The grass-clover balance
The proportion of clover in a grass/clover sward is commonly visually over-estimated, typically by twofold. The images below should be used as a guide to achieving the optimum balance as described later.
If clover does become dominant (i.e. when very little grass is visible), then it can out-compete the grass component and unbalance the sward. This may lead to increased weed ingression and greater vulnerability of the clover to winter damage. This is most common in the second year of a pasture, and a more desirable grass/clover balance will be seen in the third year and beyond.
Maintaining an optimum dry matter balance of 30% white clover to 70% grass as an average across the season is the key to grass/clover sward management, as this is expected to provide best exploitation of the clover’s nutritional and nitrogen fixing attributes alongside high yielding grass.
In the past, white clover growth patterns and the nature of the interaction with grass have tended to cause significant seasonal variation of clover content in swards – from as little as 5% in the spring up to 60% in summer – but clover breeding work at IBERS is producing varieties that are more compatible with modern ryegrasses and have more even seasonal growth curves.
Bloat is the excessive build-up of gas (carbon dioxide and methane) in the rumen resulting in distress and possible death due to the exertion of pressure on the animal’s diaphragm, heart and lungs. Some legumes, including white clover, present an increased risk of bloat (due to the rapid breakdown of protein in the rumen) if the correct management is not applied.
Effective management procedures to minimise or eliminate the risks of bloat in livestock grazing clover-dense swards should include:
- Limiting access to swards when stock is first introduced
- Avoiding turnout of hungry stock
- Feeding high dry matter forage such as hay/straw prior to turnout
- Offering hay/straw at intervals (e.g. to dairy cows at milking times)
- Feeding an anti-bloat feed additive
- Monitoring livestock
Forging an effective combination
The benefits of combining grass and clover in swards for both grazing and conservation are long established and well proven.
The most important benefits can be summarised as:
- Improved forage quality and feed value due to a boost in digestibility, intake potential, and protein and mineral content of the sward
- Reduced reliance on fertiliser due to nitrogen fixation
- ‘Soil structuring’ by white clover root systems that can help to overcome problems of soil compaction
Accumulated experience and scientific evidence indicate that the optimum balance is achieved with a clover content of 30 – 35% of the total annual dry matter yield of the sward.
Breeding for compatibility
A compatible grass/clover mixture is one with a clover content that is sufficiently large to optimise the nutritional and nitrogen fixing attributes of the clover when growing with a high yielding companion grass.
Grass and clover varieties differ in their aggressiveness towards each other due to their abilities to compete for nutrients, water and light. At IBERS, breeding for general compatibility is high on the agenda and varieties are routinely tested for this attribute.
White clover developments
Plant breeders at IBERS have been focused on a range of resource-efficient criteria in addition to more conventional breeding objectives.
New varieties are emerging from the IBERS white clover breeding programme that are better able to cope with potential climate change scenarios. Improved drought tolerance has been introduced into the breeding programme through the development of hybrids between white clover and Caucasian clover (Trifolium ambiguum), a drought tolerant, rhizomatous species. These new varieties are white clover-like in appearance and combine the agronomic performance of white clover with the drought tolerance of Caucasian clover. A further potential advantage of these hybrids is better persistence of large leaved varieties under different management systems.
AberLasting is the first white clover variety to be developed at IBERS with rhizomatous root characteristics, transferred from the more drought-tolerant Causasian clover species.
Phosphorous use efficiency
In response to concerns over future availability of phosphorous and the environmental impact it has in rivers and watercourses, the IBERS white clover breeding programme is developing varieties that can grow in low-phosphorous soils and to make better use of the nutrients available. New varieties from the programme, now in trial, are offering the possibility of reducing fertilizer P inputs to swards.
Nitrogen use efficiency
One of the main attractions of white clover is its high protein content, which can be greater than 25% of dry matter. Where conditions are optimum for overall yield, the protein content can exceed the nutritional requirements of grazing ruminants and are lost through their excreta. This can make a significant contribution to overall agricultural losses.