Red Clover

• Growth habit of red clover
• Disease break
• Longevity of red clover
• Making silage
• Environmental benefits

Summary

• High yields of multi-cut silage are possible from red clover swards without input of artificial nitrogen fertilizer.
• Red clover is a relatively short-term ley maintaining high levels of production for three-to-six years.
• Red clover has poor ensiling characteristics, with much lower water-soluble carbohydrate (sugars) and somewhat higher buffering capacity than perennial ryegrass.
• These ensiling limitations can be overcome by a 48-hour wilt or by using a suitable preservative to produce silages with high intake characteristics.

Introduction

Red clover in a mixture with perennial ryegrass and white clover can produce very high yields of multi-cut silage without artificial nitrogen (N) fertilizer. Red clover has enormous capacity to ‘fix’ atmospheric N into plant-available N in the soil supplying the equivalent of around 300 kg per ha per year.

High N fertilizer prices is stimulating an interest in using red clover for multi-cut silage, particularly on out-blocks of land. Very high yields are possible. Annual yields of 15 tonne (t) DM per ha have been recorded at Teagasc Grange and Solohead under zero-N fertilizer input. It is a relatively short-term ley maintaining high levels of production typically for three-to-six years. The red clover tends to die out of the sward over time. The high costs of reseeding and risks associated with reseeding explains why it has not been widely used and there is relatively little experience of it on Irish farms.

Red clover is typically grown on mixed arable and livestock farms in the UK and Denmark where it is a fertility-building part of an arable rotation, while also producing cheap feed for livestock. Lucerne/alfalfa fills a similar role in regions with warmer and drier conditions during the summer. Red clover is more productive in regions with damper and temperate climates and is highly-productive under Irish conditions.

Growth habit of red clover

Red clover produces a number of erect shoots that grow from the crown of a taproot (somewhat similar to a dock plant; Figure 1). It has a poor capacity to spread out in the sward or replace shoots that are damaged by machinery or disease, which accounts for its relatively short lifespan in swards. In contrast, white clover produces stolons that grow along the surface of the soil in much the same way that ivy grows up a wall. This stoloniferous growth habit accounts for the greater persistence of white clover in swards compared with red clover. Red clover swards should be sown as an arable crop. At Teagasc Solohead, after burning off with glyphosate, the ground is prepared with one or two ‘runs’ of a disc harrow followed by two runs of a power harrow. The seed is sown during the second run of the power harrow.

Figure 1. The growth habit of red clover

Disease break

The decline in the use of red clover in the UK since the 1960s was caused by greater availability of cheap N fertilizer during the 1970s and by diseases; stem eelworm and Sclerotinia fungus. Stem eelworm is a disease of many arable crops and causes stunting of red clover. Sclerotinia is also a disease of other leguminous arable crops such as beans and causes ‘clover rot’ in red clover. The risks of these diseases in Ireland are very low at present. These diseases can be controlled by a four-year break; for example by growing red clover in rotation with maize.

Alternatively, where red clover is grown with perennial ryegrass and white clover the crop can remain productive even after the red clover has died out of the sward because the red clover is progressively replaced by white clover, which becomes dominant in the sward over time. This creates a disease-break between stands of red clover while extending the interval between reseeding. It is important to plan for a disease break to avoid these diseases building up in the soil, particularly for out-blocks used continuously for silage production.

Longevity of red clover

Cost-effectiveness depends on longevity of the crop. At Teagasc Grange between 2002 and 2007, very high yields were recorded over each of six years. An annual yield of 15.9 t per ha was recorded in 2007, the final year of the study, although the red clover content of the sward had declined from 75% in 2002 to 44% in 2007. The saving in fertilizer N over six years was €1,800 per ha at 2019-2021 prices or 2.5 times that at 2022 prices.

Choice of red clover cultivar is important. We do not have a recommended list for red clover in Ireland. Fearga, AberClaret, AberChianti, Milvus and Lemmon are red clover varieties that have performed well on UK recommended lists, although seed of some of these cultivars is in short supply at present. Companion ryegrasses (perennial or/and hybrid) with heading dates in late May are ideal.

Seed mix

A typical seed mixture contains 4 kg red clover along with 8 or 9 kg ryegrass and 1 kg white clover. A large-leaved white clover cultivar should be included because it is more persistent and will replace the red clover as the red clover dies out of the sward. Bearing in mind that red clover seeds are three to four times the size of white clover seeds; 1 kg of white clover seed contains more-or-less the same number of seeds as 4 kg red clover seed. Hence, while it might seem that there is much less white clover in the seed mixture, you can end up with the same number of red and white clover plants in a newly-established sward.

Dock control

Dock control during reseeding is very important. Post-emergence herbicide control of docks can give very effective control that persists for the lifetime of the sward (10 years) and is vastly more cost-effective and eco-efficient than trying to control docks in older swards. The availability of suitable post-emergence herbicides was restricted during 2021. It is anticipated that they will be available under derogation in 2022.

Other measures to promote longevity 

Other key management practices for promoting longevity is zero N fertilizer input, maintaining adequate potassium (K) and phosphorus (P) fertilization, avoiding damage by machinery and animal hooves and adequate lime application. Nitrogen fixation is a biological process and therefore soil pH in the range of 6.5 to 7.0 is optimum.

Red clover herbage harvested for silage contains around 25 kg of K and 3 kg of P per t DM. Harvesting 15 t DM per ha per year removes a huge amount of K in particular and this needs to be replaced by slurry or artificial fertilizer. To avoid luxury uptake of K and subsequent problems with milk fever in cows, K fertilization should take place in advance of each crop. For example, if you plan to harvest a first-cut crop of 5 t DM per ha in mid-to-late May, this requires 125 kg/ ha of K and 15 kg/ha of P during March or early-April. It is best to hold off spreading slurry until ground conditions are such that avoids any damage to the crop. A light grazing in the spring and in the late autumn is possible although this can shorten the longevity of the red clover content of the sward.

Making silage

The optimum harvest date for first-cut is around 20 May and subsequent harvests at 6-to-8 week intervals. Red clover has poor ensiling characteristics, with much lower water-soluble carbohydrate (sugars) and somewhat higher buffering capacity than perennial ryegrass. These limitations can be overcome by a 48-hour wilt and therefore the weather conditions at harvest are an overriding consideration. A good wilt can be achieved by spreading out the sward immediately after cutting and rowing in after 48 hours or so.

Experience at Teagasc Solohead, along with evidence from elsewhere, is that red clover makes reasonably good quality silage with slightly lower digestibility and crude protein than fertilized ryegrass swards (Table 1). High volumes of cheap (in terms of N fertilizer input) silage are produced with very high intake characteristics, which is a redeeming characteristic of red clover silage. High intake compensates for somewhat poorer nutritional characteristics and cattle perform well on red clover silage once it is well-preserved. Where a wilt is not possible it is necessary to use a suitable preservative at harvest, such as molasses.

Target harvest dates are 15 to 20 May (yielding approximately 5-to-6 t DM per ha), early to mid-July (4-to-4.5 t DM per ha) and late August or early September (3-to-3.5 t DM per ha). Large quantities of herbage (>2.5 t DM per ha) can build up during September and October and it is important to remove this before the winter. Otherwise there can be large losses of herbage due to senescence and heavy covers can damage the clover content over the winter. The approach used at Teagasc Solohead is to zero-graze under good ground conditions during late October and November leaving swards ‘cleaned out’ to the butt over the winter.

Table 1. Analyses of first-cut red clover silage at Teagasc Solohead in 2021 

Environmental benefits

Red clover swards under multi-cut silage have low environmental footprints compared with alternatives. Greenhouse gas and ammonia emissions are low due to zero input of N fertilizer. Although red clover can fix huge amounts of N (equivalent to 300 kg/ha) this N is almost entirely taken up by the crop, is carried to the silage pit and ultimately ends up in the slurry tank. Recycling of this slurry back to the red clover swards using low emissions application techniques results in low nitrate losses to water.

Red clover is potentially useful where N fertilizer is constrained by environmental or financial considerations. Longevity is key to competitiveness and time will tell how well red clover swards persist on commercial farms.

James Humphreys, Dan Barrett and William Burchill 
Teagasc, Moorepark Animal & Grassland Research and Innovation Centre, Fermoy, Co. Cork