Clover swards require a high level of management if they are to provide a viable alternative nitrogen source to chemical fertiliser, farmers were told during a recent industry webinar hosted by Germinal UK.
At an optimum 30% of sward composition, both red and white clovers can fix 150kg to 250kg/ha, or 120 to 200 units/acre of nitrogen, according to Adam Simper.
However, he stressed that these figures relate to clover plants grown under trial plots.
Replicating this on farm is much more challenging with variation in soil type, weather and management systems.
But on fertile soils with pH levels between 6.0 and 6.5, along with index 2 for phosphorous (P) and potash (K), clover can provide adequate nitrogen to reduce the reliance on chemical fertiliser.
“While clover can potentially fix up to 250kg/ha of nitrogen, it is more likely that 75kg to 100kg will be transferred to the surrounding crop,” Simper told the audience.
“But just because there is clover in the sward, don’t automatically assume that plant is fixing nitrogen. Farmers should dig a few soil cores in fields to check if clover is working for you.”
Clover fixes nitrogen using rhizobia, which are little nodules on the roots. When checking these nodules, if they have a reddish or pink colour, they are actively fixing nitrogen from the atmosphere.
But if swards are being fed too much nitrogen via chemical fertiliser, this can make the clover plant lazy and the plant’s ability to fix nitrogen is greatly reduced.”
Clover has different varieties with small-, medium-, and large-leaf types. Nitrogen-fixing is again influenced by the type of clover and its intended use. “Choosing the right leaf size is important for persistency. For intensive sheep systems, choose small leaf clover varieties,” said Simper.
“The smaller the leaf size, the lower the yield, but the greater stolon density. Stolons are how the plants tiller out. The more clover stolons present, the more the plant is suitable for intensive grazing by sheep.
“Large-leaf clovers have a lower stolon density and can be grazed out by overgrazing with sheep. Medium and large varieties are better suited to cattle grazing, with large varieties used for silage.”
Guidance on establishing clover swards was also outlined. Conventional ploughed reseeding from April to August is recommended, but this method can take a field out of use for six to 10 weeks until it is ready to graze.
Over-seeding is a viable alternative to ploughing, as the field comes back in to production earlier. But sowing times should be restricted to either April, July or August.
Over-seeding in May and June should be avoided as ryegrass will be at peak growth, providing too much competition for clover seedlings.
Regardless of seeding method, weeds need to be controlled first as there are limited “clover-safe” spays on the market. Clover seeds should be drilled once soil temperature exceeds 8°C.
“White clover is an extremely small seed that is often drilled far too deep. Roll soils before drilling to provide a firm seed bed, then roll afterwards to improve soil contact.
“Seed rate for white clover should be 1.5kg/acre. Red clover has a larger seed, with sowing rate increased to 3kg/acre. Drilling depth should be no deeper than 10mm.
“If stitching or over-seeding remove as much trash before seeding, then spread lime to encourage germination.
“Do not apply fertiliser to the seedbed after over-seeding as this will encourage existing grasses to outcompete seedlings.
“Put stock back on to the sward to graze down existing grasses, allowing light get down to the base of sward. Remove animals after five to seven days.
“It will take somewhere between six and 12 months for a newly established clover sward to start fixing high levels of nitrogen.”
The persistency of clover depends on how well a sward is managed. The plant has low growth rates in spring when compared to ryegrass.
This means chemical or organic nitrogen will be required in March and April to boost growth rates.
However, clover comes into its own from May onwards as temperature and day length increase. At this point, chemical nitrogen applications should cease.
Never turn hungry cattle into a clover sward as the risk of bloat is high
Rotational grazing was recommended for clover management, with a strong emphasis on avoiding over-grazing. Advice to farmers was also that clover swards should not be grazed in winter. “White clover needs sunlight, so don’t allow heavy grass covers to build. Graze white clover down to a residual of 4cm in summer, then remove stock and rest the sward.
“If cutting for silage, the same rules should apply. Do not cut the plant too close to the ground.
“With red clover, keep grazing residuals higher and do not cut swards below a height of 8cm. Red clover grows from a crown, rather than a stolon like white clover.
“If the crown is damaged by over-grazing or cutting too tight, the plant will die out. Allow clover swards to flower at least once per season, as this will help build energy reserves in the plant roots.
“Wilt red clover silage for 24 to 48 hours, but do not ted the sward out.
“Tedding and raking will cause the clover leaf to shatter. Use a mower with rubber conditioner and leave in the swathe to wilt.”
There are some downsides of grazing clover swards. Livestock are at an increased risk of developing bloat if not managed correctly.
Red clover in particular contains phyto-oestrogens, which can prevent breeding ewes from ovulating.
“Never turn hungry cattle into a clover sward as the risk of bloat is high. Providing some fibre if necessary and put bloat oil in the water.
“Once cattle are settled on the swards, the bloat risk is lowered.
“With red clover, keep breeding ewes off these swards for at least six weeks either side of the breeding season to avoid problems with fertility.
“Red clover is also more susceptible to disease such as sclerotinia, which rots the crown of the plant.
“Therefore, when planning out a rotation, do not replace old red clover swards with a new red clover reseed as the pest and disease levels will build up in soils.
“Use a break crop for a five- to seven-year period in between each red clover sward. White clover can be used as a break crop,” concluded Simper.