By Molly Shaw

White and red clovers are choice pasture species for good reason—they’re highly productive and persist well in grazed situations.  But no one species is best at everything, and there is a persuasive argument for considering other species for the qualities they can bring to a pasture mix.

If a farmer was designing the perfect legume pasture species, the wish list might look something like this:  fast to establish; tolerant of heavy soils, acidic soils, and droughty soils; weed suppressing; delicious to stock; high in nitrogen but resistant to nitrogen leaching; good regrowth after grazing; maybe even providing food for pollinators and other beneficial insects.  Oh, and highly productive too—did we mention that?  And cheap seed?

Well now, we can’t be the best at everything, and it turns out that that’s true with pasture species as well.  Starting in 2008, researchers funded by the UK Department for Environment, Food and Rural Affairs as well as industry partners conducted an extensive study that ran for three years on five research farms and dozens of farm fields across the UK.  They chose 12 species of legumes and 4 pasture grasses to grow alone and in mixes, analyzing their traits and providing almost 200 pages of Project Results [1].  Twenty-two legumes were reviewed and the 12 most promising were chosen for the study.  If a species could not take two of three factors (cold, mowing, or autumn sowing), it was excluded.  Where species were very similar in habit, only one was included to avoid redundancy.   The four grasses in the study were perennial ryegrass, Italian ryegrass, meadow fescue, and timothy.  Highlights from their research are reported in the following summary.

Twelve legumes were grown separately and in a mixture of all the species together (the “all-species mix”).  Dozens of measurements were taken on the plantings and species were ranked in their performance on 7 criteria.  Table 1 summarizes the conclusions.

Table 1:  The 12 legumes were scored according to 7 criteria, then ranked.  In this chart a higher number means better performance.  Colours have been assigned to number ranges to make the conclusions easier to see.  Numbers 1-4 are red (generally poor), numbers 5-8 are orange (mid-range performance), and numbers 9-12 are green (best performance).  For the average ranks, 7.9 and above is green, since the top five species scored quite similarly.

Criteria explained:

• The Early Development rank was obtained from the average of four variables: speed of emergence, proportion of sown seed emerging, seedling biomass at 60 days, and seedling relative growth rate.  All species were sown in the spring.  Species that emerge slowly tended to grow their bulk later on.  Large-seeded species grew fast early on, but because they tended to be tall and their growing points high up, they re-grew more slowly after grazing.
• Variables going into the Productivity rank were plant height in spring, plant regrowth after mowing (low cut to simulate grazing), crop cover (ground cover), crop biomass measured multiple times, and persistence in the All-Species-Mix after three years.  Crop cover, measured in the early spring, reflects how well the species grew over winter.  Grasses were strong in this measure, as were red and white clover, followed by black medic and Lucerne.  The biomass measurement showed the same strong legume species, but among the grasses the perennial ryegrass and Italian ryegrass pulled ahead of meadow fescue and timothy.  The All-Species-Mix was at least as good as any top species alone, both in crop cover (ground cover) and in biomass.  Regrowth after a low cut (simulated grazing) was better in plants with low regrowth points.  White clover is the prime example, since it grows creeping along the ground with horizontal stems, and had the best regrowth after grazing.  Regrowth was also better in plants that were larger before the cut, which would have had more root resources to mobilize for regrowth—red clover is an example of this trait, as well as the All-Species-Mixes.  Lucerne and black medic had relatively lower grazing tolerance than one would expect based on their biomass.
• Not surprisingly, highly productive species (good ground cover, high biomass) had fewer weeds in the ley.  Weed suppression was scored by assessing weed cover at the end of summer, in the spring, and the change in weediness over two years.
• Flowering was an attempt to quantify value to pollinators.  Plants were scored by how early they flowered and how long they continued flowering.
• The Pre-crop Value, Resistance to decomposition, and Performance of the following crop were assessed in light of legumes being used by farms to supply fertility (mainly nitrogen) for the next crop, and to condition the soil.  A ley with lots of biomass just before incorporation, especially biomass high in nitrogen, is good for the performance of the following crop (in this case a grain).  When a green manure is ploughed down there is a flush of readily-leachable nitrogen released, often before the following crop has enough roots to utilize it.  Resistance to decomposition is important to reduce leaching of nitrogen, an issue that is becoming more and more critical in New Zealand.  Slower N release (plants higher in lignin and polyphenols, also measured as higher C:N ratio) puts the available N more in line with later crop use, and so reduces leaching….but also makes the plants less digestible to grazers.  Birdsfoot trefoil stood out as being high in N-rich biomass yet slower to decompose, and therefore good for the next crop and for the environment.
• The data was hard to generalize beyond that—there was more variation in N accumulation and release between sites than between trial species.  Theoretically, grain yields in the following crop should be highest in legumes and mixtures that (1) grow a lot of biomass, and (2) have enough C (lignin, polyphenols) that their N release is delayed, matching the needs of the following crop better, a situation that was observed at one of the trial locations but not all of them.

There were five trial sites in the study.  At all sites grass tended to become more dominant over the years, more so in farms that were drier and not grazed.  In grazed situations there were more legumes remaining in the mix, although the “balance of power” wasn’t the same everywhere.  At one site, Alsike clover, white clover and red clover dominated the legumes; at another it was birdsfoot trefoil and black medic; Lucerne, red clover and white clover at a third site.  Among the legumes, black medic did better in grazed situations than without the grazing, while Lucerne was the opposite.  This means it’s important to trial these things on your own farm to see what does best under your management scheme and climate.

Trade-offs:  Ecologists talk about a concept called “functional complementarity,” which means that one species can’t be best at everything.  Plants have to make trade-offs.

For instance, the ones that were best at early development and productivity decomposed really fast after plow-down, leading to more potential for N to leach.  Lignin is woody tissue and polyphenols are tannins, and apparently when a plant makes those it is at the expense of fast growth.

Another pattern the researchers noted was that species that emerged and grew slowly (those with small seeds) tended to increase their biomass over the next two years, whereas those that emerged quickly (large seeds) tended to be smaller than their counterparts over the time, exemplified particularly well with crimson clover (an annual species).  Fast starters dominated early, then fast re-growers took over after mowing/grazing.  Lucerne was an interesting exception, emerging quickly and increasing in biomass over the two years, but for the most part quick starters petered off over time.  Small seeded species also tended to cope with grazing better.

The researchers used the hundreds of measurements they took on their trial sites with grass and legume species (grown separately and in one giant mix) and used those numbers to plug into an ecological model.  The purpose of this exercise was to guess how mixes of two, three or four species would perform, and find the ones that complemented each other optimally.  Not surprisingly, the best “designer mixes” were mostly the combinations of the highest-performing species, with secondary performers thrown in at times to fulfil a lacking ecological niche.  “Stronger as a team” could be the motto.

Why try mixtures?

They are resilient.  In comparison with the monocultures, the all-species mix showed increased ground cover and above-ground biomass, and consequently reduced weed biomass.  The biomass advantage of the all-species mix was more pronounced on poorer soils.  When under stress, a mix with plants of different strengths and weaknesses (heights, root structures) outperformed the more simple leys of species that are high producers under optimal conditions.

Species profiles:  Farm managers will differ in what qualities they consider the most important for their individual farms.  The following notes will highlight where each of the species shines, and fuller profiles are available at the end of the full report document.

Alsike clover:  Neither the highest nor the lowest performer in any of the criteria, but alsike clover is more tolerant of acidic soils and heavy soils than the others.  Note that it can be toxic to horses!  And its bitter taste makes it suitable as a mix component but not on its own.

Black medic:  In contrast to the general view that black medic has low yields, in this study it shown in terms of biomass production, better even than white clover.  It self-seeds and can germinate at any time from immature seeds in the field, and it has a long flowering period (good for beneficial insects).  It is bitter so livestock don’t prefer to graze it.

Birdsfoot trefoil: The long tap root on this species gives it drought tolerance, even more so than Lucerne.  It is also more tolerant of poor drainage and flooding than Lucerne, white clover, or red clover.  It has moderate yields and is relatively weak at establishing, but provides good forage value as a non-bloating species (and livestock like it), as well as slower N-release after plow-down.

Crimson clover: This winter annual species can be used as a summer annual in cooler climates.  It is suitable for mixing with forage brassicas and is adaptable to a wide range of soil pH.  It has big seeds, giving it the strongest early development score in this study.  It has good first year biomass production and should be grazed before flowering to avoid the hairy flowers that cause livestock digestion problems, though it peters out in subsequent years.

Large Birdsfoot trefoil:  This species showed slow early growth and low productivity, and consequently it was ranked near the bottom in this study.  It is reported to have good tolerance of acidic soils.

Meadow pea:  This species was one of the poorest performers in the study, and its profile doesn’t offer much to recommend it.

Red clover:  This is a well appreciated and highly productive legume species suitable for a wide range of environmental conditions.  Its sizeable tap root gives it drought tolerance.  It outperforms white clover in first year growth, comparable to Lucerne.  It doesn’t persist as long as white clover and doesn’t take low grazing as well, being used more for two-year leys than perennial swards.  Grazing red clover carries a risk of bloat, and can cause problems with fertility in ewes due to a chemical compound with an estrogenic effect.

White sweet clover:  This is a biennial species with moderate productivity, good drought tolerance because of a large tap root, and tolerance of alkaline soils.  It can be grazed, but there are several caveats: stock have to get used to its bitter taste, it carries a bloat risk, and coumarin in the leaves can cause “bleeding disease” in stock (hence the development of low coumarin cultivars).

Sainfoin:  This is a perennial legume that establishes slowly, but can produce good biomass later on (in years 2 and 3) and is drought-tolerant due to a strong tap root.  It is not super competitive, so is difficult to include in mixtures with grasses, and may be why it performed relatively poorly in this study.  It is highly palatable to stock (they eat more of it than red clover or Lucerne) and non-bloating.  In addition, the aphenolic compounds it contains reduces parasitic worms in lambs.

White clover:  This classic legume is popular for good reasons.  It was hard to beat in terms of biomass production and grazing tolerance in ideal (moist, cool) conditions with high soil pH and good soil drainage, but black medic did manage higher overall biomass production in this trial.  Its large N contribution is highly leachable because it has a low C:N ratio.  Because it spreads across the ground by stolons it has outstanding tolerance to grazing.

Winter vetch:  Vicia sativa is an annual species, also called common vetch, spring vetch, garden vetch and tare, but distinct from hairy vetch.  It didn’t perform well in this study.  It has exceptionally low grazing tolerance, so much so that when grown on organic vegetable farms as a green manure, it is commonly killed by a low mowing at flowering.

References

1.    Döring, T.F., Baddeley, J.A., Brown, R., Collins, R., Crowley, O., Cuttle, S., Howlett, S.A., Jones, H.E., McCalman, H., Measures, M., Pearce, B.D., Pearce, H., Roderick, S., Stobart, R., Storkey, J., Tilston, E.L., Topp, K., Watson, C., Winkler, L.R., and Wolfe, M.S., Using legume-based mixtures to enhance the nitrogen use efficiency and economic viability of cropping systems. Project Report No. 513. 2013, Agriculture and Horticulture Development Board, HGCA Division.