Research identifies top sward species for minerals

A study at Rothamsted Research North Wyke, Devon, has identified which species to include in a sward mix to improve levels of trace minerals for livestock.

Post-doctoral scientist Tegan Darch wanted to see if farmers could reduce their reliance on mineral supplements and supply them instead from multispecies swards.

She found much of the existing information on minerals in pasture systems was anecdotal or piecemeal, or focused on a small number of species. Data on iodine and selenium, among the most deficient minerals in livestock diets, were particularly limited.

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Dr Darch was most interested in the potential for pasture grazed by beef cattle or sheep, whose diets would not necessarily include concentrates.

“Feedback from livestock experts is that they are seeing too many problems with mineral toxicity,” she says. “There are complex interactions between minerals, and supplying lots of one mineral is not necessarily the best thing to do.

“Too high a level of one mineral can reduce the absorption of another. For example, if iron, sulphur or molybdenum levels are too high, even if an animal is taking in enough copper, the copper can’t be absorbed.”

The experiment

Dr Darch investigated 21 species of grasses, legumes and forbs (herbs). They were grown as single-species stands in pots, rather than at field scale, to eliminate possible differences in management and climatic factors.

She used AHDB recommendations for trace element supplementation of beef cattle and sheep to compare plant species.

Findings

There were significant differences between botanical groups for many minerals tested:

• Forbs were highest in iodine and selenium
• Grasses were highest in manganese
• Legumes were highest in copper, cobalt, zinc and iron.

“If farmers are soil-testing and plant-testing, and they know they are deficient in certain minerals, they can use this information to decide between candidate species, and tailor their seed mixes to the minerals of most concern,” says Dr Darch.

Comparing species concentrations to livestock intakes, the research showed that the legumes bird’s-foot trefoil, yellow trefoil – also known as black medic – and alsike clover, as well as the herbs yarrow, chicory and ribwort plantain, were the best all-rounders.

“These species could be good for farmers who don’t know what minerals they are deficient in, or who are buying out of a catalogue,” she says.

Further research

Dr Darch is now conducting further experiments to see if the results are consistent across soil types.

“Although I expect that the absolute concentrations of micronutrients will vary across soil types, what I would like to see is whether the ‘best’ species are consistent, or whether there is a lot of variation,” she says.