BY: JOANNA GRIGG
The old way of formulating sowing rates for mixed pasture has been challenged by a PhD student’s research into synergies of pasture mixes.
Thinzar Soe Myint, Lincoln University, has been head down in a paddock of pasture plots taking careful measurements of ryegrass, white clover and plantain. These were grown both on their own and in different combinations with varying nitrogen and irrigation applications.
Her work has proven that resulting pasture yield composition depends on the proportions of species in the mix, not the amount of the mixture sown (i.e. total sowing rate).
In this case it’s all about proportions.
“Species proportion should be emphasised more than overall seed abundance because even low seed rate can give high drymatter yield if we combine the species perfectly and proportionally,” Thinzar said.
This grassroots research may not involve glamorous equipment nor be splashed across the tech mags but it has huge implications for pastoral farming that relies on the bread-and-butter grass, herb and legume mix to fuel animal production.
It is just one part of wider work by Lincoln University into pasture mixes and persistence (five experiments), which is ongoing and has covered at least 20 years of work. All papers are available on the NZ Grasslands Association website (www.grasslands.org.nz).
Thinzar said the way a pasture mix is normally designed is based on information about the performance of the species or cultivars when grown by themselves as monoculture plots. The National Forage Variety Trial is an example. But evaluating them in mixtures should have a more important role, she believes.
“Previous trials of mixes rely on the agronomist’s best guess of sowing rates or proportions of component species. This approach provides little or no predictive ability.”
Her experiment showed that optimal proportions of species in the seed mix, to maximise annual dry matter yield in the first year after sowing, were about 35% perennial ryegrass, 25% white clover and 40% plantain, based on seed count. These proportions were equivalent to about 8 kg/ha of perennial ryegrass, 3.5 kg/ha white clover and 7.5 kg/ha of plantain cultivars, used at the lowest total sowing rate tested of 1000 seeds/sq m.
There was no difference in dry matter yield for high sowing rates (2000 seeds/sq m) so low sowing rates are fine. Most NZ sowing recommendations are 25-30 kg/ha with grass, so the 19-20 kg/ha recommendation is lower, Thinzar said.
As well as the lower overall sowing rate the experiment results suggest that ryegrass and plantain be sown at 7-8kg each. Normally farmers sow ryegrass as the dominant component and the plantain is added at only about 1-3kg/ha.
The experiment was repeated at two overall sowing rates of 1000 and 2000 seeds/sq m to confirm that the species population eventually sorts itself out through the process of size/density compensation in plant communities.
“This gives us confidence that the mixing effects are going to be consistent across a range of total sowing rates currently recommended in New Zealand.”
The maximum yield was 28.7 tonnes for the first year after sowing. This experiment will run for three years so a better idea of persistence will emerge.
“This must be emphasised as the botanical composition of even simple pasture mixtures changes considerably after sowing from one year to the next.”
The maximum yield was achieved with nitrogen (N) fertiliser (275kg N/ha) as it boosted ryegrass and plantain. A parallel objective of this experiment was to test the effect that soil nitrogen status has on the dry matter yield of individual species in a pasture mix, and on the pairwise and more complex interactions between species in mixed swards.
“We did this by applying it in the first 14 months of the experiment to the high N treatment, and no N fertiliser as a low N treatment, and then seeing how the plants sorted themselves out.”
Rather than testing rates of N the experiment was looking at the change in the soil N environment to a level that would affect yield and composition. In the first 14 months, the ratio of ryegrass and plantain stayed the same (i.e. the same relative balance) but white clover reduced. This impacted on the synergy growth of white clover and ryegrass, and white clover and plantain. Less white clover didn’t impact on the synergy of ryegrass and plantain however.
Thinzar and her supervisor Dr Alistair Black see the bigger-picture implications of this work.
“This result supports the widely accepted management recommendation to use strategic N fertiliser to encourage pasture growth in the cool season when white clover growth and N fixation is low,” Black said.
There will always be the dilemma between high sowing rates to maximise yield and weed suppression but also reducing ryegrass content of the seed mix to promote slow-establishing species like white clover and timothy.
Thinking of sowing rate in terms of seeds per square metre gives an impression of what the intraspecies and interspecies competition between plants might be like. He would like the wider seed industry to think about the approach used in this experiment, which gives more informed and unbiased evaluations. Such a trial design involves a regression analysis approach to measure the relationship between yield (or any other trait) and the proportions of the mixture ingredients.
“Formulating pasture mixtures based on a plant community-level approach gives more reliable information of not only performance of plant community but also that of each constituent.”
As a general message for farmers chasing yield, Black said the experiments support choosing pasture mixes over monocultures.
“The other message is you don’t need to worry about sowing rate because, even with low sowing rates, the perfect combination of species mixes with right proportions will give high dry matter yield.”
He adds that while finding synergy between species is the goal, too many species can reduce yield.
He recommends choosing forage species and cultivars that match the local environment, the grazing system, and feed requirements of stock on the farm.
“We already know a lot about what species to grow where: for example, perennial ryegrass and white clover for mild temperature and moisture environments, cocksfoot and sub clover for drier niche areas.”
He implores farmers not to lose sight of these fundamental principles of designing pasture seed mixtures and good pasture management in general.
“And we should remember that New Zealand has a history of government funded research in grassland farming systems spanning over 70 years.”
“The danger is forgetting that much of it is still relevant today.”
Research paves new way to set sowing rates
The Research: Designing optimal seed mixes for temperate pastures.
Funding: Assisted by the Hill Country Pastures Programme on behalf of Beef + Lamb NZ, Ministry of Business, Innovation and Employment, Seed Force, and PGW Seeds.
Supervisors: Alistair Black and Derrick Moot, Lincoln University
PhD student: Thinzar Soe Myint is in her third year of experiments for her thesis. From 2012 to 2018 she was Assistant Lecturer in Yezin Agricultural University, Myanmar, and she plans to return there to study tropical pastures – something she describes as more challenging than temperate! A NZ Aid Scholarship provides financial support.
What she did: Sowed perennial ryegrass, plantain and white clover species on their own, in a binary mix, and three-way mix. The swards were harvested eight times (once cut and the rest grazed by sheep). Swards were irrigated November to January (240mm total) and Urea applied to some plots (50kg N x 2 then 25kg every grazing).
Findings: The interim results (year two of a three-year study) show the optimal mix for maximum dry matter in year one is 35% perennial ryegrass, 25% white clover and 40% plantain, based on seed count. This is equivalent to 8.3 kg perennial ryegrass, 3.6 kg white clover and 7.6 kg plantain (19.5 kg total seed) per hectare at the sowing rate of 1000 seeds/m2. This is lower than traditional recommendations of 25-30kg/ha and is a higher plantain:ryegrass ratio.
Synergies exist between species but must be tested in plot experiments to measure the relationship between yield and the proportions in the mixture.
There was no difference in dry matter yield for high sowing rates (2000 seeds/sq m versus 1000 seeds/sq m).
