Anticipated changes to European and UK agricultural grasslands in the next 20 years include more frequent and severe droughts, changes to seasonality and being impacted by new weeds.
Although drought reduces grass growth, choice of species in the sward can help maintain growth for longer, while grazing management can support fast regrowth after drought. This blog explores the impact of a changing climate on grassland and how to increase grassland resilience to drought.
Climate change impact on agricultural grasslands
Grassland productivity is highly dependent on the weather. At first glance models exploring the impact of increasing CO2 concentrations in the atmosphere on plant growth indicate some positive results. Plants can be more efficient with more CO2 available to them, which could result in higher yields. Increasing temperatures could also mean a longer growing season, with spring starting earlier and autumn lasting longer. The opportunity for more productivity also means there will be higher potential for grassland to act as a carbon sink, as with more production, more photosynthesis will be occurring, providing a greater carbon sink effect.
However, the increases in yields, lengthening of growing season and greater carbon sink effect can only work up until a certain point, as when it gets really warm the grassland will become a carbon source.
Grassland is also sensitive to the amount of precipitation. The climatic models cannot accurately predict how the precipitation patterns will shift. There could be less precipitation, or very unevenly distributed precipitation, which could override all the yield boosting effects. Large parts of Europe can also expect more frequent and more severe drought. The inter annual variability will also be strong – one year will not be like another.
The impact of drought on grassland
A grassland sward adjusts to drought by first attempting to avoid dehydration, then reducing growth rates and finally going into dormancy.
Grasses initially resist drought by losing less water to the atmosphere, but this only works up until a certain point. When it becomes really dry, plants have to tolerate dehydration by reducing their water needs. This means lower growth, so they produce less biomass that needs water. If drought conditions continue, they then reduce their biomass via the senescence of leaves, so they lose tissue that requires water.
The final step is dormancy, which means the death of the plant except organs close to the root, which allows fast regrowth after the drought conditions pass.
Summer drought means low or no growth during the summer season, but there can be a longer vegetation period in the autumn. This can be a benefit to farms grazing livestock, but a challenge if making silage, as it is difficult to make good silage in short days.
There are big differences in species and how they deal with drought. Perennial ryegrass does not perform well in dry conditions. Summer droughts can be anticipated to cause decreasing dry matter yields and gaps in sward as some grass dies, leading to unwanted species infilling.
Increasing grassland resilience to drought
There are several approaches which can increase grassland resilience to drought. These include:
- Retaining soil organic matter content in soil can help maintain grass growth for longer in dry conditions
- Avoiding soil compactions can help increase water infiltration
- Adjusting and adapting fertilisation and utilisation dependent on the site and soil
- Adapting the plant community for the site to provide drought resistance.
Plant species with deep rooting systems have an advantage when it is dry, as they are better able to exploit the water available in the soil. However, if there are only deep rooting plants this might lead to water stress as they all competing for water, so mixed species with different rooting depths is optimal.
Grass species for drought resilience
Festuca arundinacea (tall fescue) will maximise water uptake and also performs well in wet conditions, making it a good option for a future with more variability in the amount of precipitation from year to year. There are soft leaf varieties which are better suited for feeding cattle than non-improved festuca, which is not particularly palatable.
However, tall fescue is not easy to establish and requires a high soil temperature and careful first utilisation. Perennial ryegrass will push it out of the sward as it grows much faster in spring, making it difficult to establish in existing ryegrass swards.
Dactylis glomerata (cocksfoot) is also quite deep rooting, but not as deep as festuca. It reduces water loss by closing stomata. The feed quality is not as good as perennial ryegrass, but there are systems that perform well with cattle on these grasses.
Diversification of grassland communities
Inclusion of many species of grasses, legumes and herbs in a sward will improve resilience to changing climatic conditions. These will use different layers of soil for extracting resources, so will work under different conditions.
For example, if there’s ryegrass in the sward it will grow well in spring, but if it gets dry in the summer cocksfoot or tall fescue will grow much better. Some species are not persistent against intense utilisation, so will need reseeding after a while.
The benefit of diverse swards is you get overyielding, where the mixture can exceed the best performing single species.
Pasture management in a changing climate
Extensively grazing livestock on a sward which is kept short can be problematic in drought conditions as plants have shallow roots and are not getting a rest period. One solution is to transition to a rotational grazing approach – which was first defined by André Voisin in 1950s. The principles of this include:
- Recovery period principle – a sward needs to rest before the next utilisation, harvest maximum herbage growth rate
- Occupation principle – short occupation with no regrowth during utilisation. If cows on for longer than three days grass will start to regrow, not good for the plant if grazed again after regrowth begins
- Maximum performance principle – animal demand and pasture performance match. For example, the high yielding dairy cows get access to the best pastures
- Regular performance principle – short occupation to achieve low variations in feed quality. If cows are utilising the same space for two weeks, they are getting the best quality forage for just the first few days of that period
Avoiding a plant being grazed after regrowth has begun is particularly important. This is because during the first leaf stage of grass growth the plant is not photosynthesising, so all energy for growth is coming from the base of the plant. If the plant is grazed at this point, then the plant’s carbon storage is depleted, which slows down regrowth.
Grazing systems in droughts
In drought the feed quality will decrease, as will the quantity as plants lose leaves to conserve water. There will likely be feed shortages, but as grasses store carbohydrates in the plant base, it remains important to leave enough residual when cutting or grazing to ensure a fast regrowth after the drought.
It is important to stick to the rest period. One option is to use deferred grazing, so some fields from spring are not grazed, leaving a lower quality food source available to use during a summer drought. Mob grazing systems can also be used.
As growth slows, there will be limited nitrogen uptake during drought. Also, nitrogen fixation only works in moderate drought and stops in long or severe droughts. However, nitrogen availability will be high after the drought and plants with past water stress will overcompensate.
After drought
Root biomass is much higher when plants experience drought, which supports fast regrowth. But there will always be gaps which fill with weeds if they are left bare without reseeding.
Unfortunately, reseeding can be difficult in terms of timing. Germination requires presence of light, sufficiently warm temperature and moisture, so there may not be enough time to successfully reseed in autumn, in which case it will have to wait until spring.
In summary, there are several approaches that can be taken to increase grassland resilience to drought. These include diversification of species in the sward and grazing management to support rapid regrowth. The information in this blog was inspired by the Climate Farm Demo webinar presented by Killian Obermeyer, from Landwirtschaftliches Zentrum fur Rinderhatung in Germany. To learn more, watch the full webinar here.
