By Molly Shaw
With many Canterbury irrigators toppled by wind this spring, even low-land farmers will get practice coping with droughty conditions—conditions that will likely become all too familiar in the next 3-4 decades as Canterbury weather patterns change along with global weather changes. Within our life-times, predictions are for NZ to see increased clouds from the west. This will mean more rain on the west coast along with strong nor’west winds and warmer temperatures on the east coast, making for drier pasture and crop fields.
Not all plants sit defenseless in the face of scorching winds. Many plants around the world have mechanisms to cope with dry soils while continuing to grow. White clover, our main pasture legume, just doesn’t happen to be one of these resilient individuals—though, of course, it has many other endearing features that make it the most popular pasture legume in NZ. It pairs with ryegrass, establishes relatively quickly, and is highly productive. It just doesn’t happen to be drought-tolerant, as dryland farmers know all too well.
White clover’s growth pattern leaves it susceptible to drought. It has a small tap root that dies out after about 18 months, and it spreads by above-ground stolons with short anchor roots. Under ideal moisture conditions the plant does great, but when it’s dry the stolons dry out and can eventually die. Sometimes there is enough life in the tap root to grow again after rain, but sometimes that doesn’t grow back either.
To tolerate dry soils, legumes and other plants can use a number of mechanisms. For example, they can grow long roots to reach deep water that is available even after the soil surface is dry, or they can adjust the chemistry of their roots and leaves to be able to suck water even from relatively dry soils. In the end, drought tolerant plants keep up productivity under dry conditions better, and regrow faster when rain does come. Other types of plants get even more creative when dealing with drought, from alternative photosynthetic pathways that allow them to grow at night, when it’s less droughty, to elaborate structures like plant hairs to reduce water loss from leaves.
Researchers at Lincoln University have explored three basic strategies to improve the drought-tolerance of our pasture legumes:
Replace white clover with another similar nitrogen-fixing species
Breed white clover varieties that tolerate drought better than our main varieties
Breed white clover hybrids with other legumes to get more drought-tolerant nitrogen fixers.
Rainer Hofmann, plant physiologist and Senior Lecturer in Plant Biology at Lincoln University, has spent a significant portion of his career working on this puzzle, and explains the current status of each of the options above.
Replace white clover with another similar nitrogen-fixing species: Strawberry clover has been one focus of the research program. While white clover grows a small taproot that dies out in a few months, leaving the plant to spread by above-ground stolons with shallow feeder roots, the tap root of strawberry clover is deeper and longer-lived, leading to higher productivity during dry conditions. The trade-off is that it establishes slower, 6-9 months instead of white clover’s 3-4 months. Since its early growth is slower it can’t be seeded with ryegrass, but is better paired with something less competitive like tall fescue. Dryland farmers may decide these trade-offs are worth it, considering the alternative is a dead pasture of white clover, but farmers with access to irrigation are unlikely to find this too attractive.
Breed white clover varieties that tolerate drought better than our main varieties: Kopu II, a productive white clover cultivar used extensively in NZ, has been crossed with Tienshan, a white clover variety found high up in the Chinese mountains and known for its drought-tolerance. Sure enough, the offspring are more drought-tolerant than Kopu II. The trade-off is that under ideal moisture conditions, the hybrids are only about 2/3 as productive as Kopu II. Under dry conditions, Kopu II’s productivity can drop precipitously to 20-30% of its ideal, while the hybrids only drop a couple percentage points from their maximum yield. This means that the hybrids have predictable yield which is higher than Kopu II in droughts, but lower during ideal moisture conditions. Breeders are currently working to stabilize the traits of the new hybrids and trial them all over NZ. It is predicted that one will be ready for commercial use in about 6 years.
Breed white clover hybrids with other legumes to get more drought-tolerant nitrogen fixers: In Rainer Hofmann’s view, this has been the most promising strategy so far. White clover has been crossed with another clover species (Trifolium uniflorum). Resulting plants still spread with stolons along the soil’s surface, but a larger tap root. Plants have a similar growth habit to white clover but more resistance to drought. Again, the trade-offs are slower establishment and lower yields under ideal growing conditions (about 65-75% the yield of Kopu II under ideal moisture conditions). AgResearch has further adopted this breeding project and expects to have commercially-viable seed available in 5 years.
Drought-proofing for today: Breeding projects and new plants for the future are all well and good, but what can farmers do this year for nitrogen fixation on dry pastures? Two tried-and-true practices may not sound exciting but they do work.
Lucerne is a nitrogen-fixing species with a huge tap root, hefty N-fixation, and good productivity—higher than that of white clover. The challenge is that Lucerne is so aggressive that it doesn’t pair well with the customary ryegrass used extensively in New Zealand. It can be planted with grasses (e.g. cocksfoot) but the grazing management will be different than a white clover/ryegrass mixture. It can be done though, and many farmers do it! The easiest (and more widespread) use of lucerne is as a monoculture for grazing and hay production. In these systems grazing animals get the extra fibre they need in their diet from pure grass swards or dry hay.
Shelter belts have fallen out of fashion in Canterbury lately, but they were planted before large scale irrigation was used, at a time when water conservation was more critical. The old shelter belts reduce water loss from pastures, but take a long time to grow. An alternative is miscanthus grass, a bunch grass species that takes only a year to establish, grows about 3 meters tall, and dies back to the ground in the winter. Planting just the northern and western edges of a pasture block protects about 2/3 of the block from the drying effects of scorching northwest winds during the summer months when this protection is the most critical, and center pivot irrigation can run right over the wind breaks without damaging either the equipment or the grass.
Trade-offs: Listing to the options for drought-proofing pastures, the “trade-off” theme emerges. If pastures could always get more than 80 cm of rain per year, evenly spaced and more abundant in the summer months, then our beloved white clover would be the best option. That ideal doesn’t come true for dryland farmers. For farmers with irrigation, it comes with a cost, and irrigating at current rates may not be feasible in the future. Taking measures to drought-proof pastures now is like paying for car insurance for a sixteen year old son driving a brand new Mustang. Yes, there is a real cost now, but it is likely to be worth it in the future.