With the continual deluge of rain, farmers have questioned will prolonged waterlogging kill my pastures?

Already there are enquiries of what to sow to replace lucerne stands that haven’t survived and the expectation is that cocksfoot pastures may have also died.

The following information  has been taken from a recently published MLA factsheet SFS wrote called, ‘How do I know if my perennial grasses need rescuing?‘ or check out the YouTube Video 

• Plants start to show signs of waterlogging stress after 14 days with varying reductions in shoot/root dry matter (cocksfoot up to 56% and tall fescue 38%) and tiller development after 21 days of continual exposure (Mui et al, 2021). Perennial ryegrass, phalaris and tall fescue are likely to recover from periods of waterlogging lasting up to 14 days by employing various tolerance mechanisms, whereas cocksfoot will not. Cocksfoot is suited to well drained soils.
• What happens is when the soil is waterlogged and pore spaces fill with water, there is no oxygen for the grasses to access to respire. Respiration is the process plants undertake to form carbohydrates and so growth ceases. In addition, gases like carbon dioxide and ethylene increase and are harmful to roots. Plants vary in their ability to survive these conditions.
• Associated pugging through grazing waterlogged or wet soils can also be damaging to pastures. Research in southwest Victoria indicated that a single pugging event with cattle can reduce ryegrass tiller density by as much as 52% through crushing and bruising the plants (Zie et al, 2001)
• When soil is saturated, it loses its strength or ability to remain cohesive. Increasing water content causes greater separation of clay particles and softens the cements that hold the clay particles together (Agriculture Victoria, 2020). The soil loses its strength to support livestock weight. As a result, heavy animals, particularly cattle can pug a pasture.

It is likely that grasses that are relatively waterlogging tolerant like phalaris, perennial ryegrass and tall fescue have been severely stressed with waterlogging events in last three weeks.

It is therefore worth checking pastures. Sown pasture species might have failed in paddocks or in low lying areas and plans can be made for oversowing next autumn.

Paddocks where stock were contained may have become overgrazed and these pastures might need a couple of weeks rest to recover carbohydrate reserves.

If you suspect the perennial ryegrass population has declined, it might be worth letting remaining plants seed to encourage perennial ryegrass recruitment next autumn.

See the MLA factsheet SFS developed on How do I optimise seedling recruitment?

References
Mui NT, Zhou M, Parsons D and Smith, RW (2021) Aerenchyma formation in adventitious roots of tall fescue and cocksfoot under waterlogged condition. Agronomy 11, 2487.

Agriculture Victoria (2020). Soil strength. In Victoria resources online. Online (verified March 2022)
http://vro.agriculture.vic.gov.au/dpi/vro/vrosite.nsf/pages/soilhealth_soil_strength

NY ZN, Ward GN and Michael AT (2001). Long-term effects of pugging on soil and pasture in SW Victoria In: Proceedings of the 10th Australian Agronomy Conference, January 2001, Hobart, Tasmania.

Photo: Severe case of waterlogged pasture with pugging damage
Credit: L Miller, SFS