Saturday, December 2, 2023

Winter forage grazing a washout for soil

Neal Wallace
Erosion risk grows when damaged soil is left exposed after grazing.
Chief scientist with Terra Informatics Dr Mitchell Donovan says there is ‘literally no debate’ that riparian planting reduces the amount of soil entering waterways.
Reading Time: 3 minutes

New research has found that a disproportionate loss of soil comes from winter forage grazing, concluding that where they can, farmers should consider alternative wintering systems.

Research for Our Land and Water looked at the national impact on soil loss from winter forage grazing and found that while the practice accounts for less than 1% of land area, it is responsible for 15% of soil loss within some catchments.

Chief scientist with Terra Informatics Dr Mitchell Donovan says this loss of soil – or, as he prefers to call it, natural capital – has environmental and financial costs, and may also affect future yields from those paddocks.

A geomorphologist, Donovan studies earth-surface processes and landform evolution. He says rather than trying to reduce erosion through reactive management such as detainment bunds and sediment traps, farmers who can, may want to consider preventing erosion by shifting away from wintering practices altogether.

However, “I’m not a farmer and I recognise this is not necessarily feasible for all farmers given their location and ability to grow grass in winter”, he says.

The problem is caused by several issues: a mixture of bare ground cover, damaged and pugged soil following grazing, as well as slope and rainfall.

Donovan says the erosion risk is particularly concerning when damaged soil is left exposed after forage crop grazing.

Buffers and riparian planting can prevent soil loss entering waterways, but Donovan says this won’t stop soil from being lost or eroded within a grazed paddock.

Riparian planting sieves nutrients and other elements, limiting the amount entering waterways.

“There is literally no debate that this reduces soil from entering waterways,” he says.

The research calculated soil loss as a function of seasonally variable rainfall erosion, ground cover, soil properties, grazing practices and terrain information within two catchments across New Zealand.

The modelling captured long-term rainfall and land-use conditions, but not year-to-year variability.

It also found that soil loss from various land use in two river catchments, the Clutha in Otago and Waikato, was not uniform.

“These analyses generally indicate that broad and sweeping statements about specific land use classes are generally unfounded unless specific context and conditions are met.

“Exceptions to this are forage crop paddocks, landslide scars, and to a lesser degree, exotic forestry land, which had relatively high surficial erosion rates.”

Researchers concluded that while individual forage crops are not a significant contributor to overall sediment yields within a catchment due to the relatively small area they cover, conditions of these paddocks are such that even if 1% of land is in forage crops, they could account for 10-20% of surface erosion across the catchment.

“Thus, when considered as a whole, winter-forage crop paddocks represent a ‘low-hanging fruit’ when considering options for reducing sediment loads in catchments.”

The study looked at the impact of specific land use in the two catchments.

Exotic forestry in the Waikato catchment accounts for 17% of land cover and contributes 22% of soil loss.

While natural or native forests occupy over 20% of the land in the Waikato catchment, it contributes only 15% of soil loss.

It showed proportionately less impact than winter grazing, but it suggests that despite high ground cover for part of a 30-year cycle, forestry land may contribute substantially to long-term sediment loads when it is exposed following harvest.

In the Clutha catchment, pastoral grasslands occupy 44% of erodible land area, but contribute only 29% of surface erosion.

“This likely reflects a combination of lower rainfall erosivity, lower soil erodibility and relatively lower grazing intensity compared to the Waikato catchment,” Donovan says.

The Clutha catchment has relatively high erosion contributions (53%) from ungrazed natural grasslands, which occupy only 35% of the catchment’s erodible land area.

This land class is often located at high elevations on steep lands with highly erodible soils and is often left natural due to the high rates of erosion.

But the catchment does have relatively high contributions from winter forage cropping as well as landslides. 

“These analyses generally indicate that broad and sweeping statements about specific land use classes are generally unfounded unless specific context and conditions are met,” Donovan says.

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