Robo-hoof and rainmaker test soil and water impacts

July 2006
Farmers need to manage their stock in a way that looks after soil and water quality, protecting productivity while minimising environmental impact.

Pugging, caused by too many animals grazing on wet ground, compromises the physical quality of soil, with impacts on pasture growth. A 1% increase in large soil pores gives a 1.8% increase in pasture production in spring.

Surface run-off from land with grazing animals can contaminate waterways with faecal bacteria, nutrients such as phosphorus and nitrogen which can trigger the growth of algae and aquatic weeds, and sediment, which impairs fish spawning and causes flooding.

Water quality must be maintained for all users and to maintain New Zealand exports reputation as clean and green.

At Invermay, a rain machine and an artificial cattle hoof are being used to simulate a range of grazing pressures under rainfall, to measure effects on soil and surface run-off. The aim is to produce a toolbox of best management practices that farmers can use to minimise contamination of waterways while optimising soil quality and maintaining productivity.

Dr Richard McDowell, soil and water impacts expert

Soil compaction and treading damage decreases subsequent pasture production potential and increases the risk of surface run-off of contaminants. Main influences on the extent of damage are soil moisture status, grazing intensity (animal numbers per ha), grazing duration and livestock loading (animal type and age).

Cattle have a greater compaction effect on soil quality than sheep and deer. Soil compaction and treading damage increases the movement of sediment and nutrients from soil (phosphorus and nitrogen), and nutrients and faecal bacteria from dung into waterways.

Invermay scientists have developed an artificial hoof, applying equivalent force to the soil as a two-year-old Friesian. The aim is;

1) To separate out the effects of treading (which affects the soil), from the effects of fertiliser and dung losses (surface run-off detrimentally affects water quality).

2) To measure the vulnerability and response of different soils to treading damage.

To measure the effects on water quality we cut turf from paddocks stocked at different rates and grazed for different lengths of times and place them beneath a rainfall simulator which applies 20-130mm of rain per hour, says Dr McDowell. The soils response and surface run-off is compared.

The artificial hoof can also be used in the field, towed behind a truck.

The greater the stocking rate, the greater the treading damage and - due to compaction and pugging - the greater the surface runoff and quantity of nutrients lost.

At an Invermay Open Day in May, visitors were shown the rainfall simulator in action and invited to rank eight samples from best to worst soil and water quality based on grazing time and intensity, the condition of pasture and the concentration of E. coli, and sediment in the captured surface run-off. About 30% got it right with women out-performing men.

Dr David Houlbrooke, soil compaction expert

AgResearch has published a booklet focused on density and duration of grazing and the impact on pasture production in the following month, when a soil is susceptible to treading damage. One hundred cows grazed on a hectare for eight hours, for example, might result in a pasture penalty of 30% versus 15% for three and a half hours.

A similar chart is now being developed for water quality, i.e. a certain number of cattle for a period of time causes a percentage decrease in the quality of the water coming off.

New Zealand soils range from quite resilient to pugging (volcanic ash soils that cover much of the Waikato) to the extremely vulnerable Pallic soils (common in Otago, Southland, Canterbury, Manawatu and Wairarapa). These are structurally poor and vulnerable to degrading of structure, especially under impact by heavy machinery or stock.

Good farmers have a reasonable feel of the relationship between overgrazing with cattle and compromised pasture production due to a loss in soil physical quality, says Dr Houlbrooke.

Methods to correct soil compaction include natural recovery processes, mechanical aeration (sub-soiling) and cultivation with re-grassing. However, such methods only address an existing problem meaning there will be a pasture yield penalty.

Soil compaction and treading damage can be avoided or diminished during wet periods by the strategic use of feed or stand-off pads (particularly during spring) or by preferentially grazing free draining or drier paddocks. Otherwise, soil compaction prone soils should be grazed with low animal density for a short duration on a pasture with higher residual cover.

Managing treading damage on dairy and beef farms in New Zealand.

Betteridge, K., J. Drewry, A. Mackay and P. Singleton (2003).

Palmerston North: Land and Environmental Management,

AgResearch Ltd. 34 p.

Contact: Dr David Houlbrooke

Soil scientist

03 4899045