Irrigation and increased numbers of cattle on the rolling topography could potentially create soil compaction and water runoff issues, but farmers and AgResearch scientists are working together to create best-management practices for a region undergoing major change.
AgResearch Invermay scientist Dave Houlbrooke told a recent field day that because of those changes, farmers had less room for error.
"Farmers have to be more accurate."
For the last five years AgResearch has been running trials on the Ludemann farm at Windsor, where plots have been exposed to different stocking rates, irrigation treatment, cropping and fertiliser.
In tandem with that was a recent study on North Otago downlands soil quality which found the change in soil properties such as macroporosity, a measure of compaction, occurred in newly irrigated land within two years.
AgResearch Invermay scientist Jim Paton said 60 sites were monitored over five years.
Twenty of those sites were dryland farms, a mixture of sheep, beef and some cropping, with the balance undergoing land use change following irrigation.
He said the study found irrigated land had lower macroporosity than dryland but that there was no significant differences in soil properties between established irrigated land and new irrigated land after the first two years.
But comparing soil properties with dryland, he said it suggested damage occurred early.
"Soil compaction is caused by the compression of large pore space under animal grazing. The difference between the dryland and irrigated surveys reflects the higher soil moisture content in irrigated soil, increasing the susceptibility to treading damage during grazing."
Another study assessed the effects of irrigation and type of grazing animal on soil quality and production for pasture and cropping.
The study involved 32 plots and compared irrigated versus dryland, sheep versus cattle and pasture versus forage crop, measured in terms of soil quality, plant production and water water use efficiency.
Dr Houlbrooke said the plots were grazed according to feed availability and irrigated when soil moisture level fell below 50% of available water capacity.
Three years of results show that cattle grazing and irrigation was decreasing soil physical quality as measured by soil macroporosity.
The level of compaction was greater for forage crops grazed by cattle, reflecting the intensive grazing at potentially wet times of the year.
This was combined with agronomic and management differences, such as long fallow periods and decreased organic matter inputs.
Decreased soil macroporosity was associated with a reduction in soil function, especially drainage, root penetration, water and air transmission along with lower plant yield and surface runoff.
Dr Houlbrooke said water use efficiency was consistent within seasons and between irrigated and dryland plots, at between 18kg and 31kg of dry matter per millimetre of water applied.
Richard McDowell said the North Otago soils and landscape were vastly different from Canterbury, which had borne the brunt of criticism over groundwater pollution.
He said the issue for North Otago was surface water runoff, which could lead to algae growth in streams, ponds and rivers.
Phosphorous content in runoff was accentuated by heavy stocking, especially cattle, irrigation and feeding winter forage.
Dr McDowell said data from the first year of the trial showed losses were greater from blocks grazed by cattle, compared to sheep, and from irrigated plots, rather the dryland.
Losses from winter forage crops in July and August were comparable to losses for the entire year from dairy grazed pasture.
Irrigation was a major contributor to nutrient loss and Dr McDowell said it was important to ensure only required amounts of water were applied.
Soil compaction and stock manure were also factors in phosphorous surface runoff from forage crops, and Dr McDowell said scientists had trialled restricting grazing to four hours and increasing the soil's phosphorous absorption capacity by spraying aluminium sulphate after grazing.
Results showed those steps significantly decreased phosphorous runoff, but the use of aluminium sulphate was untested.
The final trial looked at managing flat land, comparing cattle and sheep grazing, mown and pasture fallow, perennial ryegrass and tall fescue, irrigation and dry land and use of nitrogen.
AgResearch Lincoln scientist Todd White said the benefits of irrigation were highlighted in the trial, with pasture yields increasing about 80%, but pasture grazed by sheep benefitted more than that grazed by cattle or the mown trail.
Adding nitrogen was most successful on the irrigated and mown blocks.
There were fewer earthworms under cattle grazing, compared to sheep, mown plots and areas left fallow, while irrigation increased worm numbers, especially in blocks grazed by sheep or left fallow.