TRC study into stock rates called good news

Intensive dairy farming at double the current average stocking rate does not damage soil ecosystems, concludes a five-year study in Taranaki, the first research of its kind done in New Zealand.

The results fill a critical knowledge gap and may challenge popular beliefs that intensive pastoral grazing causes gradual ecological decline.

The work was funded by the Taranaki Regional Council and conducted by the council's terrestrial ecologist, Shay Dean, between 2002 and 2007 at the DairyNZ research farm at Whareroa, near Hawera.

Reporting the results at a meeting on Tuesday, the TRC's environment quality director, Gary Bedford, said the soil "held up markedly well. The biodiversity was actually best in some of the most intensively stocked pastures."

Councillor Michael Joyce said it was "refreshing to see a good news story rather than the bad news stories usually associated with intensive dairying".

Over the past 40 years, the number of dairy cows in Taranaki has increased from 350,000 to 480,000 and the average stocking rate from 1.43 to 2.8 per hectare.

Miss Dean said the only other studies in this country on high stocking rate impact had focused on sheep in natural and semi-natural grasslands and the effect on invertebrates.

TheWhareroa ecosystem work took advantage of an opportunity to use five 1000 square-metre paddocks that had been set up for another study. One was fallow land; another was pasture "cut and carried" with no stock grazing; the other three were stocked at three, four and five cows per hectare. The grazed plots were fertilised each October with 700kg of 30 per cent potassic superphosphate.

Each of the four treatment plots was rotationally grazed or mown at 3-4 week intervals. The stocked plots were grazed by between six and 50 cows for 24 hours. As indicators of soil and pasture ecosystem health, the abundance and diversity of plants, earthworms, insects, spiders, mites and nematodes among the plots were compared. No changes were found.

Measurements were made to identify possible variations in soil chemistry, density and porosity. Grazed paddocks had lower soil porosity compared to the fallow and mowed paddocks. This suggested that over five years, higher stocking rates may cause small increases in bare ground and small decreases in clover. Various statistical tools were also applied to ascertain trends of changes in ecological parameters.

"In terms of pastoral and soil ecosystem and health and biodiversity, this study has found that higher stocking rates also do not have significant detrimental effects on the diversity or abundance of either surface dwelling (including insects and spiders) or soil dwelling (nematodes, earthworms, mites and springtails) creatures. Indeed, in some cases, the higher stocking rates were found to have resulted in increased diversity and abundance of some invertebrate groups," Miss Dean's report says.

"These results suggest that in the short term at least, stocking rates of up to five cows per hectare do not pose a significant threat to biodiversity in Taranaki agricultural lands. It is surmised that to some extent at least, this finding, which is very encouraging in terms of the region's ecological state, reflects the robust nature of the region's soils."

Dr Bedford said the study is significant as a stocktake of the health and biodiversity of pastoral soil in Taranaki, for "state of the environment" monitoring purposes and "for its indication that monoculture land uses such as intensive pastoral grazing need not be synonymous with general ecological decline, as popular perception might otherwise have it."

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