Unrestricted dairy cattle grazing of a pastoral headwater wetland and its effect on water quality

•First study to directly quantify the grazing activity of cattle with access to a wetland.•The water quality effect of cattle grazing was monitored for 2 years.•Cows did not spend much time grazing this deep (1–2 m) wetland.•Wetland grazing only resulted in a small pollutant pulse on one occasion.•Cattle exclusion from such deep wetlands is unlikely to improve water quality.

Unrestricted cattle access to pastoral wetlands can result in increased pollutant loads from agricultural areas. We quantified the time a herd of dairy cattle spent grazing a pastoral headwater wetland and its associated impact on water quality. Over a two year period a herd of ∼220 dairy cattle rotationally grazed a paddock containing a permanently saturated pastoral wetland. Flow and turbidity were continuously monitored at a v-notch weir and baseflow and event samples were collected and analysed for TSS, Escherichia coli and various forms of N and P. Cattle were detected grazing the wetland paddock on 18 days by time-lapse cameras. Cattle only entered and grazed the saturated wetland area for 30–40% of the time and there were usually only a few animals in the wetland at one time. Cattle that did graze the wetland tended to remain near the edge. We attribute the low level of wetland grazing to the cattle recognizing the risk of entrapment in the deep (up to 2 m), boggy wetland soil. A measurable increase in pollutants (attributable solely to cattle generated disturbance) occurred only once. This occurred when a cow became entrapped in close proximity to the water quality monitoring location on a day when wetland flow was elevated. Exclusion of cattle from our study wetland by fencing is therefore unlikely to substantially improve downstream water quality. Further research is required to determine whether this is the case for similar wetlands in different environments.