Grazing strategies for reducing contaminant losses in surface runoff from winter forage crop fields located in hill country and grazed by sheep

ABSTRACT

A paired catchment study approach was used to quantify fluxes of Nitrogen (N), Phosphorus (P), sediment and Escherichia coli (E. coli) in surface runoff from fields where sheep were wintered on swede (Brassica napus) and kale (Brassica oleracea) crops. The effectiveness of a strategic grazing approach that protected critical source areas (CSAs) was examined to determine if these fluxes could be reduced. Averaged over two years, estimated fluxes of N, P and sediment in surface runoff recorded in winter and early spring (June-September) were up to 0.64, 0.22 and 51 kg ha⁻¹, respectively, and up to 2 × 10¹¹ MPN ha⁻¹ for E. coli. With the exception of E. coli, these fluxes were low compared to those reported in other studies and largely attributed to the low amounts of surface runoff recorded. This, in turn, can be explained by relatively benign weather conditions recorded during the study period and the relatively light grazing pressures imposed by sheep. Compared to standard grazing practice, protection of CSAs reduced N, P, sediment, and E. coli fluxes by 38%, 48%, 55% and 63%, respectively. These reductions can largely be attributed to lower concentrations of contaminants in surface runoff when the CSA was protected.

KEYWORDS:

• Wintering sheep
• forage crop
• surface runoff
• contaminant losses
• critical source area

Acknowledgements

Grateful thanks are given to Ahmed Elnaggar and Mitchell Donovan for help in setting up the field experiment and Ronald Alderton for giving access to his farm. Alasdair Noble provided statistical advice and analysis. This research was supported by the Sustainable Farming Fund, administered by the New Zealand Ministry for Primary Industries (Contract ref: A25626). Special thanks to Craig Simpson from Watershed Solutions and Nicole Foote from the New Zealand Landcare Trust for coordination of the project and the development and distribution of high-quality extension material to the wider community. Finally, a big thank you to the Steering Group members particularly Beef + Lamb New Zealand, Ballance AgriNutrients, Otago Regional Council, Environment Southland, and Greater Wellington Regional Council. The manuscript benefitted from the constructive comments from two anonymous reviewers, whose feedback are gratefully acknowledged.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

This work was supported by Ministry for Primary Industries [Grant Number A25626].