Pathogen Pathways – Best management practices
5. Conclusions
This report describes the key pathways, arising from the Pathogen Transmission Routes Research Program (and related studies), by which faecal microbes are transferred from farm animals to waterways. Additionally, Best Management Practice (BMPs) guidelines are provided with which to reduce the faecal contamination of waterbodies.
Key findings
Faecal contamination of freshwaters can arise through the deposition of faeces by grazing animals directly into waterways. This direct deposition can occur when dairy cows cross a stream to or from the milking shed and, through sporadic incursions by grazing cattle into waterways. Bridging streams intersected by farm raceways is an appropriate mitigation measure for herd crossings, whilst fencing stream banks will prevent sporadic incursions.
Riparian buffer strips not only prevent cattle access to waterways, they also entrap microbes (and other pollutants) washed downslope in surface runoff, and provide shade and improve aquatic habitat. Guidelines for optimal riparian buffer design with respect to the entrapment of faecal bacteria are provided in this report.
Water quality improvements can be realised from fencing stock out of ephemeral streams, wetlands, seeps, and riparian paddocks prone to saturation. During prolonged spells of wet weather, improved water quality may result through the relocation of stock from paddocks to feed or wintering pads, and herd homes.
Soil type is a key factor in the transfer of faecal microbes to waterways. The avoidance of, or a reduction in, grazing and irrigation upon poorly drained soils characterised by high bypass flow and the generation of surface runoff, are appropriate management practices, and are likely to lead to improvements in bacterial water quality.
In addition to the identification of appropriate soil type, timing, volume, location and technique are also key factors in the optimal irrigation of effluent. Ideally, irrigation should occur when the volume to be applied does not exceed the water storage capacity of the soil, with effluent being stored until such soil moisture conditions arise. This ‘deferred irrigation’ can markedly reduce pollutant transfer to waterways. As with effluent irrigation, the identification of appropriate soil type timing, volume, location and technique are also key factors in the optimal irrigation of water. Spray irrigation results in less bypass flow and hence less microbial contamination of groundwater than the border strip technique.
Some microbial transmission pathways and potential mitigation measures remain poorly understood and, in some cases, untested. These information gaps are outlined below and provide a focus for further research.
Further research needs
Cattle attraction to waterways and wetlands has been shown to be variable and may be dependent upon a range of factors (e.g., the size and depth of the waterbody, the accessibility, season, stream bed substrate). An improved understanding of cattle behaviour in this respect would help to prioritise the fencing of waterways and wetlands.
The provision of off-stream sources of water, shade and shelter for cattle, as an alternative to permanent fencing of streams, has not been tested in New Zealand. Studies that quantify the impact of these alternative resources upon water quality would therefore be of value.
Our current understanding of microbial entrapment within riparian buffer strips is limited and would be improved through further experimental studies encompassing a range of soils, slope angles and vegetation types. In particular, an improved understanding of those factors that influence microbial attachment to particulates would aid the evaluation of buffer strip effectiveness.
Buffer strip experiments to date, have focused upon the performance of grass buffers. However, unless grass buffers are to be mowed, periodic (light) grazing is required, preferably by sheep which do not damage soils as much as cattle and are not attracted to water. No information exists with which to evaluate the impact of periodic buffer grazing upon water quality. Furthermore, tree buffers are favoured over grass buffers for their shading and habitat benefits. Again, however, there is no information available with which to determine tree buffer performance with respect to microbial entrapment.
Experimental studies are required to evaluate the efficacy of measures with which to treat microbes in drains and ditches. Constructed wetlands, vegetated ditches, and farm ponds all have potential in this respect, but little or no information is available with which to evaluate them.
Little information is available with which to evaluate either the cost-effectiveness or social implications associated with the mitigation measures outlined in this report. Such an analysis is necessary to be able to fully evaluate the worth of each measure.
In addition to improving our understanding of the performance of specific BMPs, two broader issues relating to faecal contamination of waterways are of significant importance:
- The role of water in explaining reported rates of human infection in New Zealand, in particular, campylobacteriosis and cryptosporidiosis, remains unclear. Direct exposure to water through freshwater recreation has been predicted to account for only a small proportion of reported cases of campylobacteriosis (McBride et al. 2002). However, the indirect impact of water and background levels of pathogens in the pastoral landscape, upon human infection is unknown. Waterways, for example, may play an important role as a vector for pathogens in the environment, causing re-infection of animals in farms downstream, and maintaining a general background level of pathogens on pastoral land. Identifying causes of human infection therefore requires that water as a pathogen vector or indirect cause needs to be better understood. Furthermore, the tracing of pathogen strains that cause infection and re-infection of grazing cattle is of high priority and would help to explain the persistence of some pathogens in the pastoral landscape.
- Limited overseas evidence is available to link cattle productivity with the microbial quality of cattle drinking water. Given the potential significance of this issue, New Zealand based studies are desirable.
Contact for Enquiries
Phil Journeaux
Manager
North Island Regions
Sector Performance Policy
MAF Policy
Private Bag 3123
Hamilton
NEW ZEALAND
Phone: +64 7 957 8313
Fax: +64 7 957 8315
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