THE BURDEN AND VALUE OF MANURE ON PASTURE-BASED DAIRY FARMS

Maintaining soil health is pivotal towards ensuring that the soil can function as a living ecosystem, keeping it in biological balance to ensure productive agriculture. Through the implementation of careful manure management, regenerative farming practices and sustainable cultivation, many dairy farmers contribute to the sequestration of carbon into soil, as well as replenishing soils with other nutrients which are essential for crop cultivation. Controlled manure application is used in South Africa towards the restoration of soils and has shown to reduce dependence on fertilizer inputs. Chemical fertilizers are widely considered to be of greater environmental detriment, with lower N, P and K application typically linked to better overall health of the environment.

Effluent management on dairy farms relies on the manure-based waste stream to be collected and stored in ponds before being spread onto lands or pastures using a variety of methods. It is essential that this should be carefully managed to prevent seepage and pollution of sub-surface water. Care must be taken to divert, collect and contain liquid effluent run-off from stalls and cow housing. Ground water and soil contamination with faecal coliforms, nitrates and salts can occur through leaching of run-off if not controlled properly. Therefore, effective manure management on a dairy farm is critical to using this waste stream in a sustainable manner.

To acquire more knowledge and develop guidelines, Milk SA is funding a project with the title: ‘’A feasibility analysis of low cost biological wastewater treatment options for dairy farms in South Africa’’. One of the aims of this project is to do a laboratory testing and modelling study in order to address the following aspects:

• Physico-chemical analysis of commercial dairy wastewater from pasture-based dairy farms;
• Socio-spatial assessment of the suitability of such farms for low cost biological wastewater treatment solutions, and
• Lab-based pilot studies to assess the efficacy of low wastewater treatment solutions shown to be socio-spatially suitable for these farms.

Manure and slurry application rates on soil is best managed through soil testing. Routine sampling and soil testing allows farmers to accurately determine the status and availability of nutrients and to be informed of any specific nutrient deficiency or excess. The results can further be used to determine specific crop nutrient needs which allow fertilizers to be applied ‘only as required’, thereby benefitting the farm both economically and environmentally. A summary of possible N, P and K contributions (in mg/l) for different manure management systems in a study by Trace & Save shows high variability, but nevertheless provide guidelines:

Mechanical separator: 380±477 N; 65±37 P and 465±178 K

Multi-pond:                    465±475 N; 39±27 P and 420±300 K

Single-pond:                   406±475 N; 25±21 P and 245±246 K

Bearing in mind the large Standard Deviation, the N content of the different manure management systems does not really differ, but the P and K levels do. Therefore, the possible contributions of N, P and K from manure should be tested in collaboration with the soil sample N, P and K to determine if these nutrients should be supplemented with chemical fertilizers or not.