MANAGING HEALTHY SOILS FOR SUSTAINABLE PASTURE PRODUCTION.

 Kikuyu–ryegrass cultivated pastures form the base for dairy production in the southern Cape pasture-based dairy production systems. Kikuyu is a particularly important crop to improve the biological health of a soil, but lacks quality during certain times of the year. Ryegrass usually supports with improved quality and yield. These concepts have been well-studied. However, it is always important to understand what is happening over the long term with factors impacting on soil condition to support sustainability. One such factor is soil fertility.

 To that effect fertiliser management strategies were assessed in a large survey throughout the southern Cape from Swellendam to Humansdorp. Mean soil fertility levels showed that all essential plant nutrients were within the requirements of kikuyu-ryegrass pastures. However, from an environmental sustainability perspective phosphorus and zinc levels were drastically higher than the recommended threshold in all districts. This is cause for concern and farmers should seriously take this into consideration in their fertiliser management strategies.                                                                                                                                               

 As mentioned, the conversion of natural rangeland to minimum-tillage kikuyu based pastures for dairy production in the southern Cape of South Africa, may be beneficial to soil biological quality. The question however remains what will be the status over the long term. Therefore, the objective in this part of the study was to evaluate whether 19 years of minimum-till kikuyu-ryegrass pasture had altered the distribution and quality of biological properties formerly developed under natural rangeland. In the study an irrigated minimum-till kikuyu-ryegrass pasture soil was compared to virgin soil of the natural rangeland. The soil microbial diversity was significantly altered between the cultivated pasture and the virgin soil, especially at different soil depths. A general appraisal of biological and organic soil properties indicated that conversion of natural fynbos vegetation to irrigated minimum-till kikuyu-ryegrass pasture after 19 years of cultivation was beneficial.

 Minimum tillage is generally practised to establish kikuyu-ryegrass pastures, but it has to be established if it sustains the advantage over other methods and if so, what should be the preferred method. Five tillage treatments were assessed, namely (1) kikuyu over-sown with ryegrass using a minimum-till planter, (2) eradication of kikuyu with herbicide and ryegrass sown with a minimum-till planter, (3) shallow ( 150 mm) and deep ( 150 mm) disturbance with a rotavator, (4) conventional tillage and (5) a control. The autumn, spring, summer and annual productions of treatments with the least soil disturbance were the highest. Shallow or deep tillage resulted in intermediate annual productions and the lowest was recorded for the herbicide treatment. Differences in production caused by different tillage methods were significant and changed the botanical composition. It was concluded that kikuyu over-sown with ryegrass using a mulcher and minimum-till planter was the preferred method of cultivation, both from a production and a sustainable perspective.

 Reference:

 Swanepoel, P & Botha, P., 2014. Managing healthy soils for sustainable pasture production. Information day: Milk production from planted pastures. Outeniqua Research Farm, 15 October 2014.