Weed species diversity and shifts in Conservation Agriculture-based crop rotation systems on the Highveld area of South Africa

The occurrence and infestation levels of weed species are known to vary greatly within Conservation Agriculture (CA) systems as weed species will react differently to different habitats. Two on-farm CA trial sites, near Buffelsvallei and Viljoenskroon respectively, were established during 2008/09. The objective of this study was to observe the species diversity and potential species shifts between conventional and CA based rotation systems. Monoculture maize (MM) under both conventional tillage (CT) and CA crop systems was accordingly evaluated against two-year and three-year rotation CA systems of maize with cowpea and sunflower respectively. Pearl millet was the third crop in the three-year rotation systems. Visual surveys of weed species were conducted annually at both trial sites and data for the 2012–2016 evaluation period presented regarding the weed spectrum observed. Weed spectrums differed significantly between the two localities, but weed species diversity remained constant across seasons. A weed species shift was recorded for the sandy loam soil where numbers of Commelina benghalensis L. increased rapidly after three years, whilst Crotalaria sphaerocarpa DC numbers more than doubled. New weed species, such as Helichrysum argyrosphaerum DC and Vernonia poskeana Vatke and Hildebr., also started to germinate at the sandy loam soil trial site.