Reduced tillage practices are widely considered to be more sustainable than conventional tillage practices, but many producers remain reluctant to reduce tillage due to difficulties controlling weeds. Crop rotation is often put forward as the best means to manage weeds in reduced tillage systems, but uncertainties remain around how different tillage practices and crop rotations interact. Here, we assess the effects of four different tillage practices on weed seedbank density and composition in wheat (Triticum aestivum) monoculture (WWWW), and two different rotations, wheat-medic-wheat-medic (annual medic, Medicago spp.; WMWM), and wheat-canola-wheatlupin (Brassica napus, Lupinus spp.; WCWL). We use data across a whole four-year rotation period from a longterm experiment replicated at two sites in South Africa's winter rainfall region. The four tillage practices assessed follow a gradient of soil disturbance: conventional tillage (CT, soil inversion through ploughing), minimum tillage (MT, shallow soil loosening), no tillage (NT, direct drilling with tine openers) and zero tillage (ZT, direct drilling with disc openers). Our results indicate that tillage type had no effect on weeds within the wheat monoculture. Both crop rotations generally had lower weed densities and reduced dominance of grass weeds than the monoculture, but under ZT weed seed bank density in both rotations was similar to that found in monoculture. Thus the use of ZT with crop rotation is antagonistic in this system, possibly due to more limited chemical weed control options than in CT, MT and NT, or due to crop residue cover promoting weed establishment. Subsequently, we recommend that producers in the region seeking to reduce tillage opt for NT rather than ZT, and avoid a wheat monoculture. Weed researchers and agronomists should be wary of other such antagonistic interactions between weed management practices in different systems.

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Roughly 90% of farmers in the Western Cape Province of South Africa have converted to no-tillage systems to improve the efficiency of crop production. Implementation of no-tillage restricts the mixing of soil amendments, such as limestone, into soil. Stratification of nutrients and pH is expected. A soil survey was conducted to determine the extent and geographical spread of acid soils and pH stratification throughout the Western Cape. Soil samples (n = 653) were taken at three depths (0–5, 5–15, 15–30 cm) from no-tillage fields. Differential responses (p ≤ 0.05) between the two regions (Swartland and southern Cape), as well as soil depth, and annual rainfall influenced (p ≤ 0.05) exchangeable acidity, Ca and Mg, pH(KCl), and acid saturation. A large portion (19.3%) of soils (specifically in the Swartland region) had at least one depth increment with pH(KCl) ≤ 5.0, which is suboptimal for wheat (Triticum aestivum), barley (Hordeum vulgare), and canola (Brassica napus). Acid saturation in the 5–15 cm depth increment in the Swartland was above the 8% threshold for production of most crops. Acid soils are a significant threat to crop production in the region and needs tactical agronomic intervention (e.g. strategic tillage) to ensure sustainability.

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