concentration of the cover crop mixture through biological N fixation and minimizing the potential for short-Careful cover crop management during the spring growth period term N immobilization (Ranells and Wagger, 1997). may allow farmers to maximize dry matter (DM) yield and N accumulation for the subsequent crop. A 2-yr study was conducted to deter-In cool northern climates such as Ontario (Tollenaar mine the effect of grass and legume cover crops on spring DM produc- et al., 1993), Washington (Kuo et al., 1996, 1997, and tion and N accumulation. Each year, cover crops were planted in late Maine (Griffin et al., 2000), rye can produce a substan-August and late September on a loamy, mixed, mesic Humaquept in tial amount of DM as a winter cover crop, generally the Fraser River Delta. Wheat (Triticum aestivum L.), rye (Secale from 2.5 to 5.5 Mg DM ha Ϫ1 . However, the tissue N cereale L.), and ryegrass (Lolium multiflorum L.) were planted in concentration is commonly 10 to 15 g kg Ϫ1 or less, espemonoculture and in mixtures with crimson clover (Trifolium incarnacially after seed head emergence, so the total N accumutum L.). Other treatments included pure stand of crimson clover and lation may be low. Griffin et al. (2000) reported that wheat-hairy vetch (Vicia villosa Roth.) mixture. Cover crop biomass the total DM for rye and rye-vetch mixture ranged was sampled three times in 1995 and four times in 1996 during the from approximately 3.7 to 6.9 Mg DM ha Ϫ1 just before spring growth period. Dry matter accumulation of early planted cover crops increased by 26 to 269% during the spring growth period, ranging incorporation in late May. The total DM for rye and between 0.6 Mg ha Ϫ1 for clover and 10 Mg ha Ϫ1 for wheat, wheat-rye-vetch treatments was similar to that found in more clover, and wheat-vetch treatments. Late-planted cover crops prosouthern locations, including Maryland (Clark et al., duced between 15 and 75% lower DM yield compared with early 1994; Shipley et al., 1992), North Carolina (Ranells and planted cover crops. Nitrogen accumulation increased by 3 to 74 kg Wagger, 1996), and Georgia (McVay et al., 1989), demha Ϫ1 for early planted crops and by 3 to 47 kg ha Ϫ1 for late-planted onstrating that these cover crops are well adapted to a crops. Nitrogen accumulation at final spring sampling ranged from moderately cool growing environment. 44 to 144 kg ha Ϫ1 for early planted crops and from 10 to 99 kg ha Ϫ1 Traditionally, nonlegume cover crops have been alfor late-planted crops. The low C/N ratio of wheat-vetch treatment lowed to grow until the summer crop is planted, often compared with wheat monoculture at final sampling indicated the resulting in residues with wide C/N ratios Ͼ25:1 (Ditsch potential for vetch to increase the N content of the mixture.

No abstract

No abstract

No abstract

Identifying options for the sustainable intensification of cropping systems in southern Africa under prevailing high climate risk is needed. With this in mind, we tested an intercropping system that combined the staple crop maize with lablab, a local but underutilised legume. Grain and biomass productivity was determined for four variants (i) sole maize (sole-maize), (ii) sole lablab (sole-lablab), (iii) maize/lablab with both crops sown simultaneously (intercropped-SP) and (iv) maize/lablab with lablab sown 28 days after the maize crop (intercropped-DP). Soil water and weather data were monitored and evaluated. The trial was conducted for two seasons (2015/2016 and 2016/2017) at two sites in the Limpopo Province, South Africa: Univen (847 mm rainfall, 29.2 °C maximum and 18.9 °C minimum temperature average for the cropping season over the years 2008–2017) and Syferkuil (491 mm rainfall, with 27.0 °C maximum and 14.8 °C minimum temperature). Analysis revealed three key results: The treatment with intercropped-SP had significantly lower maize yields (2320 kg ha−1) compared with maize in intercropped-DP (2865 kg ha−1) or sole-maize (2623 kg ha−1). As expected, maize yields in the El Niño affected in season 2015/2016 were on average 1688 kg ha−1 lower than in 2016/2017. Maize yields were significantly lower (957 kg ha−1) at Univen, the warmer site with higher rainfall, than at Syferkuil. In 2015/2016, maximum temperature at Univen exceeded 40 °C around anthesis. Furthermore, soil water was close to the estimated permanent wilting point (PWP) for most of the cropping season, which indicates possible water limitations. In Syferkuil, the soil water was maintained well above PWP. Lablab yields were low, around 500 ha−1, but stable as they were not affected by treatment across season and site. Overall, the study demonstrated that intercropped-DP appears to use available soil water more efficiently than sole maize. Intercropped-DP could therefore be considered as an option for sustainable intensification under high climate risk and resource-limited conditions for smallholders in southern Africa.