Asia, national campaigns to improve smallholder productivity could be built around a single crop with a standard knowledge base and lessons gained from similar growing conditions in one country could be adopted in the next. In Africa, regional campaigns with multiple technologies and varying knowledge bases are needed to deliver the same impact.

Broadly, the great diversity of circumstances that condition smallholder technology choices in Arica means that the unfolding of Africa’s Green Revolution has been slower than Asia’s. It has also meant that successes to date have been dispersed and the accumulate effects of success harder to discern. Even so, there are signs that the gap in cereal yields is beginning to close in some places. Figure 2 shows annualized growth rates in yields for two periods: 1961 to 2000, the 20th Century Green Revolution years 1961-2000 and the more recent years, 2000 to 2013. During the first period, growth in Asian cereal yield outpaced those in Sub-Saharan Africa by a wide margin. Since then, Asian yield growth rates have slowed somewhat, as have average world rates. However, growth rates in Sub-Saharan Africa have accelerated. The figure shows considerable regional differences for both periods, with the largest yield gains occurring in Southern Africa. Keeping in mind that yield improvements in Asia have been sustained for six decades, it is clear that the yield gap between Asia and Sub-Saharan Africa remains large. However, the recent differential growth rates of crop yields do suggest that the gap between Asia and parts of Africa have begun to close.

# **7. Lessons from farmers’ fields**

In this section, we focus on elements of maize and rice productivity in SSA that illustrate why the task of developing and disseminating appropriate smallholder technologies is complex in SSA, and provide some examples of technologies that have succeeded. The detailed studies are reported in Otsuka and Larson (2016).

# **7.1 Maize**

Recent empirical studies suggest that the adoption of technologies built on high-yielding fertilizerresponsive modern varieties (MVs) significantly increases crop revenue and household income (Bezu et al., 2014; Mathenge et al., 2014; Khonje et al., 2015), reduces poverty (Khonje et al., 2015; Mathenge et al., 2014; Zeng et al., 2015) and improves food security (Bezu et al., 2014; Khonje et al., 2015; Smale et al., 2015). Nevertheless, regional adoption rates of MV maize differ significantly, and in many places, most farmers use traditional maize technologies. For example, Smale et al. (2013) report the following MV adoption rates for 2006-2007: 33 percent of maize area in Eastern Africa; 52 percent in Southern Africa, excluding South Africa; and 60 percent in West and Central Africa. Drawing on data from 20012006, Kostandini et al. (2013) also report considerable differences in MV adoption rates among countries, ranging from 5 percent in Angola to more than 70 percent in Kenya and Zambia.

As in Asia, the full benefits of the maize technologies developed for Africa are only achieved with high-levels of soil nutrients, which in practice means using chemical fertilizers (Morris et al., 2007). And there is ample evidence that the MV technologies, once adopted and used in combination with fertilizer, perform as expected. For example, estimated marginal response rates of maize to fertilizer ranged from 12 kg maize per kg. of nitrogen application in Zambia (Xu et al., 2009) to 25 kg. in Kenya (Matsumoto and 11