numbers of cattle, while in Ethiopia it may be caused by the widespread use of animal draft. The low use of improved seeds in Niger is likely to be associated with the unavailability of significantly improved varieties of sorghum and millet.

Forty-one percent of households in Nigeria use inorganic fertilizer and 34% use agrochemicals. However the use of organic fertilizer is the lowest among the countries at only 3%. This low use in the country with the highest agro-ecological population pressure is again a problem for BR.

Tanzania’s use of the four inputs varies between 12 percent for agro-chemicals and 18% for improved seeds. That the use of these inputs is low but even in the country with the lowest agroecological population density is consistent with BR.

In Uganda the use of improved seeds is at 18% while that of inorganic fertilizer is only 3 percent of households. Organic fertilizer and agro-chemicals fall in between at around 12 percent. Even though its agro-ecological population density is far lower than for Tanzania, it is doing worse than Tanzania, again a challenge for BR.

Improved seed use is significantly higher in the cool areas than in the warm areas (11-25% versus 1.2-6.9%). A similar difference arises for organic fertilizer, where the cool areas have use varying between 56% and 61%, while for the warm areas they vary between 3% and 13%. In warm areas it is very difficult to accumulate soil organic matter, which decays rapidly when exposed to heat, while in cool or cold areas it is far easier to do so. The poor returns to organic fertilizer may therefore be a major barrier to intensification in the warm areas, compared to the cool area. There are hints in the literature that this may indeed be the case, with consistency of intensification with BR better in the cooler areas than in warm areas.

The use of chemical fertilizers across the cool zone and the warm semi-arid zone is much higher than in the remaining three warm zones (40% to 53% versus 6% to 26%). This may be related to the higher use of improved seeds in the cool areas already discussed, although high fertilizer use and low seed use go together in the warm semi-arid zones. The warm areas, other than the semi-arid and arid ones) have soils that are low in cation-exchange capacity which limits the payoff to chemical fertilizer. We will discuss the implications of these findings in the conclusion section.

# **5. Regression results**

In the regressions we explore the impact of AEP and UG on population density, public investment, farming systems characteristics and technology use. The coefficients of AEP do not help us test the BR framework, but provide causal relationships with respect to AEP. However, the coefficients of urban gravity, in addition to explaining what it does, do test the BR framework. They have to be consistent with the predictions of the BR framework with respect to the impact of market access on intensification.

We estimate the regressions aggregating all the variables at the EAs level. We focus on 1993 EA located in 7 AEZ. The regressions are estimated in double log form, and apart from the two variables of interest, AEP, UG and their interaction include only country dummies.<sup>11</sup> By doing so we only use the within country variations to estimate the equations and differences in policies and other country-specific factors are therefore left out.

> 11 Square terms were all insignificant, except for crop profits and therefore dropped.

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