Time of Planting and Maize Performance


Maize yields are substantially reduced each day that planting is delayed after the onset of rains
Acland (1971) reported reductions of 55-110 kg ha-1 for each day planting was delayed in
Kenya's Rift Valley Province, and as much as 170 kg ha -1 d-1 in the Eastern and Central
Provinces, where the season is shorter. Labour and draught power constraints, however, lead
many small farmers to continue to plant maize for two or even three months after the start of the
rains. Contrary to conventional wisdom that late planting demonstrates small farmers' irration-
ality, scientists now recognize that labour and power constraints limit farmers' ability to plant at
the 'optimal' time. Indeed, the appropriate variety for small farmers will often be 20-30 days
quicker maturing than the breeders' preferred full-season cultivar. In addition, some cultivars
intentionally avoid planting early in order to reduce the risks from hazards such as uncertain
rainfall or diseases and pests associated with rainfall. Interest has therefore grown in the effects
of late planting on varietal choice, and in the selection of cultivars adapted to small farmers'
power constraints.


Fertilisers and Maize

Agronomic recommendations aimed solely at yield maximisation underestimate the importance
of yield stability and hazard management to resource-poor farmers. Improved maize cultivars
tested without fertilizers in on-farm trials in Malawi, for example, were more than twice as
unstable as local maize. With fertilizer, yield stability improved for both local and improved
maize, though the latter remained significantly less stable than the local maize (Hildebrand and
Poey, 1985). Farmers also limit their use of purchased inputs such as fertilisers when they fear
damaging losses from environmental hazards. Producers in southern Zimbabwe, for example,
apply a basal dressing of compound fertilizer after rather than before the maize crop emerges, in
order to reduce their losses from poor germination (Shumba, 1985).


Experimental Conditions and Potato Varietal Selection in Rwanda

One potato research programme in Rwanda owes its success in part to the screening of germplasm
without fertilisers or fungicides. The programme recognized early that most farmers' only
commercial inputs would be occasional seed purchases; since degeneration rates (accumulation
of viruses) are low in the highlands, farmers can multiply their own seed for five to ten years. In
order to benefit the minority of farmers who can afford other inputs, scientists in the Rwanda
programme also carry out separate fungicide trials and train extension officers in their use.

In its first five years Rwanda's low-input screening programme introduced six improved cultivars
whose yields without chemical inputs were two to five times the previous national average
(PNAP, 1984, 1985). Germplasm sources for the improved cultivars included South America,
Mexico and Europe. Two previously introduced cultivars which the programme re-released in
1980 were found in all the country's major potato producing regions in 1985. In nearly two-thirds
of 360 potato fields observed in 1985, either Montsama or Sangema occupied the largest area
(Haugerud, 1988). As about half of all potato fields are intercropped with maize, beans, sorghum,
colocasia and sweet potatoes.

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