Asian Regional Maize Workshop, 7, Los Baños, Philippines : February 23-27, 1998.

Open Access

Farmers in tropical countries often grow maize in soils with sub-optimal levels of nitrogen (N), and cultivars that perform well under these conditions would be desirable. CIMMYT began breeding maize for tolerance to low N in 1986. Practical conclusions from this research are presented herein for maize breeders who target N-stressed environments in the tropics. In CIMMYT's breeding program, ideotype selection under low and high N in the tropical maize population Across 8328 BN (BN = bajo nitrogeno, Spanish for low nitrogen) over five cycles of full-sib recurrent selection resulted in selection gains that averaged 84 kg/ha per year at low N and 120 kg/ha per year at high N. Lines extracted from an advanced cycle of Across 8328 BN produced a higher frequency of low N-tolerant top crosses than lines extracted from a sister population selected conventionally under high N. An analysis of 14 maize progeny experiments showed that once the N stress in the target environment reduces yields by more than 40% of the yield under well fertilized conditions, it is significantly more efficient to use a low-N selection environment for targeting farmers' fields with low N than to use a high-N field as the sole selection environment. Statistical procedures, such as increasing the number of replicates or using improved designs, and secondary traits, such as decreased barrenness, delayed leaf senescence and a reduced anthesis-silking interval, can improve the precision with which superior genotypes are identified under low N. Diallel studies have shown that performance of inbred lines under low N is not a reliable indicator of hybrid perfbrmance under low N. Genetic variation for tolerance to low N can be found in most improved germplasm. However, selection for higher yields under low N usually increases N uptake and total grain protein yield at a lower relative rate than it increases grain yield, leading to lower grain protein concentrations. We conclude that maize breeding that systematically evaluates germplasm for tolerance to low N may stabilize maize production and the income of resource-poor farmers operating under low-N conditions. However, when promoting low N-tolerant germplasm, it should be remembered that a reduction in grain protein concentration appears associated with most yield gains ob.served under low N and that soil fertility and grain protein concentration can only be maintained if .there is no consistent net N loss from the soil-crop-animal system.

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0208|AGRIS 0201|AL-Maize Program|R01PROCE

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CIMMYT Staff Publications Collection