The oat (Avena sativa L.) panicle develops from the tip to apex, resulting in differences in grain weight. A detailed dissection of grain weight across the panicle could reveal information to cope with stress. This study aimed to determine the principal grain-filling component across the oat panicle and the effect of nitrogen (N) fertilizer dosage. Grain-filling can be decomposed into grain-filling rate and grain-filling duration, as these processes result in different responses on final grain weight depending on the crop species. Experiments were conducted consecutively for 2 yr with seven oat genotypes and four N fertilizer rates. There were differences in the grain-filling parameters depending on the grain position in the panicle, type of grain (primary or secondary), genotype, and N rate. The heaviest grains had higher initial grain weight and grain-filling rate. The average grain-filling rate had a significant linear correlation with grain weight, whereas the effective grain-filling duration did not. Grain weight, average grain-filling rate, and grain-filling onset decreased basipetally. Lower N rates had longer effective grain-filling duration but decreased the average grainfilling rate. Our results indicate that the grain weight differences across the oat panicle and genotypes are mainly explained by grain-filling rate, and there are differences in grain-filling onset with no change in the effective grain-filling duration.

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