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Differential Response of Rice Varieties to Timing of Mid-season Nitrogen Applications¹

Field studies were conducted with rice (Oryza sativa L.) on Crowley silt loam soil using the important commercial cultivars ‘Bluebelle,’ ‘Nova 66,’ and ‘Starbonnet.’ We studied the effects of rate and timing of mid-season N applications on grain yield, plant height, lodging, head-rice yield, and grain weight (g/M) of varieties (cultivars) differing in maturity and plant type. All plots received 45 kg/ha of N as ammonium sulfate 15 days after seedling emergence, just before initial flooding. The remaining N needed to give total rates of 78, 112, or 146 kg/ha was applied to drained soil at specified intervals near midseason. Fifteen maui stems, selected at random from the outside rows of each plot at the time of mid-season N application, were used for determining internode lengths.

Maximum grain yields were associated with N applied at median internode lengths averaging 21.0, 58.5, and 5.0 mm for Bluebelle, Nova 66, and Starbonnet, respectively. Delaying mid-season N applications until these respective stages of plant development, resulted in shorter plants and less lodging, accompanied by increased grain weight and head-rice yields.

Variations in the median internode length at N fertilization, timed for a combination of maximum grain yield and minimum plant height and lodging, were closely associated with plant type. Starbonnet and Bluebelle, which have short, stiff straw and erect leaves, responded better to N applied at a shorter internode length than Nova 66, a taller, broader-leaved variety. When N was applied too early, Nova 66 produced considerably more excessive vegetation than did Bluebelle and Starbonnet. Both Starbonnet and Bluebelle have plant types which approach the one currently favored by many plant breeders.

Key Words: Oriza sativa L. • Internode elongation • Morphological development • Plant type • Lodging • Rice yield • Grain weight (g/M) • Milling yield

² Associate Agronomist. Arkansas Agr. Exp. Sta. and Research Agronomist, Crops Research Division, ARS, USDA.

Received for publication February 6, 1970.

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