The response of grain yield to narrow rows can be analyzed in Westgate et al., 1997).terms of the effect on the amount of radiation intercepted by the crops. The objective of this work was to study the effect of row spacing There are times during the crop cycle that are most on grain yield and radiation interception (RI) during the critical period critical for yield determination. These times comprise for grain set in three crop species. Ten experiments were conducted the period bracketing flowering in maize (Kiniry and with maize (Zea mays L.), sunflower (Helianthus annuus L.), or soy- Ritchie, 1985; Fischer and Palmer, 1984) and sunflower bean [Glycine max (L.) Merr.] under irrigation or under dryland con- (Chimenti and Hall, 1992, Connor and Sadras, 1992; ditions without severe drought during flowering and grain filling. The Cantagallo et al., 1997) and extend to more advanced treatments consisted of two row distances combined with other factors reproductive stages in soybean (Shaw and Laing, 1966; such as plant density, cultivar, defoliation, etc. Grain yield responses Board and Tan, 1995; Egli, 1997). Higher crop growth to decrease distance between rows were inversely proportional to RI rates during these periods would allow more grains to achieved with the wide-row control treatment during the critical pebe set and thus higher grain yields (Andrade et al., riod for grain number determination (r 2 ϭ 0.62, 0.54, and 0.86 for maize, soybean, and sunflower, respectively). Moreover, when row 1999). Crop growth rate is directly related to the amount spacing was reduced, grain yield increases and RI increases during of radiation intercepted by the crop (Gardner et al., the critical periods for grain set were significantly and directly corre-1985). Therefore, the response of grain yield to narrow lated in the three crop species (r 2 ϭ 0.71, 0.64, and 0.94 for maize, rows can be analyzed in terms of the effect on the soybean, and sunflower, respectively). For the conditions of these amount of RI at the critical periods for kernel set. In

Effi cient use of N in plant production is an essential goal in crop management. An experiment was performed at INTA Balcarce, Argentina during 3-yr to evaluate the eff ect of row spacing and N level on nitrogen use effi ciency (NUE) in no-till (NT) irrigated maize (Zea mays L.). Treatments consisted of a factorial combination of row width (70, 52 and 35 cm) and N rates (0 to 180 kg N ha −1 ). Nitrogen rate and narrow rows increased total dry matter (DM), grain yield, and N accumulation. Relative responses to narrow rows decreased as N availability increased. Th e NUE decreased with N rate and increased with narrow row spacing. Narrow rows increased NUE by 12 and 15% expressed as DM or grain yield per unit of available N, respectively. Physiological effi ciency decreased with N rate and was not aff ected by row spacing. Recovery effi ciency (RE) decreased with increasing N rate, and increased for the narrow row spacings. Th e eff ect of narrow rows on RE decreased as N availability increased. Narrow rows increased NUE largely as a result of increased RE. Th ese increments in RE could contribute to increase the profi tability of maize production and to diminish the risk of environmental pollution.

efficiency of light interception per unit leaf area (Bullock et al., 1988; Westgate, 1998). However, grain yield No-tillage maize (Zea mays L.) grown without adequate N interincreases in response to narrower rows under convencepts less than 95% of the incident radiation at flowering. Reducing tional tillage and adequate N were less than 10% the distance between rows could increase radiation interception and grain yield. A 2-yr experiment was conducted at INTA Research (Hunter et al., 1970; Stivers et al., 1971; Bullock et al., Station, Balcarce, Argentina (37؇45 S; 58؇18 W), to study the effect 1988; Porter et al., 1997). of row spacing and N availability on intercepted radiation, kernel In Balcarce, no-tillage maize grown without adequate number, and grain yield of no-till maize. Treatments consisted of a N intercepts less than 95% of incident radiation at flowfactorial combination of row width (0.35 and 0.70 m) and N (0 and ering (Sainz Rozas, 1997). Decreasing the distance be-120 or 140 kg ha Ϫ1 each year) at a constant plant density. Low N tween rows could partially offset N stress effects and decreased kernel number and grain yield. Narrow rows significantly increase kernel number per square meter and grain increased kernel number per unit area and grain yield. Average inyield. Because LAI and radiation interception respond creases in response to narrow rows were 14.5 and 20.5% for kernel to N supply (Novoa and Loomis, 1981; Lemcoff and number and grain yield, respectively. However, relative increases in Loomis, 1986; Muchow, 1988; Muchow and Davis, 1988; dale, 1995) because of an increase in LAI and in the a.i. ha Ϫ1 ; and metolachlor (2-chloro-N-(2-ethyl-6-methylphe-

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