Effects of nitrogen fertilizer applied before permanent flood on the interaction between rice and rice water weevil (Coleoptera: Curculionidae).

Nitrogen fertilizer was applied to rice, Oryza sativa L., before permanent flood to determine the interaction between rice and the rice water weevil, Lissorhoptrus oryzophilus Kuschel (Coleoptera: Curculionidae), through a series of experiments conducted over a 3-yr period in Texas and Louisiana. Both absolute and relative percentage of yield loss because of L. oryzophilus feeding was not affected by fertilizer rates in the Texas experiment. Absolute yield loss increased with nitrogen rates in 2001 and 2002 in the Louisiana experiment; however, percentage of yield loss was not affected. This finding suggests that nitrogen rate does not affect tolerance of rice to L. oryzophilus injury. In the Texas experiment, differences were detected for ratoon crop yield among nitrogen rates and insecticide that were applied at preflood on the main crop, indicating a carryover of fertilizer and insecticide effects from the main to the ratoon crop. L. oryzophilus populations tended to increase with nitrogen fertilizer in the Louisiana experiment. Our results show that farmers should not increase preflood nitrogen fertilizer to increase tolerance of rice to L. oryzophilus injury.

Narrow-waveband reflectance ratios for remote estimation of nitrogen status in cotton.

Tailoring nitrogen (N) fertilizer applications to cotton (Gossypium hirsutum L.) in response to leaf N status may optimize N use efficiency and reduce off-site effects of excessive fertilizer use. This study compared leaf and canopy reflectance within the 350 to 950 nm range in order to identify reflectance ratios sensitive to leaf chlorophyll (Chl), and hence N status, in cotton. Plants were grown outdoors in large pots using half-strength Hoagland's (control) solution until some three-row plots received a restricted supply of N. Treatments comprised control, 20% of control N at first flower bud (square) onward; 0 and 20% of control N at first flower onward; and 0% of control N at fruit-filling onward. Despite leaf N values ranging from 51 to 19 g kg-1 across treatments and sampling dates, a weak correlation was obtained between Chl and N (r2 = 0.32, df = 70). In general, N stress led to increased reflectance at 695 +/- 2.5 nm (R695) and decreased reflectance at R410, and changes in leaf N were best correlated with either R695 or R755 in leaves and either R410 or R700 in canopies. The strongest associations between leaf constituent and canopy reflectance ratio were Chl vs. R415/R695 (r2 = 0.72), carotenoids vs. R415/R685 (r2 = 0.79), and N vs. R415/R710 (r2 = 0.70). The R415 measure appears to be a more stable spectral feature under N stress, as compared with more pronounced changes along the reflectance red edge (690-730 nm). Multiple regression identified a three-waveband canopy reflectance model that explained 80% of the variability in leaf N. Results indicate that remote sensing of N status in cotton is feasible using narrow-waveband reflectance ratios that involve the violet or blue region of the spectrum (400 to 450 nm) and the more commonly featured red-edge region.