Amaranthus species, commonly referred to as “pigweeds,” are among the most troublesome weeds in many crop production systems. Effective control of these species often begins with an understanding of their biological and reproductive characteristics. At two sites in Missouri, six pigweed species (redroot pigweed, common waterhemp, spiny amaranth, tumble pigweed, smooth pigweed, and Palmer amaranth) were established in 60-m rows spaced 1.5 m apart. At biweekly intervals, plant heights and dry weights were recorded for each species; seed numbers were estimated at the end of the growing season. Dry weight of Palmer amaranth was up to 65% greater than those of all other species 2 wk after planting (WAP). Palmer amaranth biomass accumulation remained greater than those of the other species throughout the season and at the end of the season was 1.2- and 2.7-fold greater than those of redroot and tumble pigweed, respectively. Palmer amaranth was approximately 10 cm tall 2 WAP (37% taller than the next tallest species, redroot pigweed) and approximately 24 cm tall 4 WAP (45% taller than redroot pigweed). In contrast, common waterhemp had not emerged 2 WAP, and plant dry weight 4 WAP was approximately 11 and 26% those of Palmer amaranth and redroot pigweed, respectively. Final plant height ranged from 58 (tumble pigweed) to 208 cm (Palmer amaranth). Redroot pigweed, smooth pigweed, common waterhemp, and Palmer amaranth plants each produced over 250,000 seeds plant−1. Spiny amaranth and tumble pigweed produced approximately 114,000 and 50,000 seeds plant−1, respectively. Common waterhemp produced 535 seeds g−1 of total plant dry weight; this seed production was 1.4-, 1.4-, 2.0-, 3.4-, and 3.4-fold greater than those of redroot pigweed, smooth pigweed, Palmer amaranth, tumble pigweed, and spiny amaranth, respectively. Because the timing for many postemergence herbicides depends on weed height, rapid growth shortly after emergence reduces the time frame for optimum control of species such as Palmer amaranth. Delayed emergence also could result in escaped common waterhemp. Escape of only a few plants could result in a rapid increase in seed populations in the soil seed bank and may select for late-emerging biotypes.

The increasing cost of commercial fertilizers and environmental problems associated with improper fertilization management have prompted the need to re-examine commercial N sources that can effectively supply N to pastures while minimizing N losses. This 3-year study evaluated the effects of selected N sources on bahiagrass (Paspalum notatum Fl€ ugge) responses, soil properties and N losses. Treatments consisted of a factorial combination of 6 N sources [(i) ammonium nitrate (AN), (ii) ammonium sulphate (AS), (iii) urea (U), (iv) urea treated with Agrotain (U + Agrotain), (v) SuperU and (vi) ammonium sulphate nitrate] and 3 N levels (0, 60 or 120 kg ha À1 year À1 ), replicated three times. Bahiagrass dry-matter yield (DMY), crude protein (CP) concentration, N uptake and recovery were not affected by N source, with the exception of AN that resulted in reduced DMY in 2010 compared with the other sources. Bahiagrass DMY, CP concentration, N uptake and recovery increased linearly as N levels increased. Nitrogen fertilization showed no effect on soil pH or soil N accumulation. Soil pore-water N concentrations from treatments fertilized with N were similar to the control plots indicating no threat to the environment. At the N levels evaluated in this study, selection of N source should be based on the fertilizer cost.

The relationship between riometer-measured absorption of 30 MHz cosmic radio waves at Thule, Greenland, and the extramagnetospheric 1-100 MeV solar proton event fluxes observed by satellite OV5-6 during 1969-1972 is studied, with night and day conditions at Thule being separated. In the 70-90 km region in which most nighttime absorption occurs, ionization conditions are, in fact, relatively simple. This leads to a nighttime relationship between absorption and the square root of the integral proton flux, which is similar to that for daytime conditions. Based on previously measured effective recombination coefficients, theoretically optimum energy thresholds are determined for use in this approximate relationship. The values are 2.2 MeV and 5.2 MeV for night and day conditions, respectively. Both the approximate absorption, and that calculated by a reformulation of the standard approach, compare satisfactorily with extensive riometer measurements at Thule. The approximate relationships are employed in a real-time system that uses satellite proton data for prediction of radio wave (riometer) absorption.

Velvetleaf plants have diurnal leaf movements, which may result in decreased interception of herbicides when applications are made near sunset. However, it is not known if leaf angle alone accounts for diurnal fluctuations in efficacy. Greenhouse experiments were conducted to determine the effect of time of day (TOD) of application and velvetleaf leaf angle on glufosinate efficacy and spray interception. Glufosinate at 90, 180, and 360 g ai/ha was applied to 10-cm-tall plants at 4:00, 6:00, 7:00, 7:30, and 8:00 P.M., respectively. Leaf angles were either manipulated physically to −90° or the plant's natural 2:00 P.M. leaf angle (approximately −10°) or were allowed to exhibit their natural leaf movements. Plant dry weight 3 wk after treatment revealed that TOD effects were observed for all leaf angle treatments after glufosinate application at 90 g/ha. At 180 g/ha glufosinate, there was no TOD effect for plants with 2 P.M. leaf angles, whereas there was a TOD effect for plants with −90° and natural leaf angles. At 360 g/ha glufosinate, biomass for the −90° leaf angle plants was similar to that for the natural and the 2:00 P.M. leaf angle plants when glufosinate was applied at 4:00 P.M. but was significantly different at or after 6:00 P.M. This suggests that at least 4 h of light is needed to provide optimum herbicide activity when spray interception is reduced as a result of leaf movements. Leaf angle decreased by as much as 70% from 4:00 to 8:00 P.M., which resulted in approximately 50% less spray interception at 8:00 P.M. than at 4:00 P.M. These data provide evidence that leaf angle plays a pivotal role in reducing glufosinate efficacy when applications are made near sundown. However, leaf angle is not the sole reason for reduced efficacy because TOD effects were observed at different leaf angles with 4 h of light, after an application of 360 g/ha glufosinate.

RESEARCHB ahiagrass (Paspalum notatum Flügge) is the main forage used for the beef cattle industry in Florida because of its reliability and persistence under adverse climatic conditions and management practices. It is the most widely planted warm-season grass in Florida, covering approximately 1 million ha. Nonetheless, overdependence on bahiagrass pastures has made the industry vulnerable to potential losses from pests and diseases. During 1996 through 2000, >150,000 ha of bahiagrass pastures were damaged by mole crickets (Scapteriscus spp.) in Florida (Adjei et al., 2001). Damage to bahiagrass pastures by mole cricket stimulated a search for other grasses adapted to the Florida environment.Brachiariagrasses are the most widely grown forages in tropical America, occupying >80 million ha (Boddey et al., 2004). Brachiaria species are popular among producers because they show rapid regrowth and good persistence under close or frequent defoliation (Rika et al., 1991). 'Mulato' is the fi rst hybrid in the Brachiaria genus and results from crossing ruzigrass (Brachiaria ruziziensis Germain & Evrard, clone 44-6) and palisadegrass [Brachiaria brizantha (A. ABSTRACTBahiagrass (Paspalum notatum Flügge) is the most used forage for beef cattle (Bos sp.) in Florida; however, damage to bahiagrass pastures by mole cricket (Scapteriscus spp.) stimulated a search for other adapted grasses. The objective of this study was to test the effect of different stocking rates (SRs) on animal performance and herbage characteristics of 'Mulato' (Brachiaria sp.) and bahiagrass pastures. The experiment was conducted in Ona, FL, from May to September, 2007 and 2008. Treatments were three SRs (4, 8, and 12 heifers ha −1 ) and two forage species (Mulato and 'Pensacola' bahiagrass). There was a linear decrease in herbage mass (from 5.9 to 3.2 Mg ha −1 ) and herbage allowance (from 2.8 to 0.6 kg dry matter kg −1 liveweight) with increasing SR. Mulato had greater in vitro digestible organic matter concentration than bahiagrass (675 vs. 534 g kg −1 ) but similar crude protein concentration (130 g kg −1 ). There was a linear decrease in average daily gain (from 0.49 to 0.22 kg d −1 ) and a quadratic response of liveweight gain ha −1 as SR increased (190, 353, and 217 kg ha −1 for SRs of 4, 8, and 12 heifers ha −1 , respectively). Mulato has potential to be an alternative forage to bahiagrass for beef cattle producers in Florida.