The objective of this study was to evaluate the radiation use efficiency (RUE) and canopy structure of elephant grass varieties (Cenchrus purpureus Schum.) of contrasting statures, under monocropping or intercropped with butterfly pea (Clitoria ternatea L.) in cut-and-carry systems. Two tall varieties (elephant B and IRI-381), and two dwarf ones (Mott and Taiwan A-146 2.37), were evaluated as monocrops or binary intercrops with the legume in a 2-year trial with eight harvests. Greater annual leaf biomass accumulation was observed in the monocrops of the tall variety elephant B (7.76 t/ha per year) and dwarf Mott (8.08 t/ha per year). Greater herbage bulk density (59 kg/ha per cm) and leaf area index (3.83) were recorded in canopies of dwarf Mott than in those composed of IRI-381 (37 kg/ha per cm and 3.48, respectively). In the first year, dwarf varieties Mott and Taiwan A-146 2.37 showed less RUE (0.89 and 0.84 g dry matter (DM)/MJ, respectively) than the elephant B (1.46 g DM/MJ). Higher non-fibre carbohydrate (NFC) contents were found in dwarf Mott under monocrop (180 g/kg) and in the intercrop systems. Tall varieties elephant B and IRI-381 showed greater efficiency in intercepting the radiation to accumulate herbage via stem accumulation. Dwarf Mott variety exhibited short stems and great leaf biomass accumulation that favoured denser canopies with higher content of NFCs in vegetal tissue. Planting butterfly pea into rows of elephant grass varieties can be adopted with no significant losses in RUE caused by light extinction, regardless of the grass stature.

Silvopastoral systems (SPSs) are sustainable alternatives for pasture intensification. Management practices such as thinning are options to diminish competition for resources between pasture and trees, ensuring appropriate forage production. The objective of this study was to evaluate the productive and nutritive traits of Piatã palisadegrass (Urochloa brizantha cv. BRS Piatã) after thinning the forest component of a SPS. Forage production and nutritive value, along with microclimate variables were assessed in a SPS and in an intensive system (INT) in São Carlos-SP, Brazil, from 2016 to 2018. The INT was a full sun pasture of Piatã palisadegrass, while the SPS was established with Piatã palisadegrass shaded by eucalyptus (Eucalyptus urograndis clone GG100) spaced 15 × 2 m but thinned to 15 × 4 m before the onset of the experiment. In the SPS, measurements were made under the tree row (SPS_1), 3.75-m (SPS_2), 7.5-m (SPS_3) and 11.25-m (SPS_4) distant from the North row. Forage accumulation in the two systems were similar, whereas the pre-grazing forage mass in most of the SPS positions were smaller than that of the INT during the autumn of 2017 and summer of 2018. Crude protein (CP) content was greater in the SPS than in the INT in most positions and seasons. Forage production was favoured by the thinning of the trees mainly in the seasons close to the event, while forage CP was consistently greater in the understory pasture. Thinning of trees in SPSs can be adopted to provide forage production similar to intensive pastures with higher protein content.

The goal of this study was to evaluate the effects of thinning eucalyptus trees on yield and nutritive value of corn for silage and palisadegrass in a crop–livestock–forest integrated system and to evaluate the total aboveground biomass yield in systems with and without trees. Plant variables, as well as the incidence of photosynthetically active radiation (PAR) and soil moisture, were evaluated between October 2016 and March 2018 in São Carlos, Brazil, in a crop–livestock–forest and a crop–livestock system. In the crop–livestock–forest system, eucalyptus trees (Eucalyptus urograndis clone GG100) were planted in April 2011, in single rows, with 15 × 2 m spacing. In 2016, the trees were thinned, and the spacing was changed to 15 × 4 m. The treatments comprised measurements at 0.00, 3.75, 7.50, and 11.25 m from the trees of the North row in the integrated crop–livestock–forest (iCLF) system and integrated crop–livestock (iCL) system. Palisadegrass (Urochloa brizantha) was sown after harvesting the corn. Corn yields were similar between treatments, with an average of 13.6 Mg ha−1. Corn for silage presented a higher percentage of grain in total biomass in the crop–livestock–forest positions (41.4 and 42.1%) than in the crop–livestock system (35.6%). No differences in forage accumulation were observed. Crude protein content in corn for silage and palisadegrass was higher in the crop–livestock–forest treatments than in the crop–livestock system. Such results indicate that thinning was favorable to production in the crop–livestock–forest system. Total aboveground biomass yield was higher in the iCLF system, indicating better land use for this type of integrated system.

Field experiments over 3 yr at Lethbridge, Alberta, determined the effect of various downy brome densities and times of its emergence on winter wheat biomass and seed yield. Downy brome reduced wheat biomass up to 59% and seed yield up to 68%. Time of downy brome emergence relative to wheat affected the magnitude of these yield reductions more than the density of downy brome. At comparable densities, downy brome caused 2- to 5-fold greater reductions in yield when it emerged within 3 wk after winter wheat than when it emerged 6 wk after wheat or in early spring. Late-emerging downy brome caused significant wheat yield or biomass losses only at densities of 200 to 400 plants m-2. Late-emerging downy brome plants were strongly shaded (70 to 90%) by winter wheat throughout much of the growing season.

The influence of weeds on crop yield is not only dependent on weed-related factors such as density and time of emergence, but also on environmental and management factors that affect both the weed and crop through time. This study was undertaken to develop the first physiologically based dry bean model that would account for the influence of weed competition. The specific objective was to develop a model that would account for the influence of weed competition on crop yield, and to use this model to test the hypothesis that crop yield losses resulted from competition for photosynthetically active radiation (PAR). To this end, a model that simulated the growth and development of dry bean was developed. The model performed daily calculations and simulated the phenology, leaf area expansion, dry matter production and distribution, and grain yield of dry bean based on weather and management information, but assumed adequate water and nutrients. The model was calibrated without weed competition at two locations and yr, and for these situations, adequately described the growth and development of the crop. Simulations were then run for five common ragweed densities and two times of emergence. Common ragweed leaf area was read into the model from input files and used to simulate weed shading. Shading of the dry bean canopy by common ragweed accounted for about 50 to 70% of the yield losses observed in field studies when weeds emerged with the crop. Weed shading did not account for the yield reduction measured from weeds that emerged at the second trifoliate stage of crop growth. The agreement between model predictions and field studies was consistent with the hypothesis that competition for PAR was a principal factor in weed-crop interaction. The ability to account for differences in weed densities, management, and environmental conditions suggested that modeling was a useful tool for evaluating the interaction among weeds and crops.

Field experiments over 3 yr at Lethbridge, Alberta, determined the effect of various downy brome densities and times of its emergence on winter rye biomass and seed yield. Downy brome reduced yields most when it emerged within 3 wk of rye, but densities of more than 100 downy brome m-2 were required to reduce yields by 20 to 30%. The greatest reductions in rye biomass (28%) and seed (33%) yields over the 3 yr occurred when 400 downy brome m-2 emerged with the crop. Downy brome, at densities up to 400 plants m-2, emerging 6 wk after rye or in early spring, reduced rye biomass and seed yield less than 10% in all years. Winter rye effectively shaded downy brome (40 to 90%) for much of the growing season.

Greenhouse and growth-chamber experiments were conducted to determine the influence of the ratio of red to far-red (R:FR) radiation and of neutral filtration of plant-available radiation on the vegetative and reproductive growth of selected nightshade species. Exposure of eastern black nightshade and black nightshade to far-red radiation resulted in greater partitioning of resources to stem tissue, resulting in taller plants. Number of flowers, timing of flowering, branching, and biomass production of black nightshade and eastern black nightshade were not influenced by the ratio of red to far-red radiation. Eastern black nightshade shoot and berry dry weight decreased as neutral density shading increased from 0 to 71%. Shoot dry weight decrease was associated with lower stem weight and production of fewer berries per plant. Neutral shading did not reduce leaf weight or leaf area per plant but increased the specific leaf area of eastern black nightshade. Internode elongation of nightshade species into a soybean canopy should primarily be associated with exposure to low R:FR irradiance ratios, whereas the thinner leaves of eastern black nightshade growing under shade should be associated with lower irradiance levels. Both responses are common adaptations of shade-avoiding plants.

Purple nutsedge is a troublesome C4 weed, characterized by high photosynthetic efficiency, compared to C3 weeds. As its dispersal is based on vegetative growth, accurate prediction of its growth could help in arriving at favorable management decisions. This article details the development and validation of predictive models of purple nutsedge spatial growth, based on temperature (thermal model), and temperature and radiation (photothermal model) measurements. Plants were grown in six experiments in the summers of 2008, 2009, and 2010, under different temperature and radiation conditions. Results indicate that under optimal temperatures, radiation becomes the main growth-limiting factor, and is highly related to the final leaf-cover area (R2 = 0.89). Comparison of the thermal and photothermal models showed that under all conditions, including varied temperature and radiation, the photothermal model performs significantly better, with differences in root-mean-square error values reaching up to 0.073, compared to 0.195 with the thermal model. Validation experiments confirmed the ability of the photothermal model to predict purple nutsedge spatial growth accurately.

Glyphosate-resistant (Roundup Ready®) soybean is widely grown to provide broad-spectrum weed control following sequential applications of glyphosate. Studies were conducted during 1998 and 1999 at Keiser, AR, and in 1999 at Pine Tree, AR, to examine the effect of glyphosate-resistant soybean populations on the number of glyphosate applications needed to maintain 90% or greater control of all weed species and on the gross profit margin from weed management. Soybean was seeded in 19-cm rows at 12 rates ranging from 185,000 to 1,482,000 seeds/ha. Each seeding rate received a single application of 0.56 and 1.12 kg ai/ha glyphosate once weeds were 5 to 7 cm tall, followed by repeat applications when control of any species fell below 90%. Soybean yield and gross profit margin at Keiser were similar both years, and a quadratic curve best described the relationship between seed yield and seeding rate, with maximum predicted and observed yield at 988,000 seeds/ha. Soybean seeding rate affected light interception by soybean and the number of glyphosate applications needed to maintain 90% control of all species. By the later stages of vegetative development, soybean at Pine Tree achieved ≥95% light interception, maximizing yield at all seeding rates. Late-season light interception at Keiser ranged from 88 to 99%, causing soybean yield to vary by seeding rate. Three applications of 0.56 kg/ha glyphosate were needed at seeding rates of 185,000 and 247,000 seeds/ha, whereas higher seeding rates sometimes required only a single application for season-long weed control. Application and herbicide costs were offset by reduced seeding costs; thus, predicted gross profit margin from weed management was optimized at the lowest seeding rate of 185,000 seeds/ha.

Nitrogen (N) management markedly affects weed competition with crops. The effect of N availability on plant competition varies with a plant's abilities to capture and use N. Accordingly, we expected the N effect on plant competition to change with the relative emergence time of competing individuals. This hypothesis was tested by growing corn and velvetleaf plants in target and neighbor roles and comparing their N responses. Sowing times were varied, so that target plant emergent dates were constant, whereas neighbor plants were sown to emerge 7 d before, with, or 7 d after targets. Seedlings were fertilized daily with 2.5 mmol (“low N”) or 10.0 mmol (“high N”) ammonium nitrate (NH4NO3). Corn had greater total weight, leaf area, and root-to-shoot ratio (RSR) than velvetleaf. Different dry weights may have reflected seed weights; corn seed weight was greater than velvetleaf. Regardless of role, corn and velvetleaf dry weight and leaf area were greater with high N than low N; in contrast, RSR was lower with high N than low N. The RSR response to N availability suggested plant resources were shifted from N foraging, toward competition for photosynthetically active radiation (PAR). In target plants of each species, dry weight and leaf area increased linearly with time between target and neighbor emergence. Conversely, dry weight and leaf area of neighbor plants decreased with time between target and neighbor emergence. The N response of neighbor plants increased with time of emergence between target and neighbor emergence. Larger plants likely required more N to sustain growth than smaller plants; plants that emerged earlier likely had greater daily N requirements than those that emerged later. These results support factoring in emergence when predicting N effects on plant competition. Optimal N provisioning for integrated weed management may differ with emergence times of competing plants.

Two field experiments were conducted in Oklahoma to determine whether changing wheat row spacing, seeding rate, and cultivar would reduce interference from rye. Wheat row spacing did not affect rye seed production. Averaged over row spacing, increasing wheat seeding rate from 67 to 134 kg/ha reduced rye seed production 21 and 25% in two experiments. At one site, grain yield of rye-infested wheat was increased 27 and 23% by doubling wheat seeding rate in 10- and 20-cm rows. Doubling the seeding rate of rye-infested wheat in 30-cm rows did not increase wheat yield. At a second site when data were pooled over row spacings, increasing the wheat seeding rate from 67 to 101 kg/ha increased yield of rye-infested wheat 21%, but yield was still 36% less than for rye-free wheat. In two additional experiments, rye seeds in harvested wheat were reduced 36% by increasing the wheat seeding rate from 60 to 162 kg/ha, whereas yield of rye-infested wheat was increased 82%. Nine cultivars were compared for competitive ability against rye in four experiments. Of 32 possible cultivar location situations, the rye-induced yield loss of ‘Jagger’ wheat was less than 16 other cultivar location situations, whereas the yield loss of ‘Triumph 64’ wheat was less than 10 other cultivar location situations. Mature wheat height, wheat yield, and wheat maturity classification were each negatively correlated to rye yield in infested wheat at two of four sites. Wheat plant density, head density, sunlight interception, and fall cover capability classification were each negatively correlated to rye yield at one of four sites.

A field study was conducted to determine the effect of increasing seed rates of spring wheat (Triticum aestivum) on redstem filaree (Erodium cicutarium) biomass and seed production and on wheat yield in a zero-tillage cropping system. Wheat was sown at 50, 100, 150, 200, and 300 kg/ha in an experiment four consecutive years to determine annual and accumulated effects over time. Redstem filaree was most competitive with wheat when it emerged before or with wheat and when rainfall was plentiful during May and June. Redstem filaree biomass and seed production were markedly inhibited by drought conditions. An increase in wheat seed rate from 50 to 300 kg/ha reduced redstem filaree biomass and seed production by 53 to 95% over the years. There was little yield benefit to increasing the seed rate of weed-free wheat above 50 kg/ha. However, in the presence of redstem filaree, an increase in wheat seed rate from 50 to 300 kg/ha resulted in wheat yield being increased by 56 to 498%. Over four consecutive years, an increase in wheat seed rate from 50 to 300 kg/ha reduced redstem filaree in the soil seedbank by 79%. Higher seed rates of wheat can be an effective component of an improved program for redstem filaree management in conservation tillage cropping systems.

Study of forest dynamics commonly requires measurement of canopy gap size. Hemispherical photographs can be analysed to provide various measures whereby gaps can be ranked in order of size. For ten artificial gaps in a Bornean tropical rain forest these measures were better correlated with gap microclimate than gap area measured physically on the ground. All these measures are however relative. For detailed (e.g. ecophysiological) studies the greater detail provided by absolute measures of photosynthetically active radiation (PAR) are required. Long term PAR values can be computed from hemiphots so long as measurements in the open nearby are available. Correction for cloudy weather is essential. Computed and measured PAR are compared for the test gaps. Both have inherent limits which means that below c. 15% canopy openness, differences in PAR between gaps cannot be assessed accurately.

Hurricane Gilbert caused disturbance to Jamaican montane rainforests in 1988. This study provides a detailed characterization of landscape-level changes in light below the canopy of these forests after the hurricane. Hemispherical photographs were taken below the forest canopy at four sites at permanent points 1 m above the ground between 7 and 33 mo after the hurricane. For each photograph photosynthetically active radiation (PAR) was computed. PAR declined exponentially in all sites during the period of measurement. During the first 24 mo after the hurricane, PAR beneath the canopy was significantly greater in sites that had been defoliated during the hurricane than in sites where few trees had been defoliated. By 28 mo after the hurricane there was no significant difference in PAR beneath the canopy among the four sites. By 33 mo after the hurricane canopy recovery was nearly complete and PAR was only slightly higher than measurements made before the hurricane. Our results were compared with studies of changes in light environment resulting from treefall gaps and under deciduous canopies. PAR during the first 18 mo after the hurricane was similar to that recorded in small canopy gaps in other forests. Widespread defoliation caused by hurricanes can thus increase PAR beneath the canopy over large areas and consequently opportunities may arise for widespread recruitment of tree species in response to increased light levels.

Mean annual quantities of solar radiation absorbed during various stages of regeneration of a tropical rain forest in the upper Rio Negro valley of Colombia and Venezuela were estimated for the consecutive intervals between clear-cut and 1,3, 10, 20, 35, 60, 80 and 200 years of growth. Forest phytomass and litter fall data from each of these stages were used to calculate the mean annual net dry matter production per unit of absorbed photosynthelically active radiation (PAR), the PAR conversion efficiency. The quantities of PAR absorbed by the forest stands were calculated from the leaf area index values with an extinction coefficient for PAR of 0.74, a PAR albedo of 0.04, and an annual mean incoming PAR of 2.86GJ m-2y-1. Efficiency decreased with increase in successional stage. During the first 10 years of regrowth, the efficiency of conversion of PAR into above-ground phylomass averaged 0.23 g MJ-1, decreasing to 0.07 g MJ-1 over the following 50 years. Inclusion of annual root production in the calculations resulted in a small increase in PAR conversion efficiency; however, efficiency was more than doubled for some periods when the annual leaf and twig lillerfall were included. Efficiency values for above-ground production were much lower than PAR conversion efficiency values estimated for above-ground production of temperate forests.

Tropical cyclones, which are frequent along the north-eastern Australian coast, can result in severe disturbances to rain forests in the region (Grove et al. 2000, Webb 1958). Branch breakages and tree falls result in high levels of light penetration to the forest floor, which is normally heavily shaded (Turton 1992). This change in microclimate stimulates the growth of normally suppressed seedlings, the germination of seeds that are triggered by sunlight (Chazdon 1988), and often, invasion by weeds. Fragmented rain forests, that are common in the region, are particularly vulnerable to impacts of cyclones because of their large edge to forest area ratio. Appropriate management of such rain forests, following catastrophic disturbance, requires a thorough understanding of recovery processes at a number of temporal and spatial scales (Grove et al. 2000).