Defoliation Treatments Research Articles

Flooding effects on rapid responses of the invasive plant Alternanthera philoxeroides to defoliation

In experiments under controlled growth conditions it was examined how flooding affected the responses of the invasive plant Alternanthera philoxeroides to defoliation. In drained and flooded conditions, plants were subjected to five defoliation levels: 0, 10, 50, 90% removal of leaf tissue and apex removal (90% leaf tissue plus apical bud removal). Plants were harvested weekly for five weeks. In drained conditions, plant biomasses including total biomass, shoot biomass and root biomass after 50% defoliation rapidly recovered to the control plant level. They were significantly lower for the 90% defoliation and apex removal treatments compared to control plants throughout the experiment. In flooded conditions, total biomass and shoot biomass after 50% defoliation, 90% defoliation, and apex removal treatments could return to control plant levels before the end of the experiment. In 90% defoliation and apex removal treatments root to shoot biomass ratios of both drained and flooded plants were initially much higher than in control plants, but the difference disappeared rapidly. The final biomasses decreased with increased defoliation intensity in drained conditions, but no significant difference was generally found in any of the defoliation treatments in flooded conditions. The rapid re-growth of A. philoxeroides plants after defoliation may partly be responsible for its invasion success. However, defoliation capable of removing 90% of the leaf tissue may be desirable in restricting the growth of this invasive species in drained conditions.

Leaf Trichome Production and Responses to Defoliation and Drought inArabidopsis lyrata(Brassicaceae)

Leaf trichomes can protect plants against herbivory and drought, but can be costly to produce. Theory suggests that selection for reduced costs of resistance may result in the evolution of inducible defences. We quantified variation in tolerance to drought and defoliation, and tested the hypotheses that (a) tolerance is associated with cost, (b) leaf trichome production increases tolerance to drought, and (c) trichome production is increased in response to defoliation and drought stress in Arabidopsis lyrata (Brassicaceae). Eight maternal half-sib families were exposed to two watering regimes and four defoliation treatments in a factorial design. Tolerance to drought varied among families and was inversely related to leaf size, but was not related to trichome density. Family mean performance in the low-watering treatment tended to correlate negatively with that in the control treatment. Trichome production was not induced by defoliation or drought stress. The results suggest that there is genetic variation in tolerance to drought in the study population, that tolerance to drought is associated with a cost, and that trichome production does not increase tolerance to drought in A. lyrata.

Responses of Three Bromegrass (Bromus) Species to Defoliation under Different Growth Conditions

Bromegrass species are important forage crops in temperate regions of world. This study compared responses of three bromegrass species to defoliation in the greenhouse and field to determine if the former could predict responses in the latter. Experiments were conducted in 2006 and 2007 in Saskatoon (52°07′N,106°38′W), Canada on meadow bromegrass (Bromus ripariusRehm.), smooth bromegrass (Bromus inermisLeyss.), and hybrid bromegrass (B. ripariusX B.inermis) following defoliation to 5 cm stubble height. When defoliated at the vegetative stage, above-ground biomass was similar among the three species in the field, but meadow bromegrass produced greater above-ground biomass than smooth bromegrass in the greenhouse. When defoliated at the stem elongation stage, meadow bromegrass produced greater above-ground biomass than smooth bromegrass in both environments. In the field, for all defoliation treatments, tiller number was greatest in meadow bromegrass, intermediate in hybrid bromegrass, and least in smooth bromegrass. In the greenhouse, however, the three species did not differ in tiller number. Similar results were found for below-ground biomass. Thus, testing the effect of defoliation in the greenhouse environment did not accurately predict the effect in the field environment.

Do artificial and natural defoliation have similar effects on physiology of Eucalyptus globulus Labill. seedlings?

• Artificial defoliation is often used to simulate defoliation by herbivory and is usually considered a good indication of a plant’s response to a given type of damage. However, the findings of studies directly comparing the two defoliation types are inconsistent. • Here, the short term effects of artificial and insect defoliation by larvae of Paropsisterna agricola on growth, biomass allocation and photosynthetic capacity of Eucalyptus globulus seedlings were compared in a glasshouse experiment. The artificial defoliation was carried out to closely resemble the spatial patterns observed for insect defoliation. • Height and diameter increments were reduced as a result of insect defoliation, whereas artificial defoliation had no significant effect on height. Increased photosynthetic capacity was observed in response to both treatments, but the magnitude of this increase was larger in insect- than in artificially-defoliated seedlings. Significant reductions in foliar carbohydrate content and total biomass were noticeable in artificially-defoliated seedlings. Although the foliar carbohydrate levels also decreased across the crown zones following insect defoliation treatment, seedlings allocated a large amount of their biomass in the branches of the damaged zone. • Despite our best endeavours to simulate insect defoliation in the artificial treatment, the latter may not reflect accurately the full strength of the effects. However, artificial and insect defoliation were similar in their direction of the responses they caused in E. globulus seedlings.

Quantification of roots and seeds in soil with real-time PCR

Study of roots and associated organisms in soil particularly in mixed plant populations, such as pastures, is limited by difficulties in quantification of root growth and function. The research evaluated the potential of DNA quantification by real-time PCR to improve our capacity to study and understand roots in such contexts. Probes and primers were developed for two common pasture species, Trifolium subterraneum and Lolium perenne (and closely related Lolium spp.), and evaluated for specificity and sensitivity in TaqMan assays on DNA extracted from soil. Further experiments examined the ability to detect DNA in dead roots, the changes in root DNA levels of plants defoliated or treated with herbicide and the relationship between DNA and root dry weight for single and mixed plant species grown in pots. T. subterraneum DNA/PCR 200 fg/µl was detected at 17.5 cycles and L. perenne at 19.5 cycles. The assay for T. subterraneum was species specific but the L. perenne assay, as anticipated from the choice of probe, also detected some closely related species. The assays were sensitive and capable of detecting equivalent to <2 mg roots/kg of dry soil and able to quantify targets in mixed populations. DNA concentration varied with plant age and genotype and DNA in dead roots found to decay rapidly over a few days. DNA concentrations in roots were found to respond more rapidly to defoliation and herbicide treatments than root mass. This approach appears to offer a new way to study roots in soil and indicates that quantifying root DNA could provide insights into root function and responses not readily provided by other methods.

Variation in arbuscular mycorrhizal fungi colonization modifies the expression of tolerance to above‐ground defoliation

Summary 1. Plant association with arbuscular mycorrhizal fungi (AMF) has been considered a factor increasing plant tolerance to herbivory. However, this positive effect could decrease with colonization density if the benefit : cost ratio of the AMF–plant association changes. We measured plant performance and tolerance to defoliation across a gradient of commercial AMF (Glomus sp.) inoculum concentration. 2. Six genetic families of Datura stramonium were grown under greenhouse conditions and subjected to five increasing levels of AMF inoculum concentration and to defoliation treatments, i.e. the presence/absence of 50% artificial damage, following a full‐factorial design. 3. AMF colonization increased linearly with inoculum concentration while foliar area, root mass, flowering phenology and seed production expressed nonlinear functions. Plant genetic variation in the benefit function of AMF colonization was also detected. We show a negative interaction between AMF concentration and plant tolerance to defoliation. 4. Synthesis. The negative correlation between plant tolerance and AMF concentration suggests that defoliation can reduce AMF benefits and that natural variations in AMF can limit the evolution of optimum levels of tolerance. Moreover, genetic variation in the shape of the reaction norms to AMF in the presence/absence of defoliation suggests that plants may evolve in response to variation in densities of AMF and herbivores.

Can brush-cutting of Pteronia paniculata improve the composition and productivity of veld in the Succulent Karoo, South Africa?

Pteronia paniculata is an indigenous, unpalatable shrub that invades mismanaged Karooveld, resulting in degraded rangelands with low species diversity and grazing potential. We conducted a series of trials in the Succulent Karoo Randteveld near Barrydale to determine if the uniform defoliation of P. paniculata dominated vegetation at two heights (0.05 m and 0.20 m above ground level) using a brush-cutter and, in one trial, application of a second cut will improve the plant species composition, productivity and grazing capacity of the veld. Brush-cutting treatments and the uncut control all resulted in a change in species composition towards greater species diversity and more palatable species and an average increase of 540 kg ha−1 (28%) in above-ground biomass over four years. It appears that there was a pervasive improvement in species composition associated with a general decline in the cover and abundance of P. paniculata over the time-scale of the present study that was not influenced by the defoliation treatments, except for the 1996-cut treatment where the cover of P. paniculata increased. The absence of propagules of palatable species in the soil seed bank and competition from P. paniculata (a long-lived, perennial shrub) are assumed to be among the main reasons for the lack of response of the vegetation community to the defoliation treatments. Brush-cutting (in the absence of reseeding), aimed at reducing the dominance of unpalatable karoo shrubs, was more costly but not significantly better than long-term resting in improving veld composition or forage production.

Defoliation Management of Bahiagrass Germplasm Affects Cover and Persistence‐Related Responses

Bahiagrass (Paspalum notatum Flügge) cultivars are valued for their persistence under grazing and low management inputs. However, they are daylength‐sensitive and have minimal cool‐season production, resulting in high winter feeding costs in forage‐based livestock systems. New germplasm is less daylength‐sensitive, possesses greater cold tolerance, and is more productive during the cool season, but its persistence under defoliation is unknown. A field experiment quantified cover and persistence‐related responses of photoperiod‐sensitive bahiagrass (diploids ‘Pensacola’ and ‘Tifton 9’ and tetraploids ‘Argentine’ and Tifton 7) and a less photoperiod‐sensitive, cold‐adapted (PCA) diploid bahiagrass (Cycle 4) at two stubble heights (4 and 8 cm) and two harvest frequencies (7 and 21 d). Argentine cover was unaffected by defoliation treatments, but harvesting every 7 d to 4 cm for 3 yr resulted in &lt;40% cover for both Tifton 9 and Cycle 4. Averaging over defoliation treatments, stem base mass of Pensacola and Argentine was 60% greater than Cycle 4. At 4‐ and 8‐cm stubble heights, the advantage was 86 and 41%, respectively. When defoliated every 7 d, Pensacola and Argentine stem base mass was 125% greater than Cycle 4, while the difference was 23% for the 21‐d treatment. Similar but less‐pronounced responses were observed for N and total nonstructural carbohydrate (TNC) content of stem base and root + rhizome fractions. Defoliation management of PCA Cycle 4 is more critical than for Pensacola and Argentine bahiagrass, and longer regrowth intervals and taller residual heights may be required to ensure its persistence.

Effects of defoliation on carbohydrate status, nodules and spring growth in white clover in a sub-arctic climate

In order to improve the basis for utilizing white clover (Trifolium repens L) in northern agriculture, we studied the effects of defoliation intensity on spring growth in a sub-arctic climate in relation to carbohydrate and nodule status. White clover plants (cv Snowy) were studied in a pot experiment in the field on the coast of northern Norway from spring 2001 until spring 2002. The experiment was repeated with some modifications from spring 2002 until spring 2003. During the growing season from summer to autumn, the plants were totally stripped of leaves down to the stolon, cut at four or seven cm height or left undisturbed. The plants were sampled destructively in autumn, early spring and late spring and sorted into leaves, stolons and roots. The plant material was weighed and analysed for carbohydrate reserves and nodule number. Defoliation during the growing season resulted in reduced concentration of reserve carbohydrates in autumn and reduced winter survival of the stolons. The most severe defoliation treatment reduced the herbage growth in spring. In contrast, the two milder defoliation treatments had no effect on herbage growth during spring or on total plant dry matter and nodule status in late spring. In conclusion, moderate defoliation during the growing season had no effect on herbage growth the following spring nor on carbohydrate, nodule or dry matter status of the plant in late spring. Moderate defoliation increased spring growth and thereby also the nitrogen demand of the plants. This probably enhanced nodule formation and development of inactive to active nodules.

GROWING APPLE (MALUS DOMESTICA) UNDER TROPICAL MOUNTAIN CLIMATE CONDITIONS IN NORTHERN ETHIOPIA

SUMMARYLack of effective chilling during the dormant season is one of the major problems when apples are growing under a tropical climate. We evaluated the response of different apple cultivars (Golden Delicious, Gala, Fuji, Granny Smith and Jonagold) grown on M9 rootstock with different dormancy-management practices. The trials were carried out between 2004 and 2006 in a tropical mountain area (Tigray, Ethiopia), where chilling conditions are poor with the aim of improving and synchronizing the bud break and the blossoming period of these apple cultivars. Two-year-old well-feathered trees were planted in two experimental trial sites in a randomized complete block design. Trees were subjected to the following treatments in two sets of experiments: one defoliation per year only; two defoliations per year, one defoliation followed by 1% hydrogen cyanamide (Dormex) treatment; one defoliation followed by 2% Dormex treatment; one defoliation followed by 4% winter oil; one defoliation followed by 0.5% Dormex and 2% winter oil; and a control with no defoliation or dormancy breaking treatments. The results show positive effects of the dormancy breaking agents on the productivity of the trees after defoliation, with comparable results for the effectiveness of both Dormex and winter oil. There were no statistically significant differences between the Dormex doses. The defoliation treatment alone was not sufficient to break dormancy for the cultivars Golden Delicious, Granny Smith or Gala but showed promising results with dormancy breaking on Jonagold. Yields increased as a result of better flowering time synchronization within a tree but even with the dormancy treatments the length of the flowering period was still spread over five weeks, where under a more temperate climate it lasted two to three weeks. The average fruit weight of Jonagold and Granny Smith can be considered as a good fruit quality while the fruit of other diploid cultivars like Golden, Gala and Fuji were rather small, which indicates that fruit thinning by hand will be a necessity for these cultivars. Red colouration of the apples on the cultivars Gala and Jonagold was excellent and meets the standards necessary for commercialization of these fruits. The sugar concentration of the fruits and the fruit firmness at harvest was high. The results of these first trials indicate that it is possible to develop new apple production in the mountain region of Tigray, Ethiopia.

Soil moisture and plant growth responses to litter and defoliation impacts in Parkland grasslands

Soil moisture can limit plant production in cool-temperate grasslands, particularly under recent increases in drought severity and predictions of future climate change. This necessitates research that examines moisture mitigation strategies under the dominant land use of grazing. We examined the effectiveness of plant litter in regulating soil moisture dynamics and associated plant production in a native grassland and tame pasture in the Aspen Parkland of Alberta, Canada during 2007/2008, specifically testing (1) how litter and defoliation (as a surrogate for grazing) alter the micro-environment at the soil surface, (2) whether litter, in combination with defoliation, alters forage production, and (3) whether production changes can be linked to observed micro-environmental changes created by litter modification. The full factorial randomized complete block design consisted of three litter treatments (litter removal, in situ control, and double litter) and three defoliation treatments (undefoliated control, light defoliation to 6.5 cm stubble height, and heavy defoliation to 2.5 cm). Soil moisture levels were assessed over four periods after summer precipitation to examine soil moisture depletion. Plant phenological development and canopy cover were also assessed throughout 2007, and biomass was measured in August 2007 and 2008. Abundant litter (up to 14,000 kg ha −1) improved soil moisture levels in both native and tame grasslands by either maintaining higher absolute moisture levels or reducing total moisture loss after rainfall. Both litter removal and double litter reduced total growing season production compared to in situ levels at the Native Site, despite highest vegetation cover in litter removal plots. High litter levels also improved growth of individual grass tillers at this site, but negatively impacted sexual production. Conversely litter removal improved overall production at the Tame Site, although litter did enhance grass tiller recovery following mid-season defoliation at both sites, in part due to positive influences on soil moisture. Treatment effects on plant production were less pronounced in 2008 than in 2007. Overall, high litter levels improve soil moisture retention and can improve or stabilize plant production particularly in native prairie sites, and can provide an important drought management strategy in the Aspen Parkland.

Defoliation frequency effects on winter forage production and nutritive value of different entries of Dactylis glomerata L.

The objective of this study was to evaluate the effect of different defoliation frequencies on winter forage production and nutritive value of improved populations of Dactylis glomerata L. Four entries: (a) base population, (b) selected plants from mass honeycomb selection (MHS), (c) selected plants from pedigree honeycomb selection (PHS), and (d) selected plants from pedigree honeycomb selection using the combined criterion CC = x 2 (1 ‐ CV)/CV (PHS(CC)) were tested under four defoliation frequencies: (1) frequent, (2) moderate, (3) infrequent, and (4) control. Dry matter (DM) production under the frequent and moderate defoliation frequencies was 17% and 7% respectively more than under infrequent defoliation in the first year. However, in the second year, DM production under infrequent defoliation was 41% and 46% higher than under frequent and moderate defoliation, respectively. The DM production of the selected populations (b, c and d) of D. glomerata consistently exceeded that of the base population (a) under various defoliation frequency regimes during winter. Among the selected entries, MHS and PHS(CC) had consistently higher DM production than PHS for the two experimental years. Under increased defoliation frequencies, crude protein content significantly increased and neutral detergent fibre and acid detergent fibre significantly decreased compared with the control defoliation treatment in both years. There were no significant differences (P > 0.05) in nutritive value among the entries. Generally, DM production was more stable under infrequent defoliation, while nutritive value met the demands of small ruminants for maintenance and lactation.

Response of understory vegetation and soil moisture to infrequent heavy defoliation of chemically thinned juniper woodland

Targeted grazing programs that use small ruminants to control invasion of undesired woody plants are increasing across western North America. However, information regarding the potential impact of these herbivory regimes on non-target vegetation is lacking. We conducted a study in central New Mexico to determine the effects of removing all above-ground herbaceous biomass during winter in plots located beneath (under-canopy) or close to (inter-canopy) live one-seed juniper trees ( Juniperus monosperma) or dead tree snags that had been chemically treated 10 years previously. In under-canopy plots (defoliated in 2005 and 2006) understory herbaceous cover was not affected by the defoliation treatment; however, biomass production of Bouteloua gracilis and other perennial grasses decreased. In inter-canopy plots (defoliated in 2006), the defoliation treatment increased total herbaceous cover in plots between live trees, whereas biomass production was not affected. Simulated infrequent targeted grazing in a juniper woodland had slight detrimental effects on non-target plants in under-canopy plots by decreasing biomass production of native grasses under dead and live trees but had positive effects on inter-canopy plots by increasing total herbaceous basal cover. Superficial volumetric water content was not affected by the defoliation treatment, suggesting that simulated targeted grazing does not increase superficial soil moisture.

Defoliation and ENSO effects on vital rates of an understorey tropical rain forest palm

Summary 1. Rain forest understorey plants suffer leaf area losses due to natural causes or when leaves are harvested as non-timber forest products. The negative effects of defoliation on plant fitness can be exacerbated during periods with strong water shortage and high temperatures, typical during ENSO (El Nino Southern Oscillation) years in Mexico and Central America. At present, the iso- lated and combined demographic effects of ENSO events and repeated defoliation on tropical rain forest plants are poorly understood. 2. We studied the consequences of repeated defoliation and an ENSO event on vital rates (mortal- ity, growth, and reproduction) of the dioecious understorey palm Chamaedorea elegans .F rom March 1997 to March 2000 (including the 1998 ENSO year), we subjected 814 mature individuals to one of five defoliation treatments (0-100% of newly produced leaves were removed twice a year), recording mortality, growth (leaf production) and reproduction (inflorescence and seed production) every 6 months. 3. Increasing defoliation strongly reduced reproduction but had smaller effects on growth and mor- tality. Among non-defoliated palms, the probability of mortality increased with light availability, likely due to drought stress during the dry season, but this was not the case for the defoliated plants, probably because leaf area removal lowered transpiration and increased the root mass-to-leaf area ratio. During the ENSO year, growth and inflorescence production were stimulated, but survivor- ship and seed production diminished significantly, independent of defoliation level. 4. Synthesis. Variation in light availability and the occurrence of severe droughts can strongly affect demographic behaviour of understorey plants such as C. elegans, significantly influencing the effects of defoliation. Thus, strong episodic disturbance events (such as ENSO, insect outbreaks, strong storms, fires and landslides) should be taken into account to adequately understand the mechanisms that determine the population dynamics of forest plants and the potential for sustainable utilization of non-timber forest products.

MANIPULATION OF THE PRODUCTION PATTERN OF EVERBEARING CULTIVARS BY DEFOLIATION TREATMENTS

An experiment was performed in 2003 to determine the effect of defoliating plants on the production pattern of the everbearing cultivar 'Flamenco'. Defoliation was achieved by mowing-off the plants with a strimmer (a mechanical rotary cutter) on three different dates. All leaf material and emerged flowers were removed but the crown was left undamaged. Results showed that the total yield of 'Flamenco' remained largely unaffected in comparison to the non-defoliated control, provided the plants were defoliated before the second half of July. The cropping pattern was shifted so that the peak occurred two weeks later in comparison to the control, filling a trough in production. In a second experiment, in 2005, three defoliation treatments were compared on four everbearing cultivars. The defoliation treatments had no significant effect on the class 1 yield of 'Flamenco' and there was a shift in the peak of production but not as marked as in 2003. With the exception of one defoliation treatment with 'Everest', the overall yields of the other cultivars were significantly reduced. It is clear that everbearing cultivars respond differently to defoliation treatments and it is also likely that the response will vary between seasons, due to different weather patterns. Further studies are needed to determine if this practice would be a useful agronomic tool, particularly for avoiding periods of high temperatures that can reduce flowering in some everbearing cultivars.

Pod Removal, Shade, and Defoliation Effects on Soybean Yield, Protein, and Oil

Soybean [Glycine max (L.) Merr.] yield formation in field environments can be either source‐limited or sink‐limited, depending on the assimilatory capacity of the mother plant relative to the assimilate demands of the developing seeds. Our objective was to evaluate yield and seed quality relationships under source‐limited and sink‐limited conditions during the seed‐filling period, as protein and oil comprise on average more than 600 g kg−1 of the soybean seed. Field experiments were conducted in five central Minnesota environments to examine the effects of multiple pod removal, shade, and defoliation treatments on soybean yield, seed size, seed protein concentration, and seed oil concentration. Pod removal, shade, and defoliation treatments each resulted in decreased yield and altered protein and oil concentrations relative to the control. Averaged across all levels for each treatment, pod removal increased seed size 19% over the control, while defoliation and shade resulted in respective seed size reductions of 7.7 and 15% when compared to the control. Despite differential treatment effects on individual seed growth, pod removal and shade treatments each resulted in increased seed protein concentration and decreased seed oil concentration. In contrast, defoliation resulted in seeds with lower protein concentration and higher oil concentration than pod removal and shade. Pod removal and shade effects on seed quality differed primarily in magnitude, as the average oil concentration across all shade treatments was 10 g kg−1 lower than the average oil concentration across all pod removal treatments. No individual treatment resulted in increased oil concentration relative to the control.

Defoliation Management of Bahiagrass Germplasm Affects Dry Matter Yield and Herbage Nutritive Value

Bahiagrass (Paspalum notatum Flügge) cultivars are daylength sensitive and have minimal cool‐season production. A new bahiagrass breeding line is less daylength sensitive and more cold tolerant, but its dry matter (DM) yield and nutritive value responses to defoliation treatments are unknown. A 3‐yr field experiment studied effects of two stubble heights (4 and 8 cm) and two harvest frequencies (7 and 21 d) on diploids ‘Pensacola’, ‘Tifton 9’, and the less photoperiod‐sensitive, cold‐adapted (PCA) Cycle 4 bahiagrass; and the tetraploids ‘Argentine’ and Tifton 7. Stubble height had minimal effect on responses measured. When harvested every 7 d, Cycle 4 had the lowest and Argentine the greatest yield (8.4 vs. 11.8 Mg DM ha−1). When harvested every 21 d, Cycle 4 yield was similar to the other diploids (10.6 Mg DM ha−1) and less than the tetraploids (12.6 Mg DM ha−1). Cycle 4 yield distribution was similar to other diploids and generally greater in spring but less in summer than the tetraploids. Across defoliation treatments, Cycle 4 in vitro digestible organic matter (IVDOM) averaged 546 to 562 g kg−1 and was greater than all entries in 2006 and all but Tifton 9 in 2005. These data indicate that Cycle 4 was less productive than other bahiagrasses when harvested every 7 d during the warm season, as productive as other diploids at 21‐d harvest intervals, and slightly more digestible than other currently used bahiagrasses. Inferior yield under frequent harvest may imply less defoliation tolerance, thus persistence of PCA Cycle 4 should be studied further.

Late defoliation and wheat yield: Little evidence of post-anthesis source limitation

The effect of source reduction on yield and protein content of bread wheat under well-watered and mild drought stress condition in a semi-arid climate was studied. Field experiments were conducted at the Tehran University research farm during 2003–2004 and 2004–2005 growing seasons. Mild drought stress was imposed when plants were at the second node stage by repeatedly withholding watering and re-irrigating when they showed symptoms of wilting or leaf rolling. Partial defoliations (all leaf lamina other than flag leaves were removed) were imposed at booting and anthesis; complete defoliation was imposed at anthesis (defoliation treatments were applied to all plants of each plot). Drought stress caused a significant increase in the remobilization of pre-anthesis reserves to the grain. Defoliation did not significantly affect remobilization. Grain yield and 1000-grain weight was reduced slightly by drought stress, but in most cases it was not significantly reduced by defoliation. Significant changes were not observed for grain protein content between defoliated and control plots. The results suggested that grain yield of the cultivar used under the condition tested is more controlled by sink than source strength.

Barley yield losses due to defoliation of upper three leaves either healthy or infected at boot stage by Pyrenophora teres f. teres

This paper evaluates, in the greenhouse and under natural conditions, barley yield losses due to defoliation treatments of the upper three leaves either healthy or infected at the boot stage by Pyrenophora teres f. teres. Defoliation was assumed as a loss of a similar leaf area caused by net blotch disease severity of 100%. Contribution to grain yield was defined herein as a difference between defoliation treatments and a treatment where plants lost all their upper three leaves. In contrast, yield losses referred to differences in yield between defoliations and the control. In the greenhouse, removal of the antepenultimate leaf did not affect any yield component. For main stems, defoliating upper three leaves reduced grain yield by 30% and this was mainly due to flag leaf removal. These losses were similar to those induced by net blotch disease under natural conditions, but were of 42% for all tillers. Grain yield losses due to disease severity were not equivalent to the defoliation effect of a similar healthy leaf area. On the other hand and for a significant contribution to grain yield, flag leaf was dependent on the presence of the other two leaves. Inoculation and defoliation of 21 cultivars induced similar grain yield losses of 32%. However, biotic stress reduced by 40% the contribution of their upper three leaves. Under field conditions, yield losses were not significant until barley plants lost more than one upper leaf and flag leaf contribution was equivalent to that of the remaining leaves. Characteristic roots, defined as leaf coefficients for plant performance, were 0.13, 0.06 and 0.01 for the flag, penultimate and antepenultimate leaves, respectively. Because antepenultimate leaves become trivial at the boot stage, we propose that coefficients of the remaining leaves should be used when modelling yield losses due to barley foliar diseases.

Late Summer Management Can Improve Forage Yield Distribution and Nutritive Value in Temperate Grassland

Chemical defoliation (glyphosate spraying) at the end of summer and fertilization are practices proposed to improve winter forage availability in native grasslands of the Flooding Pampa of Argentina. However, there is no preceding work comparing the effect of either practice on the yield and botanical composition of these grasslands. In 2002 and 2003, three defoliation treatments were applied in mid‐February: control (C) without summer defoliation; severe summer defoliation (D) to 2‐ to 3‐cm stubble height and glyphosate, isopropylamine salt of N‐[phosphonomethyl] glycine, G, applied at 2.2 kg a.i. ha−1. Additionally, the effect of three rates of N, 0, 35, and 70 kg N ha−1and P, 0, 11, and 22 kg P ha−1 applied in combination with the three late summer defoliation treatments were researched. Early changes during fall in the biomass components ryegrass (Lolium multiflorum Lam.), warm‐season grasses (WSG), dead matter, and litter were evaluated after defoliation treatments. The effect of defoliation treatments along with N and P fertilization was followed on seasonal and annual forage yield, botanical composition, in vitro dry matter digestibility (IVDMD), and crude protein (CP). Glyphosate application increased annual ryegrass contribution while reducing WSG and legumes, mostly narrow leaf trefoil (Lotus tenuis Waldst &amp; Kit.) and annually yielded 29% less forage dry matter (DM) than the intense mechanical defoliation. Forage IVDMD and CP were increased by D and G in fall the first year. In the second year CP was low in G in winter and spring. Changes in botanical composition can explain the differences observed in quality DM traits. Fertilizer increased forage yield 2 to 10 kg DM per kg N and 8 to 24 kg DM per kg P applied. Severe defoliation applied in late summer increased fall and winter forage availability and quality without the negative effect on legumes observed with glyphosate that had a negative impact on DM accumulation and quality in spring.