Defoliation Of Grasses Research Articles

Sweet Immunity in Action: Unlocking Stem Reserves to Improve Yield and Quality. A Potential Key Role for Jasmonic Acid.

Common agronomic practices such as stem topping, side branch removal, and girdling can induce wound priming, mediated by jasmonic acid (JA). Low light conditions during greenhouse tomato production make the leaves more sensitive to the application of exogenous sugar, which is perceived as a "danger" in accordance with the concept of "Sweet Immunity". Consequently, source-sink balances are altered, leading to the remobilization of stem starch reserves and enabling the redirection of more carbon toward developing fruits, thereby increasing tomato yield and fruit quality. Similarities are drawn with the mobilization of fructans following defoliation of fodder grasses (wounding) and the remobilization of fructan and starch reserves under terminal drought and heat stress in wheat and rice (microwounding, cellular leakage). A central role for JA signaling is evident in all of these processes, closely intertwining with sugar signaling pathways. Therefore, JA signaling, associated with wounding and sugar priming events, offers numerous opportunities to alter source-sink balances across a broader spectrum of agricultural and horticultural crops, for instance, through the exogenous application of JA and fructans or a combination. This may entail reconfiguring and reversing phloem connections, potentially leading to an enhanced yield and product quality. Such processes may also disengage the growth-defense trade-off in plants.

Tree and grass competition in savannas: Interactive effects of soil moisture, grass defoliation and grass species identity on the growth and survival of tree seedlings

AbstractWe experimentally examined the influence of grass competition, grass species identity (taxa) and water availability on the seedling growth and survival of two dominant tree species (Vachellia (formerly Acacia) robusta and V. tortilis) of the Serengeti National Park in Tanzania. Despite being widely distributed, the species have an opposing overstorey dominance across a rainfall and productivity gradient, with V. robusta dominating the more productive mesic sites and V. tortilis in the lower productivity, drier sites. We investigated the role of different grass species, which vary in distribution and abundance across the rainfall gradient, in influencing the growth and survival of Vachellia seedlings. We found a significant effect of grass competition but no effect of grass species identity on the growth or survival of seedlings. Seedling survival was highest in the absence of grass competition, intermediate when grasses were defoliated to simulate grazing and lowest when grown with ungrazed grasses. Grass competition had a more negative effect on the stem diameter of V. tortilis than V. robusta. All V. tortilis seedlings grown under a combination of drought conditions and unclipped grasses died by the end of the experiment. However, reduced grass competition by simulated grazing improved V. tortilis seedling survival to comparable levels achieved by V. robusta species. Our study advances our understanding of tree and grass competition across environmental gradients and suggests that the presence of grass and soil moisture have species‐specific effects on tree seedling growth and survival in African savannas.

Defoliation height and potassium fertilization of Mulato II grass

A pot trial in greenhouse conditions was carried out to identify at which defoliation height Mulato II grass (Urochloa hibrida cv. Mulato II) should be managed and to determine whether potassium fertilization is necessary in soil with high potassium content. The experiment was carried out in a greenhouse in a randomized block design with six treatments and five replications. Treatments consisted of a 3 × 2 factorial arrangement, with three defoliation heights (50, 65, and 80 cm) and with or without potassium maintenance fertilization. The production characteristics and chemical composition of the forage plant were evaluated. There was no interaction effect between defoliation height and fertilization with or without potassium on any of the analyzed variables, except for mineral content in Mulato II grass. The tillers and leaves number, shoot dry matter, leaf+sheath, root system, and residue were influenced by defoliation heights and potassium fertilization, except for the leaf blades and root dry matter, leaf blade/stem+sheath ratio, and leaves number, which were not influenced by potassium fertilization. Higher shoot dry matter was observed at the heights of 65 and 80 cm; however, comparing these two heights, leaf + sheath dry matter was lower at 65 cm. Regarding the grass’s chemical composition, there was an increase in neutral and acid detergent fiber and indigestible neutral detergent fiber contents as the defoliation height increased, which resulted in lower production of potentially digestible dry matter, which can compromise the potential use of the forage by animals. Among evaluated treatments, Mulato II grass defoliation is recommended for a maximum height of 65 cm. Potassium fertilization increases the yield and the potentially digestible dry matter content of Mulato II grass, even when cultivated in soil with high potassium content.

Defoliation height and potassium fertilization of Mulato II grass

A pot trial in greenhouse conditions was carried out to identify at which defoliation height Mulato II grass (Urochloa hibrida cv. Mulato II) should be managed and to determine whether potassium fertilization is necessary in soil with high potassium content. The experiment was carried out in a greenhouse in a randomized block design with six treatments and five replications. Treatments consisted of a 3 × 2 factorial arrangement, with three defoliation heights (50, 65, and 80 cm) and with or without potassium maintenance fertilization. The production characteristics and chemical composition of the forage plant were evaluated. There was no interaction effect between defoliation height and fertilization with or without potassium on any of the analyzed variables, except for mineral content in Mulato II grass. The tillers and leaves number, shoot dry matter, leaf+sheath, root system, and residue were influenced by defoliation heights and potassium fertilization, except for the leaf blades and root dry matter, leaf blade/stem+sheath ratio, and leaves number, which were not influenced by potassium fertilization. Higher shoot dry matter was observed at the heights of 65 and 80 cm; however, comparing these two heights, leaf + sheath dry matter was lower at 65 cm. Regarding the grass’s chemical composition, there was an increase in neutral and acid detergent fiber and indigestible neutral detergent fiber contents as the defoliation height increased, which resulted in lower production of potentially digestible dry matter, which can compromise the potential use of the forage by animals. Among evaluated treatments, Mulato II grass defoliation is recommended for a maximum height of 65 cm. Potassium fertilization increases the yield and the potentially digestible dry matter content of Mulato II grass, even when cultivated in soil with high potassium content.

Depletion of carbohydrate reserves limits nitrate uptake during early regrowth in Lolium perenne L.

The mechanisms linking C/N balance to N uptake and assimilation are central to plant responses to changing soil nutrient levels. Defoliation and subsequent regrowth of grasses both impact C partitioning, thereby creating a significant point of interaction with soil N availability. Using defoliation as an experimental treatment, we investigated the dynamic relationships between plant carbohydrate status and NO3--responsive uptake systems, transporter gene expression, and nitrate assimilation in Lolium perenne L. High- and low-affinity NO3- uptake was reduced in an N-dependent manner in response to a rapid and large shift in carbohydrate remobilization triggered by defoliation. This reduction in NO3- uptake was rescued by an exogenous glucose supplement, confirming the carbohydrate dependence of NO3- uptake. The regulation of NO3- uptake in response to the perturbation of the plant C/N ratio was associated with changes in expression of putative high- and low-affinity NO3- transporters. Furthermore, NO3- assimilation appears to be regulated by the C-N status of the plant, implying a mechanism that signals the availability of C metabolites for NO3- uptake and assimilation at the whole-plant level. We also show that cytokinins may be involved in the regulation of N acquisition and assimilation in response to the changing plant C/N ratio.

Herbage accumulation, plant-part composition and nutritive value on grazed signal grass (Brachiaria decumbens) pastures in response to stubble height and rest period based on canopy light interception

Signal grass (Brachiaria decumbens cv. Basilisk, syn. Urochloa decumbens Stapf R.D. Webster) has been widely grown in the Brazilian tropics over the last 40 years, but management recommendations have been largely empirical and not based on canopy targets. This study was designed to characterise and explain the impact of canopy-based grazing strategies on herbage accumulation, plant-part composition, and nutritive value of signal grass. Treatments were factorial combinations of two stubble heights, 5 cm (SH5) and 10 cm (SH10), and two grazing frequencies, grazing initiated when 95% (LI95) and 100% (LI100) of incoming light was intercepted by the canopy. Rest periods were imposed during summer and autumn of both experimental years. Leaf blade accumulation was greater for LI100 than LI95 (9.5 v. 8.8 t/ha) associated with increased stem accumulation (4.6 v. 3.5 t/ha for LI100 v. LI95). The SH10 pastures produced more stem than SH5 pastures (4.4 v. 3.6 t/ha), with no difference in leaf blade accumulation. In general, SH10 pastures had more residual leaf blade mass post-graze, whereas SH5 pastures combined with higher grazing frequency (SH5-LI95) became more prostrate over time, increasing leaf blade proportion in post-graze forage. Over time, stubble height had more influence than grazing frequency on leaf blade proportion at pre-graze, and SH5 pastures had leafier canopies than SH10 pastures. Digestibility was less under LI100, especially when associated with SH5 stubble (SH5-LI100), regardless of season of the year. To provide optimal leaf blade yield and overall forage digestibility, particularly during warm, rainy seasons, defoliation of signal grass should include pre-graze height varying from 18 to 30 cm (95–100% of light interception) and mean stubble height close to 10 cm.

Influence of defoliation on CO 2 efflux from soil and microbial activity in a Mediterranean grassland

Defoliation of grasses affects carbon (C) input from plants into the rhizosphere and so may affect C turnover in soil. We examined the effect of grassland's defoliation on root-derived CO 2 efflux and microbial activity in Mediterranean Phaeozem. In situ partitioning of total CO 2 efflux into root-derived and microbial-derived CO 2 fluxes was performed by mesh-exclusion technique. Microbial basal respiration, N mineralization and the activity of enzymes involved in the cycling of C, N, P and S (also used to calculate microbial functional diversity) were measured in soils of defoliated and control plots. Cumulative CO 2 efflux in defoliated plots was 18% lower in 2006 and equal to control plots in 2007. The contribution of microbial CO 2 to total CO 2 efflux from soil ranged from 71% to 86% without significant differences between defoliated and non-defoliated plots. The lack of correlation between root-derived CO 2 and soil temperature after defoliation indicates that photoassimilate supply is the major determinant for root-derived CO 2. Microbial-derived CO 2 efflux was 20% lower in defoliated plots after accounting for temperature and humidity differences between the two treatments. Defoliation suppressed the activity of enzymes involved in the cycling of C, S and P and decreased basal respiration rates of soil microorganisms by 19%. In turn, defoliation stimulated activity of enzymes involved in the cycling of N, as indicated by the increase of potential nitrification rates and of leucine-aminopeptidase activity. Stimulation of N mineralization promotes a fast regeneration of defoliated plants. We confirm the presence of strong links between plant and microbial activity in a grassland community, as well as close coupling of aboveground photosynthetic activity with root-derived CO 2.

Effects of rain, nitrogen, fire and grazing on tree recruitment and early survival in bush-encroached savanna, South Africa

Moisture, nutrients, fire and herbivory are the principal factors governing tree–grass cover ratios of savannas. We investigated tree (Acacia mellifera) recruitment after fire and under conditions of maximum-recorded rainfall, nitrogen addition and grazing in a completely-crossed field experiment. We employed a similar garden experiment with the exception of the fire treatment. Tree germination in the field was extremely low, probably due to below-average natural rainfall in plots that only received natural rain, and insufficient watering frequency in irrigated plots. Due to low germination in the field experiment, no treatment significantly affected tree recruitment. In the garden experiment, frequent watering, nutrient control (i.e. no nitrogen addition) and grazing enhanced tree recruitment with significant interactions between rain, nitrogen and grazing. We infer that above-average rainfall years with frequent rainfall events are required for mass tree recruitment. Grass defoliation makes space and resources available for tree seedlings. Nitrogen enrichment increases the competitive ability of fast-growing grasses more than that of the N2-fixing tree component. In contrast to conventional wisdom that grazing alone causes encroachment, we suggest that there are complex interactions between the above-mentioned factors and ‘triggering’ events such as unusually high rainfall.

Impact of defoliation on the distribution of 15N-labelled synthetic sheep urine between shoots and roots of Agrostis capillarisand soil N pools

An experiment was carried out to test the hypothesis that defoliation of a semi-natural grassland species influenced the fate of the urea nitrogen (N) from sheep urine. The distribution of the 15N added as urea in a synthetic sheep urine (SSU) at 50 g N m−2, to Agrostis capillarisand fallow soil was followed over a period of 56 days in the plant and soil in a glasshouse microplot experiment. The grass was subject to three regimes; regular twice weekly defoliation, a single defoliation (both to 40 mm), and uncut. Regular cutting increased the total N concentration of shoots and reduced the biomass of roots, though 15N recovery in the shoots was not appreciably different from the uncut grass. A single defoliation immediately before SSU addition decreased 15N recovery. More 15N was recovered in the surface 50 mm of soil beneath the regularly cut than in the single and uncut treatments. The soil mineral N pool was the largest sink for SSU 15N. Microbial N increased threefold with the addition of SSU, but effects of the grass treatments on the proportion of SSU 15N in the microbial biomass could not be detected. It was concluded that regular defoliation of grass compared with no cutting could lead to greater losses of NO3− by leaching and denitrification.

Intervalo e intensidade de desfolhação nas taxas de crescimento, senescência e desfolhação e no equilíbrio de gramíneas em associação

O experimento foi realizado no INRA (Institut National de la Recherche Agronomique) em Theix, França. Duas gramíneas (azevém perene e festuca)foram semeadas em caixas (0,13 m²) e submetidas a três intervalos (3,5, 7 e 14 dias) e três intensidades (160, 320 e 640 bocados.m-2 a cada desfolhação) de desfolhação, com quatro repetições por tratamento. Quatro meses após a semeadura, a cada data de desfolhação, as caixas foram oferecidas, individualmente, a quatro ovelhas secas e retiradas após a realização de um determinado número de bocados. Para cada gramínea foram estudadas as taxas de crescimento, senescência e desfolhação. Os resultados mostraram que o intervalo e a intensidade de desfolhação tiveram efeitos diferenciados nas taxas de crescimento, senescência e desfolhação e no equilíbrio da associação.

Biomass Partitioning Following Defoliation of Annual and Perennial Mediterranean Grasses

Gutman, M., I. Noy-Meir, D. C. Pluda, N. Seligman, S. Rothman and M. Sternberg 2001. Biomass Partitioning Following Defoliation of Annual and Perennial Mediterranean Grasses. Conservation Ecology 5(2):1. https://doi.org/10.5751/ES-00308-050201

Seed Production in Sideoats Grama Populations with Different Grazing Histories

Frequent and intense defoliation of grasses has been associated with the evolution of “grazing morphotypes” that exhibit a variety of vegetative traits correlated with improved grazing resistance. While recovery from a seed bank is not considered an important grazing resistance mechanism, relatively little is actu ally known regarding seed (caryopsis) production in grazing morphotypes of caespitose grasses. The goal of this research was to compare components of seed production in 2 populations of sideoats grama (Bouteloua curtipendula var. caespitosa Gould & Kapadia) from nearby sites with different histories of livestock grazing. This was done using vegetative propagules of genotypes from both populations in a greenhouse study. The study was con ducted in 2 flowering seasons under conditions considered favor able for seed production. The population exposed to livestock grazing showed a genetically based decrease in seed production relative to the ungrazed population. Lower seed production per plant in the grazed population was at least partially due to reduced numbers of tillers and panicles per plant and spikes per panicle that may be associated with selection for grazing tolerance. The grazed population also exhibited lower average seed production per spike indicating lower inherent floral fertility. Seed production was not closely correlated with vegetative traits associated with increased grazing tolerance, nor was there evidence of obvious physiological trade-offs related to decreased seed production in the grazed population. Lower seed production potential in populations of sideoats grama intensively grazed by livestock may lead to reduced potential for seedling colonization.

Effects of grazing by free‐ranging cattle on vegetation dynamics in a continental north‐west European heathland

Summary 1. Abandonment and eutrophication are major threats to traditional pastoral landscapes and their wildlife in Europe. Social and economical developments have rendered traditional pastoralism impracticable. More knowledge is needed about the effects of grazing with free‐ranging herbivores, which is increasingly used as a substitute for the traditional herding system. 2. We studied the effects of free‐ranging cattle on the recovery of Calluna heather, tree encroachment and plant species richness in six habitats in a grass‐rich Dutch heathland during a 10‐year period. The habitats differed in soil fertility, initial heather–grass ratio and developmental stage of Calluna. 3. Despite preferential grass defoliation, cattle grazing neither reduced grass cover in grass heath nor prevented grass invasion in heather. Grass invasion failed only in a nitrogen‐poor turf‐stripped Calluna heath. Grazing induced a substantial Calluna recovery in grass heaths on podzolic soils, but its recovery failed in grass heath on a phosphorus‐rich medieval arable field. As a consequence, the grass–heather mosaics generated by free‐ranging cattle were restricted to habitats of intermediate soil fertility. 4. Grazing did not prevent encroachment by pine and birch. Removal by the site manager prevented conversion of 10–20% of the open heathland to forest. 5. During the first 5 years, grazing induced a significant increase in species richness in all habitats. During the second 5 years, species richness stabilized in grass heath and heather–grass mosaics and it declined in the pioneer Calluna heaths. 6. We found indications of various nutrient‐mediated grazing effects on the competitive balance between grass and woody pioneers. These suggest that nutrient‐mediated feedback might be an important explanatory mechanism for the described vegetation mosaic cycling in heathlands. 7. Free‐ranging grazing did not remove the high atmospheric nutrient inputs of the whole area. Substantial amounts of nutrients were redistributed from the grass lawns to the forest. 8. Free‐ranging grazing combined with tree cutting appeared to be a suitable management regime for the maintenance of species‐rich open heathlands with dynamic grass–heather mosaics. 9. Without tree cutting, free‐ranging grazing would have created dynamic tree–grass–heather mosaics in open heathland. Wood–pasture landscapes are fundamentally different from the open heather‐dominated heathlands produced by the traditional sedentary farming system. Integrated grazing of heathland, nutrient‐rich farmland and forests accelerates this change. Alternative grazing regimes are discussed.

Plant and soil microbial responses to defoliation in temperate semi-natural grassland.

There is much interest in understanding the nature of feedback mechanisms between plants and soil organisms in grazed ecosystems. In this study, we examine the effects of different intensities of defoliation on the growth of three dominant grass species, and observe how these plant responses relate to the biomass and activity of the microbial community in the root zone. Our data show that grassland plants with varying tolerances to grazing have markedly different growth responses to defoliation, and that these responses vary with the intensity of cutting. Defoliation of grasses which are tolerant to grazing, namely Festuca rubra and Cynosurus cristatus, leads to a reduction in root mass and an increase in the allocation of resources to shoots. In contrast, defoliation of a grass with low tolerance to grazing, Anthoxanthum odoratum, had little effect on root mass, but increased the relative allocation of resources below-ground. In all plant species, defoliation led to an increase in soil microbial biomass and C use efficiency in the root zone. This response was greatest in the root zone of A. odoratum and is likely to be related to changes in root exudation pattern following defoliation. The significance of these changes in relation to soil nutrient dynamics and plant nutrient uptake during regrowth require further exploration.

Picloram, fertilizer, and defoliation interactions on spotted knapweed reinvasion

Spotted knapweed (Centaurea maculosa Lam.) management may be enhanced by integrating strategies that stimulate and maintain competitive grasses. The objective of this study was to determine if picloram, fertilizer, and timing and frequency of grass defoliation could be integrated to minimize spotted knapweed reinvasion. Sixteen chemical treatments [4 picloram rates (0.00, 0.14, 0.28, and 0.42 kg a.i. ha- 1) and 4 fertilizer rates (source: 16-20-0, N-P-K; material: 0.0, 66, 132, 198 kg ha-1)] were applied in the spring of 1994 to 4 by 4 m plots and factorially arranged in a randomized-complete-block design. Within each plot, 6 grass defoliation treatments were randomly applied to 1 by 1 m sub-plots. From 1994 through 1997, 60% of the aboveground grass biomass was hand clipped and removed from the plots during the spring, summer, fall, alternating spring/fall, all 3 seasons. A control received no grass defoliation. The experiment was replicated 4 times at 2 sites dominated by spotted knapweed. At peak standing crop in 1997 spotted knapweed density, grass and spotted knapweed biomass; and percent cover of spotted knapweed, grass, litter, and bare ground were measured. Data were analyzed as a split-plot using analysis of variance. Four years after treatment all rates of picloram reduced spotted knapweed density, biomass, and cover, and increased grass yield. Nitrogen and P fertilizer tended to increase spotted knapweed density and biomass. Nitrogen and P fertilizer plus defoliation in all 3 seasons caused a greater increase in spotted knapweed reinvasion at the site with Kentucky bluegrass (Poa pratensis L.) than the site with timothy (Phleum pratense L.) and smooth brome (Bromus inermis Leys.). Fall-only defoliation and no defoliation appear to deter spotted knapweed reinvasion better than defoliation in all 3 seasons and alternately in the spring and fall.

Seed production in sideoats grama populations with different grazing histories

Frequent and intense defoliation of grasses has been associated with the evolution of "grazing morphotypes" that exhibit a variety of vegetative traits correlated with improved grazing resistance. While recovery from a seed bank is not considered an important grazing resistance mechanism, relatively little is actually known regarding seed (caryopsis) production in grazing morphotypes of caespitose grasses. The goal of this research was to compare components of seed production in 2 populations of sideoats grama (Bouteloua curtipendula var. caespitosa Gould Kapadia) from nearby sites with different histories of livestock grazing. This was done using vegetative propagules of genotypes from both populations in a greenhouse study. The study was conducted in 2 flowering seasons under conditions considered favorable for seed production. The population exposed to livestock grazing showed a genetically based decrease in seed production relative to the ungrazed population. Lower seed production per plant in the grazed population was at least partially due to reduced numbers of tillers and panicles per plant and spikes per panicle that may be associated with selection for grazing tolerance. The grazed population also exhibited lower average seed production per spike indicating lower inherent floral fertility. Seed production was not closely correlated with vegetative traits associated with increased grazing tolerance, nor was there evidence of obvious physiological trade-offs related to decreased seed production in the grazed population. Lower seed production potential in populations of sideoats grama intensively grazed by livestock may lead to reduced potential for seedling colonization.

Grass Defoliation Intensity, Frequency, and Season Effects on Spotted Knapweed Invasion

Preventing the invasion of uninfested rangeland is central to managing spotted knapweed (Centaurea maculosa Lam.). Intensity, frequency, and season of grass defoliation determine the ability of grasses to tolerate grazing and resist weed invasion. We hypothesized that as grass defoliation intensity increases, spotted knapweed cover, density, and biomass would increase, that increasing defoliation frequency would increase the intensity effect, and that spring defoliation would cause a greater increase in spotted knapweed than summer defoliation. We hand clipped grasses in 1 m2 plots at 2 spotted knapweed infested Idaho fescue (Festuca idahoensis Elmer) sites in western Montana. Clipping treatments were 4 intensities (0, 30, 60, and 90% of the foliage), 3 frequencies (1, 2, and 3 at 14-day intervals), and 2 seasons (spring beginning in mid-May and summer beginning in mid-July), factorially arranged in a randomized-complete-block design with 4 replications for a total of 24 treatments per replication. Treatments were applied in 1995 and 1996. By 1997 grass cover and density were reduced by defoliation intensity of 90%. Defoliation frequency greater than once caused a reduction in grass cover and density at the 60% intensity. Spring defoliation caused a greater reduction in grass cover and density than summer defoliation. Grass biomass was reduced by the 30% defoliation treatment. Grass defoliation intensity greater than 60% caused an increase in spotted knapweed cover and density. Defoliation more than once increased spotted knapweed cover. Spring defoliations increased spotted knapweed cover compared to summer defoliations. Spotted knapweed biomass was not affected by defoliation treatments. Our study suggests that an annual single grass defoliation of 60% or less, regardless of the season, will not increase spotted knapweed invasion on rangeland.

Grass defoliation affecting survival and growth of seedlings of Acacia karroo, an encroaching species in southwestern Zimbabwe

Two experiments were conducted, one in the field and the other in the greenhouse, to investigate the effects of the intensity and frequency of grass defoliation on the survival and growth of Acacia karroo seedlings. In the greenhouse, seedlings growing with heavily clipped grasses had higher biomass production than those competing with moderately clipped grasses. Root/shoot ratios were higher in treatments with undipped grasses. There was a negative relationship between grass root production and A. karroo biomass production. The field experiment was carried out in two paddocks, one previously heavily‐grazed and the other lightly‐grazed. Grazing in both paddocks was simulated by artificial defoliation. Generally more A. karroo seedlings emerged under lightly defoliated treatments. Clipping frequency had a strong effect (P=0.066) on the survival of emerged seedlings during the wet season. There were no differences in survival rate at the end of the dry season. Though grass defoliation was shown to enhance seedling growth under controlled conditions, no evidence was found to suggest that seedling establishment during the first year is influenced by the intensity of grass defoliation.

Selective grazing by dairy cows in the presence of dung and the defoliation of tall grass dung patches

AbstractTwo studies investigated the effect of contaminated pasture on selective grazing, overall grazing behaviour and the process of defoliation of dung patches through experiments targeted at four major questions: (a) how does relative utilization of tall and short grass change as the sward is grazed down? (b) what effect does herbage mass and sward height have on the relative utilization of short and tall grass? (c) how are tall grass patches actually utilized by cattle? and (d) how is overall grazing behaviour influenced by contamination of the sward?Experiments were conducted in mid to late season using Friesian dairy cattle. In experiment 1, two -pasture types (topped sward (T)v.grazed-only sward (G)) were used. The distribution of bites on tall grass from both pasture types indicated that the grazing animals tended initially to graze short grass when they met a new sward, and then select tall grass as the swards were progressively grazed down. This switch happened earlier in the defoliation process in the topped sward. In experiment 2 observations were conducted on previously grazed and previously ungrazed swards. The distribution of bites on tall grass showed a similar trend to that found in experiment 1 and as the sward was gradually grazed, biting rate significantly declined. There was also a significantly higher total grazing time on the previously ungrazed sward (no contamination by dung). Comparing data based on a consistent biting rate (calculated as the time for 20 consistent bites) and natural biting rate (calculated as the total time for 20 bites) suggested that the grazing animals had increased difficulty in handling tall grass which may explain the declining biting rate as the swards were being grazed down and more bites were directed at tall grass. The defoliation of tall grass dung patches appeared to be concentrated around the edges of the patch. The average area of sward affected by a single dung pat was 1·04 m2 measured at the pre-grazing stage and was markedly reduced to 0·51 m2 at the post-grazing stage. In conclusion, selective grazing is likely to exist due to the presence of dung and conditioned by dung distribution and sward type and this in turn modifies biting rate during grazing down of a sward.

Herbage yield, tiller number and root system activity after defoliation of prairie grass (Bromus catharticus Vahl)

The effects of defoliation upon root and shoot systems of prairie grass (Bromus catharticus Vahl) were examined in both field and pot studies. The varieties used were 78–32 (HY), a high‐yielding variety; 79–42 (LY), a low‐yielding variety; and the commercial variety Grasslands Matua. In the field, the presence of roots in early and late spring was estimated by measuring uptake of [32P]phosphate by roots; herbage yields and tiller numbers were recorded. In a pot study, root and shoot dry‐matter (DM) yields were analysed. In the field, roots were detected to a depth of 1·2 m. After defoliation to a height of 0·1 m, root presence decreased more than 50% at depths of 0·6 m for LY and 1·0 m for Matua in early spring and at several depths for each variety in late spring. After a second defoliation, the apparent growth rate of shoots decreased by 35% in relation to the first regrowth period. In pots, shoot DM and root DM of control plants (undefoliated) had the following allometric relationship of the form: In (shoot DM) = 0·61 + 1·14 ln (root DM) (r2= 0·81). After defoliation, compared with undefoliated controls, the relative growth rate of shoots and total herbage yields were higher, but root and stubble DM were lower in all three varieties. Pooled root DM means were 10·3 and 6·8 g plant−1 and pooled stubble DM means were 12·7 and 7·6 g plant−1 for control and defoliated plants respectively. HY produced heavier tillers than LY, pooled means being 0·94 and 0·53 g DM tiller−1 (field study) and 3·44 and 2·05 g DM tiller−1 (pot study) for HY and LY respectively. HY had 5–6 green leaves per tiller, whereas LY had 3–4. Developed green leaves were heavier in HY (58 g m−2) than in LY (48 g m−2). It is suggested that differences in both leaf parameters may be related to higher herbage yields for HY than LY.