No description available.

Switchgrass (Panicum virgatum L.), a warm-season grass, grows most rapidly in mid-summer when cool-season species such as tall fescue (Festuca arundinacea Schreb.) may have limited growth due to high temperature and low soil moisture availability. The objectives of this study were to investigate physiological factors and to determine management strategies that could optimize growth of switchgrass. The influences of two successive drought cycles on performance and water relation parameters of switchgrass and tall fescue were studied in growth chamber conditions. Water was withheld from conditioned plants until elongation of tillers stopped. Then pots were rewatered and a new drought cycle followed. Control plants remained well watered during this time. Both conditioned and control plants were then subjected to a challenge water stress. Total leaf elongation and soil water content (SWC) were measured daily. Leaf water potential (Ψ), osmotic potential (Π), relative water content, and concentrations of K, Na, Ca, and total free sugars were measured at the end of each water-stress cycle. Osmotic potential at full turgor (Π₁₀₀), symplastic water content (SYM) , and modulus of elasticity (∊) were determined from pressure-volume curves at the end of the two conditioning cycles. Conditioned plants of both species elongated more during the challenge water-stress than control plants and had lower SWC and Ψ when their leaf elongation ceased. Conditioned plants exhibited osmotic adjustment, accumulating free sugars and K, as a result of drought stress. Switchgrass SYM did not change, while Π₁₀₀ decreased, suggesting active salt accumulation. Increased ∊ somewhat counteracted the beneficial influence of osmotic adjustment. Tall fescue SYM increased, while Π₁₀₀ did not change. Decreased ∊ improved drought tolerance of tall fescue. Field experiments were conducted to investigate the influence of date of first harvest and cutting height on yield distribution and canopy characteristics of ‘Pathfinder’ (2-yr study), ‘Cave-in-rock’, and ‘Blackwell’ (1-yr study) switchgrass. Seasonal distribution of dry matter production was established by measuring first-harvest yields and regrowth. The canopy of Pathfinder was characterized by determining number and weight of tillers, light penetration, leafiness, specific leaf weight, and leaf area index in profiles of the canopy before harvest and in regrowth. First-harvest yields increased as date of first cut was delayed and cutting height was lowered. Cutting at 20 cm decreased the yield of first cutting in a second growing season. Plots not harvested in the first growing season gave higher yields in the second growing season compared with previously harvested plots, suggesting that any harvest may decrease subsequent yield potential. Regrowth decreased as date of first cut was delayed. A cutting height of 30 cm produced greater regrowth than cutting at 20 cm. Yields and canopy characteristics suggest that removal of growing points decreased second-harvest yields and weakened regrowth potential in the following year. To maximize regrowth to be used for grazing during July and August, switchgrass hay should be cut after 10 June and before 21 June. / Ph. D.

No description available.

Ph. D.

Cool-season perennial grass growth is suppressed during hot, dry summers. This "summer slump" reduces the availability of grazable forage. Tall-growing perennial warm-season grasses that produce 65 to 75% of their yield in midsummer may provide needed herbage during this period of reduced cool-season forage production. However, establishment of perennial warm-season species is slow and inconsistent when compared to cool-season species. Before these warm-season species can be relied on for summer forage production, a successful establishment methodology must be developed. A study was conducted to evaluate the influence of atrazine [2-chloro·4(ethylarnino)-s-triazine] and carbofuran on establishment of switchgrass (Panicum virgatum L.) and Caucasian bluestem [Boyhtiochloa caucasica (Trin.) C. E. Hubbard]. Treatments of carbofuran at 0 and 1.1 atrazine/ha placed in the row with the seed and of atrazine broadcast at 0, 1.1, and 2.2 kg/ha were imposed in all possible combinations. Another study investigated the influence of limestone, P, and carbofuran on the establishment of switchgrass. Treatments included carbofuran at 0 and 1.1 kg/ha, limestone at 0 and 4.48 Mg/ha, and P at 0 and 22 kg/ha in all possible combinations. Seedling growth rate, leaf appearance rate, plant height, and leaf elongation rate (LER) of seedlings were recorded. Seedling weight and populations were determined at the sixth leaf stage of development. Yields of forage and botanical compositions were also measured in the establishment year. Establishment was further evaluated with yield measurements the year after seeding. Carbofuran application increased first year yield, seedling weight, population, leaf appearance rate, and seedling growth rate. Atrazine reduced seedling population, weight, leaf appearance rate, LER, and yield. Carbofuran partially moderated atrazine influence. The 2.2 kg rate of atrazine tended to injure the switchgrass more than the Caucasian bluestem. In the study investigating limestone and P, carbofuran increased yield, seedling weight, seedling population, LER, seedling growth rate, and leaf appearance rate. The greatest yield, leaf appearance rate, and LER occurred with applications ofP and carbofuran. Even though initial soil pH was 5.1, limestone broadcast prior to planting did not influence seedling measurements and yields in either year. These plantings were made during 1985 and 1986, which were the driest summers in recent years. In spite of the moisture stress, acceptable stands of perennial warm·season grasses were established in both experiments in both years. Including 1.1 kg granular carbofuran/ha in the row, and broadcast application of 1.1 kg atrazine/ha at planting will improve establishment of perennial warm·season grasses. Where the P levels in the soil are low, a broadcast application of 22 kg P/ha improves chances for successful establishment. / Ph. D.

Please read the abstract in the section 00front of this document / Dissertation (MSc (Botany))--University of Pretoria, 2005. / Plant Science / unrestricted

Species possess characteristics that are considered adapted to burning and these allow them to outcompete species and dominate in fire prone environments. It has therefore been proposed that fire might have played a critical role in the observed expansion of the grasslands, during the late Miocene. The aim of this study was (i) to investigate whether plant response to fire was a result of physiology or (ii) whether it was due to phylogenetic history. This was achieved by doing a pair-wise comparison between Panicoideae (and Panicoideae) and non-Panicoideae (Danthonioideae and Aristidoideae) species. Pre-fire characteristics, that would enhance fire frequency and assist with plant recovery after burning, were compared across phylogenies and photosynthetic type. Post fire plant recovery was then followed in a field and pot comparison which examined the re-growth of the leaf canopy area, leaf mass, above-ground biomass and the cost of this to the below-ground biomass. The pre-fire characteristics showed both a photosynthetic and phylogenetic response. It was found that the species showed a greater canopy death during winter and had a lower moisture content than the species. These characteristics would potentially contribute towards a larger fuel load in the species. However, the comparison of the dead standing biomass at the end of winter and the below-ground biomass, showed a phylogenetic response with the Panicoideae having a proportionally larger dead standing biomass and below-ground biomass than the non-Panicoideae. These results suggest that not only did the Panicoideae have a larger potential fuel load but that they also shunted carbon below-ground, enabling a fast recovery after being burned. The post-fire results were more strongly determined by phylogeny than by photosynthetic type. The Panicoideae recovered faster and more completely than the non-Panicoideae grasses, possibly contributing to their success and expansion under conditions of increased fire frequency. Although recovery of the and Panicoideae were similar, frequently burnt grasslands are dominated by the Panicoideae. Hence, this dominance cannot be explained by differences in their fire responses and may be determined by the post-fire environmental conditions that potentially advantage species possessing the photosynthetic pathway. Panicoideae dominance is limited to mesic environments where fire is the likely driver of grassland expansion while more arid environments are dominated by non-Panicoideae species. Representative species from these non-Panicoid subfamilies showed poor recovery after fire. This suggests that factors other than fire were the likely drivers of these xeric grassland expansions. The ability of these subfamilies, and particularly the species, to cope with drought remains a likely selective mechanism that requires further research.

A detailed investigation into the harvesting of the standing grass crop within camps by cattle showed the following: 1. Cattle have a definite order of preference for the components of the Dohne Sourveld grass sward. The most palatable species in order of preference (based on Corrected Species Importance rating) are Themeda triandra, Tristachya hispida, Andropogon appendiculatus, Alloteropsis semialata and Heteropogon contortus. The least preferred species is Elionurus argenteus. 2. The key species, which is defined as the most abundant and productive palatable species, has been identified as Themeda triandra. 3. In harvesting the grass crop, cattle not only have a set preference for species, but also have a set grazing sequence pattern. This can be divided into three main phases. On entering the camp the animals first select the species of their choice, "creaming" these plants (grazed less than 50%) to obtain the most nutritious feed. In their daily search for food they spread their grazing over an increasingly wider area and over a wider range of species. When they can no longer obtain their daily requirements from this "creaming" operation of the more preferred species they return to the most preferred species , which are then heavily grazed (more than 50%) producing the common pattern of area grazing within a camp. These areas increase in size as the cattle graze down the species of their choice, moving to less preferred species as soon as they can no longer obtain their daily intake requirements from the more preferred species. Finally when they have no alternative they graze the least preferred species. 4. Increasing the stocking density does not alter the grazing sequence pattern or change the order of preference for the component species in the sward. It only increases the rate of utilization of the more preferred species and the various phases in the grazing sequence pattern are reached more rapidly. 5. At higher stocking densities the palatable species are more evenly utilized than at lower stocking densities. 6. Cattle graze a greater variety of species when the sward is young and actively growing. As the maturity of the sward increases the cattle concentrate on the more preferred species and are more selective in their grazing. However, their order of preference remains the same. Species that are highly preferred early in the season retain their ranking when the sward is mature. The only exception to this rule is Alloteropsis semialata. Early in the growing season this species is high on the preference list, but as it matures less is taken until when fully mature, it is rejected by cattle. 7. Elionurus argenteus is the least preferred of all the species in the Dohne Sourveld. Cattle will graze this plant as a last resort when grazing a camp early in the season, but reject the plant when it is fully mature. 8. To retain vigour and production of the Dohne Sourveld results of this investigation support the view that grazing management should be based on the principle of controlled selective grazing. Having shown that the more preferred species are over-utilized when attempting to utilize the least preferred species any grazing management that calls for total utilization of all standing vegetation should be rejected. 9. The results of this investigation supports a multicamp system for the Dohne Sourveld where high stocking densities are employed. This will result in the rapid and even utilization of the more preferred species. Management should be based on short period of stay and long period of absence. Cattle should be moved as soon as the key species have been well utilized, but not overgrazed. Cattle should only be returned to the camp when key species have made adequate regrowth following grazing. 10. A method, that is simple to use, is proposed for determining utilization of the veld and key species at different times of the year using the relationship between percentage of ungrazed plants and utilization of the sward and of Themeda triandra, the key grazing species in the sward. 11. Results show that when rested camps are grazed after May, the number of Themeda triandra inflorescences are drastically reduced. Veld that is specifically rested for the production of Themeda triandra seed should be rested from March of the year preceding expected seeding.

Indiana University-Purdue University Indianapolis (IUPUI) / Carbon dioxide (CO2) level in the atmosphere has increased steadily since Pre-Industrial times. The need for a better understanding of the effects of elevated CO2 on plant physiology and growth is clear. Previous studies have focused on how plants are affected by either elevated CO2 or salinity, one of many environmental stresses for plants. However, little research has been focused on the interaction of these two factors. In my project, three common grass species were exposed to both elevated CO2 and salinity, so that the effects of either of these factors and the interaction of the two on these species could be examined. The CO2 levels were set to 400 µmol mol-1, close to the current concentration, or 760 µmol mol-1, projected to be reached by the end of this century. Salt solutions of 0, 25, 50, 75, and 100 mM NaCl with CaCl2 at lower rates (1% of each respective molarity for NaCl) were used to water the grasses, which are unlikely to experience prolonged exposure to salt conditions beyond this range in their natural habitats. The three common grass species studied in my experiment were Kentucky bluegrass (Poa pratensis L.) and red fescue (Festuca rubra L.), both C3 cool season grasses, as well as buffalo grass (Buchloe dactyloides (Nutt.) Engelm.), a C4 warm season grass. Each treatment had five replicates, bringing the total number of experimental pots to 150. Various growth parameters were monitored, and all data was statistically analyzed for statistical significance. My results showed that elevated CO2 had a stimulating effect on most growth parameters, particularly when plants were given more time to grow. In a 100-day growth experiment, CO2 affected the number and dry biomass of plants of all species, regardless of their C3 or C¬4 photosynthetic pathways. Salinity consistently inhibited germination and growth at all stages, from germination through plant emergences, numbers of established plants, and dry biomasses at harvest. Interactive effects of CO2 and salinity did occur, though often in seemingly specific instances rather than forming clear and consistent trends. My findings suggested that growth of common grasses would be enhanced by the rising level of CO2 in the atmosphere, but the effect would be modified by environmental stresses, such as salinity.