Palatable Perennial Grasses Research Articles

Applying two remotely-sensed methods for monitoring grazing impacts in the Australian arid zone

Objective monitoring methods that reliably identify grazing impact are required for long-term sustainable management in the arid rangelands. In Australia such methods must contend with highly unpredictable rainfall and large paddocks incorporating spatially complex land types with differing grazing preferences. Retrospective analysis of data collected following very dry and very wet periods removes vegetation dynamics generated by lesser rainfall events and should increase our ability to separate grazing effects from seasonal variability. Two remote-sensing methods were tested for their capacity to quantify trends over 30 years in vegetation-cover dynamics on a pastoral lease in central Australia with a history of heavy grazing. Following destocking by 2002, one section became a conservation reserve and another transitioned to a research station. During drought, the Dynamic Reference Cover Method (DRCM) showed that ground-cover deficit was less negative on both areas towards the end of the study. This improvement was attributable to increased ground cover after removal of grazing, followed by a reduced, but variable, grazing intensity on the research station and the spread of an introduced palatable perennial grass. Ground-cover response following rainfall was highest in 2011. Likewise, increased ground cover meant that the percentage Cover Production Loss (%CPL) index, calculated using grazing gradient methods (GGMs), was considerably less than a decade earlier following similar rainfall. Results from an associated recovery index (R) were inconclusive. Landscape heterogeneity potentially affected calculation of cover deficit using DRCM but, because heterogeneity was stable over time, reported change between dry years reliably indicated a trend owing to grazing. Interpreting trend from successive %CPL values in wet periods was complicated on the research station by altered waterpoint locations being superimposed on pre-existing degradation; however, the method should be effective in large paddocks with stable waterpoint locations. Despite their limitations, both methods can assist in objectively judging the long-term sustainability of grazing practices in contrasting seasonal conditions.

Enhancement of palatable perennial grasses by disturbance and seed addition in degraded native grasslands of the dry pampas in Central Argentina

Land use has changed the original composition of the natural grasslands in the dry pampas of central Argentina, to the detriment of palatable perennial grasses (PPG). Long-term elimination of anthropogenic disturbances has not been enough to effectively recover the PPG, which suggests that availability of seeds and/or habitats may be limiting their recruitment. Therefore, our objectives in a degraded grassland site were to assess: (1) the germinable soil seed bank and (2) changes in botanical composition after addition of PPG seed following canopy disturbance. In autumn 2015, we randomly selected 10-paired plots, measured the aboveground biomass and took soil samples to assess the germinable soil seed bank. Afterwards, we disturbed the resident vegetation in each paired plot and either added PPG seeds or left the plot without any seed addition as a control. We quantified the seedling density of seeded and spontaneous species in the spring of 2015 and 2016, and the aboveground biomass at the end of 2017. Annual grasses and forbs (AGFS) dominated the soil seed bank, while PPG and unpalatable perennial grasses (UPG) occurred at low seed density and richness. Addition of PPG seeds resulted in the successful establishment of one key seeded PPG (Poa ligularis). In the spring 2017, the aerial biomass of PPG was highest in the seeded plots, where P. ligularis accounted for 94%. Our results showed that PPG were poorly represented in the soil seed bank of the degraded grassland and that disturbance of the resident vegetation combined with PPG seed addition enabled successful recruitment of a key PPG. So, enhancement of the pastoral value of degraded dry pampas grasslands would be possible through disturbance of the resident vegetation and addition of PPG seed.

Herbaceous responses to herbivory, fire and rainfall variability differ between grasses and forbs

Herbivory and fire are known to shape plant community structure and function in savanna ecosystems worldwide, yet these drivers are increasingly being altered in their behaviour, or completely excluded. Furthermore, herbaceous responses to rainfall in semi-arid and arid savannas may outweigh the effects of herbivory and fire, especially in nutrient-rich ecosystems. Despite considerable recognition of herbaceous responses to drivers in savanna systems, few studies consider grasses and forbs as distinct herbaceous functional entities. To address this shortcoming, we used long-term herbaceous vegetation data collected from herbivore and fire exclusion treatments in the Kruger National Park (KNP) in South Africa. We investigated response patterns in herbaceous abundances per functional group for three sampling years, which represented different rainfall conditions (i.e., average for 2010, above average for 2001, and below average for 2015. Dynamic shifts in forb versus grass dominance were revealed across treatments. Surprisingly, palatable annual forb communities seemed more resilient to herbivores, or their removal, dominating over palatable annual grass communities. Furthermore, unpalatable perennial forbs were revealed to be more resilient to herbivore presence than their absence. Palatable perennial grass abundances were consistently enhanced by above-average rainfall conditions, whereas equal dominance of palatable grasses and forbs was evident during the drought year. This highlights that palatable perennial functional groups (not grasses alone) provide important ecosystem functions, such as forage stability, and hence functional redundancy to absorb disturbances such as droughts. Moreover, rainfall variability and herbivory are considered the main drivers of palatable perennial functional groups in this nutrient-rich ecosystem. Palatable perennial grass abundances differed significantly from other alleged unfavourable herbaceous functional groups (e.g., unpalatable perennial grasses, annual grasses, and annual and perennial forbs) and, although they presented contrasting patterns for each sampling year, fire and herbivory treatments, palatable perennial grasses remained the dominant functional group in this ecosystem type. Therefore, this study did not support previous findings that conditions such as drought, fire and herbivory favour unfavourable functional groups at the expense of palatable perennial grasses. Observed patterns provide evidence of a dynamic and less predictable coexistence between grasses and forbs.

Long-term response of an herbaceous sward to reduced grazing pressure and rainfall variability in a semi-arid South African savanna

Reduced grazing pressure is expected to promote recovery of degraded semi-arid savanna. Transformation of degraded livestock ranches to a wildlife reserve in 1982 created a system with about one-third of the grazing pressure, which was maintained until 2011. Herbaceous composition and condition were monitored on four occasions between 1982 and 2011. Baseline measurements were taken following eight years of above-average rainfall conditions. Palatable perennial grasses and sward condition deteriorated markedly in response to severe, sustained drought during the early 1990s. Perennial grasses did not respond to a single record high rainfall season (2000) but recovered following a sequence of four above-average years (2008–2011). This monitoring study illustrated that recovery of a degraded semi-arid savanna may require decades of reduced grazing pressure for some degree of recovery in the absence of intervention. An appropriate time frame for management of these systems is in the order of a century.

Testing for Thresholds in a Semiarid Grassland: The Influence of Prairie Dogs and Plague

State-and-transition models for semiarid grasslands in the North American Great Plains suggest that the presence of herbivorous black-tailed prairie dogs (Cynomys ludovicianus) on a site 1) creates a vegetation state characterized by increased dominance of annual forbs and unpalatable bunchgrasses and increased bare soil exposure and 2) requires long-term (> 40 yr) prairie dog removal to transition back to a vegetation state dominated by palatable perennial grasses. Here, we examine 1) how the recent history of prairie dog occupancy on a site (1–10 yr) influences the magnitude of prairie dog effects on vegetation composition and 2) how occupancy history affects vegetation dynamics following extirpation of prairie dogs. We used a natural experiment in the shortgrass steppe of northeastern Colorado, USA, where prairie dogs were extirpated from multiple sites during an outbreak of epizootic plague. On sites occupied by prairie dogs for 1–4 yr prior to extirpation, plant cover and composition recovered to conditions similar to unoccupied sites within a single growing season. Larger reductions in perennial C4 grasses occurred on sites occupied for the prior 7–10 yr compared to sites with shorter occupancy histories (< 6 yr). On sites occupied for the prior 7–10 yr, C4 perennial grasses recovered after 5 yr following prairie dog extirpation; in addition, C3 perennial graminoids and forbs remained more abundant (compared to sites with no history of prairie dogs) throughout the 5-yr period. Our findings showcase that prior site occupancy (up to 10 yr) by prairie dogs did not induce irreversible shifts in vegetation state in this semiarid grassland. Rather, vegetation changes induced by prairie dogs represent primarily a phase shift in landscapes where prairie dog populations are regulated by epizootic plague.

Effect of prescribed fire on forage production and nutritive value of the perennial grass Saccharum griffithii

The effect of fire on growth and nutritive value of Saccharum griffithii Munro ex Boiss-ravennae (Linn.) Murr. hybrid, a low palatable perennial grass in the semi-arid region of the highlands of Balochistan, Pakistan, was evaluated during 2008 and 2009. Prescribed fire treatments were applied in February 2008 and 2009 at the dormant stage of vegetation. Different growth parameters (tiller height, tiller number/plant), forage production (fresh, dry) and nutritive value (crude protein, ash) were compared with control (unburnt) plots. Fire increased the number of tillers, and the fresh and dry forage production in both years. Fire also improved the crude protein contents. Prescribed fire at the dormant stage of vegetation can be used as a vegetation tool to improve the forage quality and quantity of the less preferred grass species Saccharum griffithii Munro ex Boiss-ravennae (Linn.) Murr. hybrid.

Resilience and recovery potential of duneland vegetation in the southern Kalahari

Many dune fields around the world have undergone alternating periods of mobilization and stabilization in response to changes in wind power and rainfall. However, in modern times disturbances associated with land use are believed to be a dominant factor contributing to the activation of stabilized vegetated dunes in drylands, while the reduction in human activities such as grazing and farming may lead to stabilization of once active dune fields. The Kalahari region of southern Africa has recently begun to exhibit visible signs of dune mobilization, a process that could lead to an activation of aeolian transport in the region with important implications for the biogeochemistry of downwind terrestrial and marine ecosystems. It is still unclear whether the region is poised at a tipping point between its current state (i.e., vegetated fixed linear dunes), and a “degraded” state (i.e., barren and active dunes). Here we investigate the ability of the landscape to recover from the degraded state by assessing the resilience of duneland vegetation and evaluating the vegetation and soil characteristics. Using field observations and soil seed bank experiments, we show that palatable perennial grass cover is reduced while the seedbank is depleted on grazed dunefields. Conversely, the interdunes generally exhibit relatively rich seed banks. Soils from grazed and ungrazed sites show that plant available nutrient contents are not significantly different; therefore, soil nutrients are likely not a major factor limiting the recovery of perennial vegetation in this region. It is observed that the perennial grasses reestablish on the recovering dunes after grazers have been excluded, indicating that the landscape is still able to recover after years of denudation and that any irreversible shift to a stable degraded state is likely during extended periods of disturbance and/or climatic shifts that promote the degraded state. We also find that changes in grass cover, grass community composition, and seed bank can serve as indicators of whether the system has irreversibly shifted from a vegetated to a bare dune state.

The influence of controlled fires on a plant community in the south of the Caldenal, and its relationship with a regional state and transition model

Fire plays a fundamental role in the structure of vegetation communities in the Caldenal. However, this effect has decreased considerably since the introduction of domestic cattle, which reduces the amount of fine combustible material. Objectives of this study were (1) to analyze the diversity and composition of a typical vegetation community in the south of the Caldenal after exposure to different fire frequencies, and (2) to show the convenience of using controlled fire for reversing the processes of scrub formation to states with more diverse and productive grassland communities. Forty-five herbaceous and woody species were recorded. Treatment C (control) showed the greatest richness and diversity in December 2006 and 2007. After the last controlled fire, the composition of the vegetation community differed significantly between treatment C and treatments T1 and T2 (with burns every 3-4 and 7-8 years, respectively). This change was mainly due to the proliferation of palatable perennial grasses. Vegetation composition was similar for C and T2 (ANOSIM, R=0.01, p=0.32 and R=0.06, p=0.01, respectively) in 2006 and 2007. However, treatment T1 showed significant changes (ANOSIM, R=0.42, p=0.01). Nasella clarazii, which is a characteristic palatable grass in the region, was the perennial forage grass with greatest cover in treatment T2, whereas it was scarce in treatments C and T1. Unpalatable grasses and bushes grew at sites without fire, e.g. treatment C in this study. Our results show that use of controlled fire of intermediate frequencies would favor the aerial cover of palatable perennial grasses in states that are most useful for sustainable cattle production.

Safe pasture utilisation rates as a grazing management tool in extensively grazed tropical savannas of northern Australia

The concept of safe pasture utilisation rates is frequently promoted as a tool for use in setting livestock numbers in perennial grass pastures in northern Australia’s tropical savannas to achieve a grazing intensity that is ecologically and economically sustainable. However, recommended pasture utilisation rates have been defined and applied in several ways, and this has led to some confusion among managers, researchers and advisers about their appropriate use. In order to reduce this confusion, this paper reviews the ecological basis and use of safe pasture utilisation rates as a management tool, concentrating on two common ways (i.e. strategic and tactical) in which they are applied. The main objective of both approaches is to limit the intensity of grazing of perennial grasses during the growing season when they are most sensitive to defoliation. When used in a strategic way, safe pasture utilisation rates provide an indication of long-term safe stocking rates that will avoid pasture deterioration in most years. Alternatively, they are used in a tactical sense to adjust stocking rates to track forage production, usually on an annual basis. Using the tactical approach, stocking rates are set at the end of the growing season in order to use a ‘safe’ proportion of the standing forage available at that time during the subsequent 12 months. Thus, stocking rates for one growing season are based on the quantity of forage available at the end of the previous growing season. In areas with high year-to-year variability in pasture growth this may lead to overgrazing during the growing season. This paper concludes by suggesting several strategies to better manage pasture utilisation levels under a tactical management approach to ensure that palatable perennial grass populations persist in the long term. A call is also made for researchers and land management advisers to be clear in the way recommended safe pasture utilisation rates are defined and intended to be used.

Water points and their influence on grazing resources in central northern Namibia

AbstractDebate among scientists about ecological dynamics and appropriate management of semi‐arid rangelands has led to a challenge of received wisdoms of range management and pastoral development in dryland Africa. In our study, we investigated impacts of grazing on grass composition around permanent water points along a pipeline and around a traditional hand‐dug well in an important grazing area in central northern Namibia. Grass species abundance and selected environmental variables sampled along transects radiating out from these water points were analysed using Canonical Correspondence Analysis (CCA). Significant grazing‐induced changes, manifested by palatable perennial grasses being replaced by less palatable annual grasses, were identified around water points along the pipeline. There annual grasses Schmidtia kalihariensis and Aristida stipioides dominate the vegetation as far as 5 km from the water points. No significant grazing‐induced changes in grass composition were observed around the hand‐dug well. Private ownership leading to stronger control of access to traditional wells compared to the open access water points along the pipeline seems to be a key factor preventing overutilisation of grazing resources around the former. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Changing grazing systems in central north Namibia

AbstractCurrent and historical livestock movement patterns are explored in a semi‐arid communal environment in central–north Namibia, placed in context of increasing population, large‐scale enclosures, increased water supply and changing vegetation. Farmer's knowledge of movements, rangeland potential, vegetation condition and plant indicators was gathered and analysed with various methods such as Canonical Correspondence Analysis (CCA). Local rangeland units were mapped with Landsat TM imagery and analysed with a Geographical Information System (GIS). The data suggest that good grazing was previously maintained by low herbivore pressure and frequent fires in a management regime controlled by hunter‐gatherers and limited permanent water supply.Population increase in settled areas starts migration to more fertile land units in previous prime grazing areas causing a conflict between grazing and cropping and a decrease in grazing condition triggering further migration and need for new water supply. Recent large‐scale enclosures are targeting predominantly more fertile land units with the most palatable perennial grass species and water, causing further conflict for communal farmers.Having reached the frontier of the traditional land there is no space for further expansion, resulting in the need to adapt to uncertainty with annual grasses more dependent on rainfall. Livestock movement patterns have changed drastically for large herd owners from transhumance and migration to largely permanent cattle posts. Small herd owners face increasing longer movements between kraals, water points, depending on less suitable and decreased unfenced grazing lands. Copyright © 2006 John Wiley &amp; Sons, Ltd.

The Effects of Perennial Grasses, Stocking Rate and Rainfall on Sheep Production in a Semi-Arid Woodland of Eastern Australia.

A grazing study was conducted in a semi-arid mulga (Acacia aneura) woodland to determine the relative importance of variables controlling sheep production in this environment. The study was based on six sheep stocking rates (0.3-0.8 sheeplha) with the exclusion of other large herbivores including kangaroos and feral goats. The availability of forage and its components, together with the wool growth and live weight of the sheep were measured every three months for seven years from October 1986. Annual rainfall accounted for 60% of the annual variation in live weight gain per head and 79% of the variation in wool growth per head. Stocking rate accounted for an additional 10% of the variation. Rainfall accounted for this large proportion of variation by its strong influence on the yield of green leaf which accounted for 78% of the variation in live weight gain and 66% of the variation in wool growth. Green leaf was a determinant of sheep production because of its superior crude protein content and digestibility compared to dry leaf and stem. The species comprising this green leaf had an influence on sheep production only during drier years when the presence of palatable perennial grasses became important to sheep production. The frequency and yield of perennial grasses were greatly diminished at high stocking rates. This change in forage composition caused a decline in sheep productivity as detected by a loss of linearity in the relationship between stocking rate and live weight gainlhead. We conclude that adjusting stocking rates is required in this environment for maintenance of long-term sheep productivity and landscape function. The loss of palatable perennial C, grasses over much of the region has reduced livestock productivity and increased the variability in forage availability. Stock numbers should be adjusted at critical times to avoid losing perennial grasses that are important for sheep production in drier years. New grazing strategies are required to restore these grasses to the region.

Grass species composition, yield and quality under and outside tree crowns in a semi‐arid rangeland in south‐western Zimbabwe

A two‐year study was conducted in lightly grazed areas of Matopos Research Station, Zimbabwe, to evaluate the impact of widely spaced trees on understorey grass composition, yield and quality. The study trees were Terminalia sericea and Acacia karroo. Ordination techniques using grass density and biomass as indices separated quadrats according to soil type but not grass species according to understorey or open areas or according to tree species. Grass yield under tree crowns was similar to open areas, in contrast with most reports where understorey areas had higher yields. The high understorey grass quality that has been reported from savanna areas exhibiting grass composition differences was not expressed. Selective grazing of palatable perennial grasses growing in association with tree crowns and their eventual replacement by low‐yielding and less‐palatable grasses that normally grow in open areas is proposed as an explanation of the unique tree‐grass interaction scenario of this study. In areas where this replacement has occurred, recovery may require management interventions.

Spatial and Temporal Pattern in the Grazing of Grasses by Sheep Within a Semi-Arid Wooded Landscape.

Grazing the woodlands of semi-arid and arid Australia by domestic stock has extensively collapsed perennial *grass populations and thereby fostered woody plant increase. This study examined the pattern of grazing of individual grass plants by sheep in the landscape of a semi-arid woodland, and a model was developed describing the spatial and temporal influences on the grazing pressure placed on plants. Plants of two widespread perennial grass species differing in palatability, Eragrostis eriopoda and Thyridolepis mitchelliana, were examined weekly in two contrasting periods at the CSIRO Lake Mere Research Facility. The plants were located throughout the landscape in lightly- and heavily- stocked paddocks. Patterns of grazing in space and time were determined by examining the grazing of marked tillers. No preference was shown for previously ungrazed plants and only occasionally were previously grazed plants preferred. Thyridolepis mitchelliana plants were slightly preferred over E. eriopoda plants. Landscape zones receiving water and nutrients from elsewhere were preferred for grazing but the effect was weak. The foliage biomass of herbaceous plants in the immediate vicinity of a grass plant did not influence the number of tillers grazed nor the probability of the plant being grazed. Overall the defoliation of individual plants by sheep was weakly determined by landscape location, stocking level, plant species and prevailing forage on offer. The influence of spatial and temporal variation was small; random grazing of grass plants was the rule. This finding suggests that the grazing pressure on palatable perennial grasses in the paddocks of semi-arid woodlands will be similar across wooded landscapes and that spatial variability in plant mortality could be due to. the combined effect of plant water stress, which varies spatially and temporally, and grazing pressure which varies temporally but not spatially.

Changes in Grass Density in Australian Semi-Arid Woodlands.

Changed grazing regimes since European settlement have been widely proposed as the cause of a decline of indigenous perennial grasses in the semi-arid woodlands of eastern Australia. A five year experiment using exclosures examined the effects of grazing on densities of perennial grasses. Short- lived Stipa spp. and Aristida spp. were dominant at most sites. Their densities fluctuated greatly with season and reached over 200 plants/m2 during climatically favourable periods. The long-lived Eragrostis eriopoda occurred at densities that were generally less than 5 plants/m2 and its populations were relatively stable. The response of Enneapogon avenaceus was distinctive. Though its density fluctuated considerably, successive peaks in density were higher and the species increased more in ungrazed areas than in destocked or unfenced areas. The differences between grazed and ungrazed populations became greater with successive peaks in density. Within the short periods that pastoralists are likely to be willing or able to apply such treatments, destocking or even removing all herbivores is unlikely to have a large effect on the density of many palatable perennial grass species. The rate of response to resting pastures will depend on seasonal conditions.

Changes in Herbaceous and Suffrutescent Perennials in Grazed and Ungrazed Desertified Grassland in Southeastern Arizona, 1958-1978

Cover of plants was measured in creosote bush (Larrea tridentata) and tarbush (Flourensia cernua) vegetations within a newly established cattle exclosure in 1958, and again in 1976-1977, within the exclosure and in contiguous grazed areas. Protection from cattle grazing for 18-19 years did not alter the direction of vegetation change. Unpalatable species, particularly snakeweed (Gutierrezia sarothrae) and fluff grass (Tridens pulchellus), increased significantly within and without the exclosure from less than 1% cover in 1958 to as much as 20% in 1977. There was significant, but limited, increase of palatable perennial grasses within the exclosure, particularly in shallow washes. These grasses also increased in grazed creosote bush vegetation, particularly bush muhly (Muhlenbergia porteri), but decreased in grazed tarbush vegetation. There is evidence that there was less moisture stress in the shallow soil over caliche in the creosote bush vegetation than in the deep soil of tarbush. There was high mortality of fluff grass after a prolonged freeze in December 1978. Such infrequent events confound the interpretation of data on long-term vegetational changes.