Influence of harvesting strategy on nutrient supply and production of dairy cows consuming diets based on grass and red clover silage

The main objective of this thesis was to elucidate the effects of regrowth grass silage and red clover silage on nutrient supply and milk production of dairy cows as compared with primary growth grass silages. In the first experiment (publication I), two primary growth and four regrowth grass silages were harvested at two stages of growth. These six silages were fed to 24 lactating dairy cows with two levels of concentrate allowance. Silage intake and energy corrected milk yield (ECM) responses, and the range in these response variables between the diets, were smaller when regrowth silages rather than primary growth silages were fed. Milk production of dairy cows reflected the intake of metabolizable energy (ME), and no differences in the ME utilization were found between the diets based on silages harvested from primary growth and regrowth. The ECM response to increased concentrate allowance was, on average, greater when regrowth rather than primary growth silages were fed. In the second experiment (publication II), two silages from primary growth and two from regrowth used in I were fed to rumen cannulated lactating dairy cows. Cows consumed less feed dry matter (DM), energy and protein, and produced less milk, when fed diets based on regrowth silages rather than primary growth silages. Lower milk production responses of regrowth grass silage diets were mainly due to the lower silage DM intake, and could not be accounted for by differences in energy or protein utilization. Regrowth grass silage intake was not limited due to neutral detergent fibre (NDF) digestion or rumen fill or passage kinetics. However, lower intake may be at least partly attributable to plant diseases such as leaf spot infections, dead deteriorating material or abundance of weeds, which are all higher in regrowth compared with primary growth, and increase with advancing regrowth. In the third experiment (publications III and IV), red clover silages and grass silages harvested at two stages of growth, and a mixed diet of red clover and grass silages, were fed to five rumen cannulated lactating dairy cows. In spite of the lower average ME intake for red clover diets, the ECM production remained unchanged suggesting more efficient utilisation of ME for red clover diets compared with grass diets. Intake of N, and omasal canal flows of total non-ammonia N (NAN), microbial and non-microbial NAN were higher for red clover than for grass silage diets, but were not affected by forage maturity. Delaying the harvest tended to decrease DM intake of grass silage and increase that of red clover silage. The digestion rate of potentially digestible NDF was faster for red clover diets than for grass silage diets. Delaying the harvest decreased the digestion rate for grass but increased it for red clover silage diets. The low intake of early-cut red clover silage could not be explained by silage digestibility, fermentation quality, or rumen fill but was most likely related to the nutritionally suboptimal diet composition because inclusion of moderate quality grass silage in mixed diet increased silage DM intake. Despite the higher total amino acid supply of cows fed red clover versus grass silage diets, further milk production responses on red clover diets were possibly compromised by an inadequate supply of methionine as evidenced by lower methionine concentration in the amino acid profile of omasal digesta and plasma. Increasing the maturity of ensiled red clover does not seem to affect silage DM intake as consistently as that of grasses. The efficiency of N utilization for milk protein synthesis was lower for red clover diets than for grass diets. It was negatively related to diet crude protein concentration similarly to grass silage diets.

Effect of fire and tree-grass patches on soil nitrogen in Australian tropical savannas

Fire is an important driver of nutrient cycling in savannas. Here, we determined the impact of fire frequency on total and soluble soil nitrogen (N) pools in tropical savanna. The study sites consisted of 1-ha experimental plots near Darwin, Australia, which remained unburnt for at least 14 years or were burnt at 1-, 2- or 5-year intervals over the past 6 years. Soil was analysed from patches underneath tree canopies and in inter-canopy patches at 1, 12, 28, 55 and 152 days after fire. Patch type had a significant effect on all soil N pools, with greater concentrations of total and soluble (nitrate, ammonium, amino acids) N under tree canopies than inter-canopy patches. The time since the last fire had no significant effect on N pools. Fire frequency similarly did not affect total soil N but it did influence soluble soil N. Soil amino acids were most prominent in burnt savanna, ammonium was highest in infrequently burnt (5-year interval) savanna and nitrate was highest in unburnt savanna. We suggest that the main effect of fire on soil N relations occurs indirectly through altered tree-grass dynamics. Previous studies have shown that high fire frequencies reduce tree cover by lowering recruitment and increasing mortality. Our findings suggest that these changes in tree cover could result in a 30% reduction in total soil N and 1060% reductions in soluble N pools. This finding is consistent with studies from savannas globally, providing further evidence for a general theory of patchiness as a key driver of nutrient cycling in the savanna biome.

Albert Einstein laying on grass in a field Portraits; Men

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Temporal and spatial patterns of soil water extraction and drought resistance among genotypes of a perennial C-4 grass

The objective of this study was to investigate patterns of soil water extraction and drought resistance among genotypes of bermudagrass (Cynodon spp.) a perennial C-4 grass. Four wild Australian ecotypes (1-1, 25a1, 40-1, and 81-1) and four cultivars (CT2, Grand Prix, Legend, and Wintergreen) were examined in field experiments with rainfall excluded to monitor soil water extraction at 30-190 cm depths. In the study we defined drought resistance as the ability to maintain green canopy cover under drought. The most drought resistant genotypes (40-1 and 25a1) maintained more green cover (55-85% vs 5-10%) during water deficit and extracted more soil water (120-160 mm vs 77-107 mm) than drought sensitive genotypes, especially at depths from 50 to 110 cm, though all genotypes extracted water to 190 cm. The maintenance of green cover and higher soil water extraction were associated with higher stomatal conductance, photosynthetic rate and relative water content. For all genotypes, the pattern of water use as a percentage of total water use was similar across depth and time We propose the observed genetic variation was related to different root characteristics (root length density, hydraulic conductivity, root activity) although shoot sensitivity to drying soil cannot be ruled out.

Impacts of level of utilisation by grazing on an Astrebla (Mitchell grass) grassland in north-western Queensland between 1984 and 2010. 1. Herbage mass and population dynamics of Astrebla spp

Managing large variations in herbage production, resulting from highly variable seasonal rainfall, provides a major challenge for the sustainable management of Astrebla (Mitchell grass) grasslands in Australia. A grazing study with sheep was conducted between 1984 and 2010 on an Astrebla grassland in northern Queensland to describe the effects of a range of levels of utilisation of the herbage at the end of the summer growing season (April–May in northern Australia) on the sustainability of these grasslands. In unreplicated paddocks, sheep numbers were adjusted annually to achieve 0, 10, 20, 30, 50 and 80% utilisation of the herbage mass at the end of the summer over the ensuing 12 months. Higher levels of utilisation reduced both total and Astrebla spp. herbage mass because of the effects of higher utilisation on Astrebla spp. and this effect was accentuated by drought. The tussock density of Astrebla spp. varied widely among years but with few treatment differences until 2005 when density was reduced at the 50% level of utilisation. A major change in density resulted from a large recruitment of Astrebla spp. in 1989 that influenced its density for the remainder of the study. Basal area of the tussocks fluctuated among years, with increases due to rainfall and decreases during droughts. Seasonal rainfall was more influential than level of utilisation in changes to the basal area of perennial grasses. Drought resulted in the death of Astrebla spp. tussocks and this effect was accentuated at higher levels of utilisation. A series of three grazing exclosures were used to examine the recovery of the density and basal area of Astrebla spp. after it had been reduced by 80% utilisation over the preceding 9 years. This recovery study indicated that, although grazing exclusion was useful in the recovery of Astrebla spp., above-average rainfall was the major factor driving increases in the basal area of perennial grasses. Spring values of the Southern Oscillation Index and associated rainfall probabilities were considered to have potential for understanding the dynamics of Astrebla spp. It was concluded that Astrebla grassland remained sustainable after 26 years when grazed at up to 30% utilisation, while, at 50% utilisation, they became unsustainable after 20 years. Results from this study emphasised the need to maintain the population of Astrebla spp. tussocks.

Impacts of level of utilisation by grazing on an Astrebla (Mitchell grass) grassland in north-western Queensland between 1984 and 2010. 2. Plant species richness and abundance

The occurrence of interstitial species in Astrebla grasslands in Australia are influenced by grazing and seasonal rainfall but the interactions of these two influences are complex. This paper describes three studies aimed at determining and explaining the changes in plant species richness and abundance of the interstitial species in a long-term sheep utilisation experiment in an Astrebla grassland in northern Queensland. In the first study, increasing utilisation increased the frequency of Dactyloctenium radulans (Button grass) and Brachyachne convergens (Downs couch) and reduced that of Streptoglossa adscendens (Mint bush). In the second study, seasonal rainfall variation between 1984 and 2009 resulted in large annual differences in the size of the seed banks of many species, but increasing utilisation consistently reduced the seed bank of species such as Astrebla spp. and S. adscendens and increased that of species such as B. convergens, D. radulans, Amaranthus mitchellii (Boggabri) and Boerhavia sp. (Tar vine). In the third study, the highest species richness occurred at the lightest utilisation because of the presence of a range of palatable forbs, especially legumes. Species richness was reduced as utilisation increased. Species richness in the grazing exclosure was low and similar to that at the heaviest utilisation where there was a reduction in the presence of palatable forb species. The pattern of highest species richness at the lightest grazing treatment was maintained across three sampling times, even with different amounts of seasonal rainfall, but there was a large yearly variation in both the density and frequency of many species. It was concluded that the maintenance of highest species richness at the lightest utilisation was not aligned with other data from this grazing experiment which indicated that the maximum sustainable wool production occurred at moderate utilisation.

Reducing seed viability of flaxleaf fleabane, feathertop Rhodes grass and awnless barnyard grass

In the sub-tropical grain region of Australia, cotton and grains systems are now dominated by flaxleaf fleabane (Conyza bonariensis (L.) Cronquist), feathertop Rhodes grass (Chloris virgata Sw.) and awnless barnyard grass (Echinochloa colona (L.) Link). While control of these weed species is best achieved when they are young, previous studies have shown a potential for reducing seed viability and minimising seed bank replenishment by applying herbicides when plants are reproductive. Pot trials were established over two growing seasons to examine the effects of 2,4-D, 2,4-D + picloram, glyphosate and glufosinate which had been successful on other species, along with paraquat and haloxyfop (grasses only). Herbicides were applied at ¾ field rates in an attempt not to kill the plants. Flaxleaf fleabane plants were sprayed at two growth stages (budding and flowering) and the grasses were sprayed at two stages (late tillering/booting and flowering). Spraying flaxleaf fleabane at flowering reduced seed viability to 0% (of untreated) in all treatments except glyphosate (51%) and 2,4-D + picloram (8%). Seed viability was not reduced with the first and second regrowths with the exception of 2,4-D + picloram where viability was reduced to 20%. When sprayed at budding only 2,4-D + picloram reduced seed viability in both trials. Spraying the grasses at late tillering/booting did not reduce viability except for glufosinate on awnless barnyard grass (50%). Applying herbicides at flowering resulted in 0% seed viability in awnless barnyard grass from glufosinate, paraquat and glyphosate and 0% viability in feathertop Rhodes grass for glufosinate. These herbicides were less effective on heads that emerged and flowered after spraying, only slightly reducing seed viability. These trials have shown that attempts to reduce seed viability have potential, however flaxleaf fleabane and feathertop Rhodes grass are able to regrow and will need on-going monitoring and control measures.

Optimisation of resin extraction from an Australian arid grass ‘Triodia pungens’ and its preliminary evaluation as an anti-termite timber coating

Spinifex grasses are the dominant vegetative component in Australian grassland habitats, covering approximately 26% of the Australian landmass. Our ongoing work explores the utility of both the cellulosic and resinous components of this abundant biomass for modern applications and a potential economy for our Aboriginal collaborators. This study is focused on the optimisation of a resin extraction process using solvent, and the subsequent evaluation, via a field trial, of the potential use and efficacy of the resin as an anti-termite coating material. Termiticidal performance was evaluated by re-dissolving the extracted resin in acetone and coating on pine timber blocks. The resin-coated and control blocks were then exposed to a colony of Mastotermes darwiniensis’ (Froggatt) termites, which are the most primitive alive and destructive species in subterranean area, at a trial site in northeast Australia, for six months. The results clearly showed that spinifex resin effectively protected the timber from termite attack, while the uncoated control samples were extensively damaged. By demonstrating an enhanced termite resistance, we here report that plant resins that are produced by arid/semi-arid grasses could be potentially used as treatments to prevent termite attack.

Learning from the past to predict the future: an historical analysis of grass invasions in northern Australia

An important focus of biosecurity is anticipating future risks, but time lags between introduction, naturalisation, and (ultimately) impact mean that future risks can be strongly influenced by history. We conduct a comprehensive historical analysis of tropical grasses (n = 155) that have naturalised in Australia since European settlement (1788) to determine what factors shaped historical patterns of naturalisation and future risks, including for the 21 species that cause serious negative impacts. Most naturalised species were from the Old World (78 %), were introduced for use in pasture (64.5 %), were first recorded prior to 1940 (84.5 %) and naturalised before 1980 (90.3 %). Patterns for high-impact species were similar, with all being first recorded in Australia by 1940, and only seven naturalised since then-five intentionally introduced as pasture species. Counter to expectations, we found no evidence for increased naturalisation with increasing trade, including for species introduced unintentionally for which the link was expected to be strongest. New pathways have not emerged since the 1930s despite substantial shifts in trading patterns. Furthermore, introduction and naturalisation rates are now at or approaching historically low levels. Three reasons were identified: (1) the often long lag phase between introduction and reported naturalisation means naturalisation rates reflect historical trends in introduction rates; (2) important introduction pathways are not directly related to trade volume and globalisation; and (3) that species pools may become depleted. The last of these appears to be the case for the most important pathway for tropical grasses, i.e. the intentional introduction of useful pasture species. Assuming that new pathways don't arise that might result in increased naturalisation rates, and that current at-border biosecurity practices remain in place, we conclude that most future high-impact tropical grass species are already present in Australia. Our results highlight the need to continually test underlying assumptions regarding future naturalisation rates of high-impact invasive species, as conclusions have important implications for how best to manage future biosecurity risks.

Perennial pastures for marginal farming country in southern Queensland. 1. Grass establishment techniques

Efficient ways to re-establish pastures are needed on land that requires a rotation between pastures and crops. We conducted trials in southern inland Queensland with a range of tropical perennial grasses sown into wheat stubble that was modified in various ways. Differing seedbed preparations involved cultivation or herbicide sprays, with or without fertilizer at sowing. Seed was broadcast and sowing time ranged from spring through to autumn on 3 different soil types. Seed quality and post-sowing rainfall were major determinants of the density of sown grass plants in the first year. Light cultivation sometimes enhanced establishment compared with herbicide spraying of standing stubble, most often on harder-setting soils. A nitrogen + phosphorus mixed fertilizer rarely produced any improvement in sown grass establishment and sometimes increased weed competition. The effects were similar for all types of grass seed from hairy fascicles to large, smooth panicoid seeds and minute Eragrostis seeds. There was a strong inverse relationship between the initial density of sown grass established and the level of weed competition.