School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemGrazing ecology and high producing dairy cows( 2005)This body of work (82 papers in scientific journals and 2 books) encompasses two broad areas of work. They are 1) growth, nutritive value and management of pastures grazed by dairy cows (40 publications), and 2) supplements for grazing dairy cows, with a particular focus on responses associated with supplement use and digestion in the rumen (44 publications). These two areas of research are inextricably linked and, taken together, have been termed `Grazing Ecology'. Of the 84 publications included, the candidate was the senior or sole author of 67% of them. The chronological development of the work reported includes research on stocking rates reported in the early 1980's through to the development of Diet Check, a decision support tool incorporating much of the information generated during the previous two decades, in the early 2000's. The publications cover aspects of grazing management to optimise growth, persistence and nutritive value of irrigated annual and perennial pastures for dairy cows. Most of this research has incorporated some aspect of stocking rate, whether it be stocking rate per se in long term experiments or frequency and/or intensity of defoliation in shorter term experiments. The aim was to establish optimum grazing strategies that best effected the compromise of maximum intake of pasture of high nutritive value while satisfying the requirements for maintenance of pasture growth and persistence of a balance of desirable pasture species. The research allowed the definition of the intake and nutritive characteristics of pasture grazed by lactating dairy cows under a range of management conditions. At the same time, strategies to effectively feed supplements were investigated. When more than one feed is offered to dairy cows, associative effects play an important role in the eventual responses achieved. Balance of nutrients, particularly in the rumen, and substitution of supplement for pasture in the diet of grazing dairy cows, were the main aspects of the associative effect between feeds considered in the research reported here. Substitution can have a huge effect on the responses obtained from supplements, and the type of supplement, by influencing the balance of nutrients ingested into the rumen, affects the composition of the milk produced. Finally, some attempt has been made to draw much of the information on pasture management and supplementation of grazing dairy cows together for use by dairy farmers and their advisers, and to define gaps in knowledge. This has been done by reviewing the scientific literature, and by the use of modelling to provide simple tools for tactical decision making. Although the research was undertaken in northern Victoria, many of the results apply equally in other areas of the world where pasture constitutes a major proportion of the diet of dairy cows. Victoria currently produces more than 60% of Australia's milk, with northern Victoria producing more than 40% of that. The development of dairying in Victoria mirrors much of the progress of the research reported in this collection of scientific publications. Before 1982, dairy farming was almost totally based on grazed pastures and the use of pasture supplements (hay and silage). A severe drought occurred in 1982, which prompted a serious consideration of the use of supplements for lactating cows grazing pasture. Today, dairy systems in Victoria vary to a huge extent, with the energy provided by pasture ranging from 0 to 100%. Over this period, average milk production has increased, from about 3000L/cow per lactation to more than 5000L/cow. With a fine line separating profit and loss in dairy businesses that basically depend on the price received for manufactured products on overseas markets, both grazed pasture and supplements need to be used optimally. The challenge has been to provide information and tools to allow dairy farmers to achieve this objective. I believe that my research, particularly in relation to pasture intake, substitution and associative effects, has been instrumental in allowing pasture-based dairy farmers to continue to remain viable in Australia, and that many of the principles developed apply wherever pasture constitutes a significant proportion of a cow's diet.
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ItemThe economic evaluation of forage research results( 2004)Three economic analyses were conducted on the results of dairy forage production experiments undertaken in Victoria. The first analysis investigated the level of pasture production increases that would have to be achieved to warrant the investment in different soil modification options. This analysis took pasture production data and using a computer program "UDDER" (Larcombe 1990) generated farm data which was then applied to development budgets. The increase in pasture growth rate required was such that it is unlikely that investment in the soil modification systems tested here will produce a satisfactory return on investment. The second analysis investigated the use of different pasture species combinations on a dairy farm in northern Victoria. A linear programming model was developed that balanced the energy requirements of the milking herd with the energy supplied from pasture and supplements. The results showed that the most profitable mix of pasture depended on the energy supply profile of the pasture and the requirements of the herd. The proportion of autumn and spring calving cows in the herd in part determined the most profitable pasture mix. The effect of grazing management on profit was the subject of the third study. A farm model was constructed that balanced the energy, protein and neutral detergent fibre requirements of the milking herd with that supplied by pasture and supplements and optimised operating profit. The results of a grazing trial conducted in south-west Victoria were entered into the model and the operating profits for each treatment compared. The results suggested that while Operating profit was related to total pasture consumption, the timing of the pasture consumption impacted on operating profit. The results also suggested that grazing frequency may have affected operating profit more than grazing intensity.
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ItemEffects of resistance to prehension and structure of pastures on grazing behaviour and intake of dairy cows( 2000)Pasture intake by dairy cows is affected by plant and sward structural characteristics of the pasture. In the meantime, grazing animals are constrained to gather their food bite by bite removing only a portion of the herbage present at the location which they bite. The extent to which grazing animals overcome the constraints imposed by the plant and structural characteristics of the pasture is the major determinant of herbage intake. The experiments which form the basis of this thesis concentrated on determining the role of sward resistance to prehension, measured in situ, as an integrating sward characteristic that determines foraging decisions of cows and the extent to which they defoliate pasture swards. Under rotational grazing systems, a cow is offered an area of pasture that is often smaller than the area from which the cow harvests its bites (defoliated area, DA). The cow therefore faces changing sward conditions during the process of grazing down into the sward and removing bites at successive lower defoliation planes. The defoliation pattern in grazing down the sward profile and the consequent herbage intake and diet composition, are examined in this thesis. A novel apparatus was designed to measure the BFF in situ at different sward profile heights. In the initial experiment, changes in BFF down the sward profile of six pasture species were examined in order to evaluate the mechanical efficiency of defoliating bites at different depths, in terms of bite weight:BFF ratio. The hypothesis tested was that cows remove 30 - 40% of the sward height at each bite due to a mechanical advantage in terms of BW:BFF. The BFF varied more between defoliation strata than between pasture species. The bite weight and BFF increased with the depth of defoliation. The mechanical efficiency of defoliating bites estimated as the BW:BFF ratio declined slightly with bite depth until a depth of about 30 - 40% of the sward height is reached, when the ratio declined more rapidly. Based on these results and those of Wade (1991), four theoretical defoliation planes (DPI, DP2, DP3 & DP4) were set each at 35% of the pre-grazing sward heights to estimate the total area defoliated by grazing cows under different sward conditions. DP2 is the plane of removal of a second bite after a first bite has removed DPI. Three spring grazing experiments were conducted to explore relationships between pasture allowance and/or sward structure and intake dynamics. In the first experiment, cows were offered a herbage allowance (HA) of 50 kg DM/cow/day either as one block with continuous access for 24 hours, or as six equal break rations opened at intervals during a 24 hour period. In the two subsequent experiments, different sward types were created in order to alter the BFF. In the second experiment swards were created with two different surface heights (USH) and in a 2 x 2 factorial, cows were offered two HA (35 and 70 kgDM/cow/day). In the third experiment, swards with three different tiller densities were created and cows were offered a similar HA of 8 kg DM/cow/3 hours. The defoliation pattern, BFF at 30, 50 and 70% of USH, DM intake, grazing behaviour and the energetics of grazing were measured. The major conclusions derived from these experiments are as follows. The average depth of defoliation (DD) increased with sward height and fell between DP2 and DP4. However, the proportion of area defoliated at each defoliation plane declined down the profile, at rates that varied with HA and tiller density but was unaffected by sward height. At a HA of 70 kg, cows barely reached DP4. The area defoliated at DP4 increased with decreasing herbage allowance and decreasing tiller density. The initial bulk density and post-grazed bulk density declined with USH, but the grazed-stratum bulk density was not significantly affected by USH. Therefore, it was concluded that the volume of canopy defoliated was the major determinant of intake. With increasing HA, the average bite weight (BW) increased, prehension bite rate declined but the overall intake rate increased. The time cost of a bite increased with BW. However, the energy expenditure on prehending a bite did not show a consistent relationship with BW. The BFF increased with sward height and tiller density. However, BFF in the leafy layer of 70% of the sward height was not affected by initial sward height or tiller density. The increase in BFF with initial sward height and tiller density was greater in the lower stemmy layer of 30% sward height. The average bite area (BA) and BW increased with HA. Intake was positively correlated with HA (R = 0.49), HM (R = 0.65) and tiller density (R = 0.51). Multiple regression analysis with herbage intake as the dependent variable indicated that, in addition to HM and HA (R2 = 0.887) , inclusion of the difference in BFF between that at 30% USH and that at 70% USH (BFFdif) as a sward characteristic provided an equation with a substantially better fit (R2 = 0.956). DMI = -3.47 + 1.80 HM + 0.225 HA R2 = 0.887 DMI = -2.73 I + 2.76 HM + 0.732 HA - 0.0416 BFFdif R2 = 0.956 It is concluded that the BFFdif has a significant value in integrating the changes in sward characteristics down the profile and is useful in improving the intake model.