School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemMeasurement of body composition of lactating ewes using dual energy x-ray absorptiometry( 2000)The measurement of body composition in live animals has previously been limited to subjective methods such as condition scoring and ultrasound. There are now many direct techniques available to estimate body composition. Dual-energy x-ray absorptiometry (DXA) is a technique that offers a rapid and non-invasive analysis of body composition. It should therefore be possible to use DXA technology to measure changes in body composition in ewes throughout lactation. However DXA has not previously been used for measuring body composition in live sheep. Experimental work was undertaken to determine the correlation between body composition given by DXA and chemical body composition. Correlations between DXA and chemical composition were excellent (R2>0.96 for live animal and R2>0.91 for carcass composition). Regression equations for body composition from DXA and chemical analysis were developed: the equations for analysis in position 1 (entire scan placed in the Right Arm region for analysis) were: fat tissue mass by chemical analysis (FTM) = (1.20 x DXA FTM) - 0.067 (R2=97.8) chemical inorganic matter = (1.08 x DXA bone mineral content) + 0.294 (R2 = 97.2) chemical lean tissue mass (LTM) = (0.933 x DXA LTM) + 1.25 (R2 = 98.9) This study concluded that DXA has great potential for use in live sheep and carcass research. The East Friesian is a relatively new breed in Australia and little is known about its productive capacity under Australian conditions. It is thought, on the basis of work undertaken in New Zealand, that the breed will provide excellent prime lamb mothers that produce large quantities of milk over long periods and are highly fecund. Lamb carcass quality is also claimed by breeders to be excellent. East Friesians were first introduced to Australia in 1996 and most ewes are half-cross, commonly East Friesian x Romney. While the East Friesian is reputedly one of the most productive dairy breeds, the lactational performance of East Friesian cross ewes in Australia is unknown. A study was therefore undertaken to compare the performance of single-bearing East Friesian x Romney ewes and the common Australian prime lamb dam, the Border Leicester x Merino. The results of this study showed that East Friesian x Romney ewes have the potential to produce significantly more milk over the first nine weeks of lactation (mean milk yield: 2.6 ± 0.1 vs 1.9 ± 0.2 kg/day for East Friesian x Romney and Border Leicester x Merino, respectively). The feed intake and changes in liveweight of the two breeds were not significantly different. Mean feed intake over the nine week experimental period was 3.39 ± 0.20 and 3.10 ± 0.16 kg/day for the EFR and BLM ewes, respectively. Mean liveweight was 76.6 ± 4.0 and 77.4 ± 3.2 kg for the EFR and BLM ewes, respectively. The question arising from this initial study was: can East Friesian x Romney ewes produce more milk by using the available feed more efficiently or by mobilising body reserves? This has significant nutritional implications, both at a research and field level. Having determined correlation equations for use of DXA on live sheep, a study of the changes in body composition (fat, lean and bone mineral) during lactation was undertaken to examine the differences between East Friesian x Romney and Border Leicester x Merino ewes. Differences were observed which indicated some differences in body composition between the EFR and BLM ewes: the EFR ewes had a significantly lower percentage of fat tissue mass and higher percentage of lean tissue mass. Some technical issues were defined and further investigation into DXA including the effect of wool cover and gastrointestinal water contents is required. However the present results indicate that DXA provides a sound basis for examining body composition in live sheep.
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ItemEffects of salinity on growth and wood and fibre properties in eucalypts( 2001)Salinity, the presence of soluble salts in soils or waters, can be separated into naturally occurring primary salinity, and secondary salinity resulting from human activities such as land development and agriculture. Secondary salinity involving high, saline water-tables affects large areas, estimated between 4.7 and 6.1 'ha (Williamson 1990, Robertson 1996), of agricultural land in Australia, and tree planting is one approach to lower saline watertables. Such plantations could become a significant fibre source for the pulp and paper industry, but it is not known whether growth in salt-affected environments influences key fibre properties important in paper production. This work therefore examined the wood and fibre properties of Tasmanian blue gum, Eucalyptus globulus ssp. globulus, and river red gum, E. camaldulensis, grown under various conditions of soil salinity. Eucalyptus globulus was studied in trial plantations in the Shepparton region of north-central Victoria. The plantations were established in 1993, and field sampling was carried out from 1995 to 1997. Salinity in the top 20 cm of soil over the period of study (1994 to 1997), according to the soil salinity classes set lutin Marcar et al. (1995), ranged from non-saline at the fresh-channel water-irrigated plot to slightly saline for the saline groundwater-irrigated plots. Tree size generally did not differ significantly between plots at any age. Differences in foliar [Na±], {K±] and [Cl-] occurred between the field plots but were not consistent between years. The highest concentrations of foliar ions were also not always associated with the plot receiving the highest salinity irrigation water, suggesting that in only some years was the soil salt level sufficient to cause a plant response. Wood basic density differed between the plots, but it could not be attributed to salinity, and may have reflected other site-specific effects. Fibre morphology parameters did not differ significantly between the plots. There were some differences between the plots in the increase in fibre length from year to year but the differences were not consistent over the entire survey period and could not conclusively be attributed to differences in soil salinity. A pilot salinity pot trial was conducted on E. camaldulensis plants, as a precursor to a more elaborate experiment planned for potted E. globulus plants. The E. camaldulensis pot trial comprised a single concentration salt (NaCl) treatment and a control (freshwater) treatment applied over a 60 day period. A marked reduction of growth occurred with salt-treated seedlings relative to control seedlings. Concomitant with the reduction in growth, salt-treated seedlings produced significantly shorter, thicker-walled fibres than the control seedlings. The pot-trial on 18-month-old Eucalyptus globulus ssp. globulus trees applied different concentration salt (lRlaCi) solutions over a 10-week period. The salinity of the potting mixture increased markedly in the salt-treated trees relative to the controls. Foliar chloride and sodium were also significantly greater in trees on the higher salt treatments than in the control trees. Diameter growth decreased with the higher salt treatments, and five trees under high salt treatments had to be harvested prior to the planned completion of the experiment, due to their poor state of health. These results indicated the salt treatments had influenced some aspects of tree physiology. A wound made to the cambium allowed pre-treatment fibres (fibres formed prior to the start of the experiment) to be distinguished from post-treatment fibres (fibres formed during the experiment) in the E.globulus pot trial. Trees on higher salt treatments produced significantly longer, thinner-walled fibres compared to controls, but this pattern also occurred in fibres formed before treatments were imposed, implying that these differences were due to preexisting differences in the trees unrelated to the salt treatment. Statistical analysis of fibres formed during treatments, taking account of pre-existing differences, found that there was no significant effect of salt treatment on fibre length or wall thickness, although this was possibly because of the low sample size relative to the variation of the experimental material. The controlled application of salt for 10 weeks during the E. globulus pot trial thus had some effect on tree physiology, but no significant effects on fibre dimensions or wood formation. This was consistent with the observation in the field trial that fibre dimensions and wood formation were not influenced by factors that did not also reduce tree growth, at least in trees up to 4 years old. Higher levels of salt could cause rapid tree decline due to the inability of the trees to exclude the salt, and processes associated with fibre formation would then also cease. The combined results from the field and pot trials indicated that E. globulus, a slightly to moderately salt-tolerant species, suffered negligible or minor growth reductions on soils irrigated to a slightly saline level, and produced fibres of similar morphology to trees grown under non-saline conditions. If soil salinity increased above the moderate level, the trees would continue to grow provided sufficient water is still available, but internal salt levels would increase to the point where tree death would result. Based on the pot trial, where such internal salt levels were achieved, the decline and death of the trees would occur before the salt affects fibre morphology. Eucalyptus camaldulensis adopted a different strategy to cope with salt stress than E. globulus. Eucalyptus globulus continued to grow provided it was supplied with water, despite its saline nature. Finally, when salt levels within the plant reached a critical level, plant health rapidly declined. When E. camaldulensis was watered with solution of a similar salinity to the highest salt treatments in the E. globulus pot trial, there was a rapid cessation in extension growth, but there was no other sign of a deterioration in plant health. The mechanism by which E. camaldulensis was able to quickly cease shoot growth, which presumably allowed it to tolerate saline conditions by restricting salt uptake, was not investigated here. Material from Eucalyptus camaldulensis that had been growing for 14 years on a dryland plantation site in southwest Western Australia was also investigated. Trees from the high salinity area did not differ significantly in average height, diameter and volume from those from the low salinity area. Basic density were significantly greater in the high salinity group of trees than in the low salinity group, but no relationship with tree growth was established. The absence of a relationship between growth and basic density was not unusual, as natural variation in basic density makes it difficult to establish environmental or experimental effects (Downes and Raymond 1997). Fibre fractional wall coverage was greater in the high salinity group of trees than in the low salinity group, as was also the case for the E. camaldulensis pot trial. In the pot trial, however, a significant growth reduction due to salinity was recorded. There were no other differences in fibre morphology between the high and low salinity groups of the Western Australian plantation. Eucalyptus globulus is less salt and waterlogging tolerant than E. camaldulensis (Bennett and George 1995a; Bennett and George 1995b) but in the field studies the growth and wood and fibre properties for each species was similar across the range of salinities encountered. The exception was basic density and fibre fractional wall coverage in the 14-year-old E. camaldulensis, both of which were greater in the high salinity group of trees. It was expected that the growth of E. globulus would be adversely affected if irrigation with the saline groundwater continued for several more years, allowing a build up in soil salinity. Based on the results from the E. globulus pot trial, once soil salinity levels exceed the tolerance limits of this species, a rapid decline in tree health will occur, and fibre formation will cease. Eucalyptus camaldulensis, with its greater salt and waterlogging tolerance, will grow in areas where other commercial species, such s E. globulus, would not thrive. However, E. camaldulensis has disadvantages for farm forestry in Australia, due to low percentage pulp yields by comparison with E. globulus (Arnold et al. 1999), and poor growth rates and tree form (Mazanec 1999). In the USA, E. camaldulensis has equalled the pulp yield of the commercially proven E. globulus (Arnold et al. 1999). Further research into improving pulp yields, growth rates and tree form of E. camaldulensis in Australia, would allow expansion of eucalypt plantations for pulp and wood production, as well as land and water care, onto previously unsuitable land.
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ItemMeasurement of the degree of cell rupture in macerated lucerne( 1978)Cell rupture is a critical process in the fractionation of green crops. Despite the importance of this process there is no accepted method for quantitatively describing the degree of cell rupture in a sample of macerated forage. Several methods were investigated and developed as measures of the degree of cell rupture in samples of lucerne leaves macerated by extrusion:- (i) Separation of cells and enumeration of intact cells. (ii) Release of cellular substances:- (a) water soluble carbohydrates (b) potassium (c) sodium (d) electrolytes (e) water (iii) Standard juice expression technique as used by other workers. A total rupture standard was considered to be essential, and was achieved in samples of lucerne leaf tissue macerated initially using a mortar and pestle, followed by ten minutes exposure to ultrasound. The performance of these techniques was evaluated in terms of repeatability and of individual and comparative performance over a large range of degrees of maceration. The cell enumeration method was not developed because a suitable agent for separating the cells of lucerne leaf tissue could not be found. All substance release assays gave repeatable estimates of cell rupture and were highly correlated with each other. It was therefore concluded, that the release of cellular substances gave a reliable estimate of the degree of cell rupture. The standard juice expression technique was insensitive to changes in the degree of rupture and was inaccurate at high levels of rupture. It was concluded that the electrolyte release assay was the most appropriate estimate of cell rupture for use in the future.
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ItemA study of factors influencing in vitro stability of nitrate reductase from wheat leaves( 1979)This review and the following chapters are concerned predominantly with the processes occurring in higher and lower plants which regulate the amount of NR present in vitro as controlled by degradation, and the level of activity of the existing enzyme. Those factors regulating the synthesis of NR will not be discussed in any detail but only mentioned where they also affect other mechanisms regulating NR. Nitrate reductase is unstable both in vivo and in vitro (101,193, 252). In vitro instability occurs since the isolation of enzymes and other cell components from plant tissue involves disruption of the plant cell. This results in mixing of substances which in situ were rigidly compartmentalized and is likely to result in the isolation of an enzyme which is modified from its native form. Factors present in plant cells which make plant proteins particularly unstable in vitro have been reviewed by Stahrran (216) and Pirie (157). They include vacuole acids, carbohydrates, hydrolytic and oxidative enzymes and phenolic components and their derivatives. In vivo variation in activity occurs in response to a number of other factors, including tissue age (103,129,166,243,264) and environment (15,72,82,1.03,129,261). Tissue age has been shown to influence the activity or stability of NR extracted from a number of species including corn (194,195,264), wheat (221), oats (194,195), tobacco (195) and barley (48). Nitrate reductase has been demonstrated in nearly all plant parts (16) and its ubiquitous presence suggested in higher plants (16,184). Nevertheless, due to the number of factors involved, detection of activity would only occur given suitable physiological and environmental conditions together with use of the correct extraction and assay procedure. Determining if the level of activity derived is an accurate estimate of the in situ activity is even more difficult. This has been attempted by correlating NR activity and grain or plant nitrogen (28, 36) . In vivo instability is indicated by the decline in NR activity with the onset of darkness, depletion of nitrate supply, and water or heat stress (11,82,121,168,235). Under appropriate conditions these factors could also affect the enzyme in vitro.
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ItemPhysiological aspects of root growth of Eucalyptus pauciflora, subsp. pauciflora and Eucalyptus nitens( 1997)This thesis examined i) morphological and physiological effects of low soil temperatures on root growth of subalpine Eucalyptus pauciflora Sieber ex Sprengel subsp. pauciflora and montane Eucalyptus nitens (Deane & Maiden) Maiden, ii) determined the variability, and in particular the day/night variability, in root elongation, and iii) explored the physiological basis for such variability. A series of experiments were undertaken with seedlings of E. pauciflora and E. nitens grown at soil temperatures of 3, 7, and 13C, and where seedlings were transferred from one temperature to another. E. nitens grew faster than E. pauciflora at 7 and 13C, but E. pauciflora grew faster than E. nitens at 3C. E. pauciflora always produced greater total and white root length than E. nitens. E. nitens roots browned faster in response to lowering soil temperatures than E. pauciflora. The osmotic potential of the roots decreased with soil temperature, but more so in E. pauciflora than E. nitens. Proline was a prominent osmoregulant in roots of E. pauciflora and arginine in E. nitens roots. It is suggested that E. pauciflora is better adapted than E. nitens to root growth at low soil temperatures because it can keep roots white longer and can maintain lower root osmotic potentials. Root growth of E. pauciflora was examined for 31 months (December 1992 - June 1995) in a mature stand at an elevation of 1545 m on Mt Stirling, Victoria, Australia. Greater night than day root elongation was recorded from eight in situ rhizotrons during the summer and early autumn of 1993. Shoot growth was also monitored during part of this study (April 1994 - June 1995). It was found that root growth commenced in the spring at soil temperatures 5 1.5C, under 550 mm of snow, at least one month before the onset of shoot growth and continued at least two months longer that shoot growth during the autumn. A period of root dormancy for at least one month a year occurred in roots of E. pauciflora. The seasonal variability in root numbers of E. pauciflora appeared to be related mainly to soil temperature and to a lesser extent to soil water content. Moreover, there appeared to be some internal periodicity in root growth which was independent of the external environment on Mt Stirling. Greater night than day root elongation was recorded in seedlings of both eucalypts in a glasshouse. Root elongation rates were greatest in E. nitens, and root elongation of both eucalypt seedlings were greater than that of the mature E. pauciflora on Mt Stirling. The zones of day and night elongation were determined in root marking experiments. Histological studies of the zone of elongation showed that cell division occurred mainly during the day and cell elongation mainly at night. Night root elongation rates were increased by increasing day-time air temperatures, light-period, and light intensity; and by decreasing water stress during the night. The turgor pressure of the root tips was greater during the night than the day. It is suggested that the amount of root growth during the night is determined directly by turgor pressure during the night and indirectly by processes during the day (light duration and intensity, and temperature during the light period) which determine the extent of cell division during the day. A greater rate of cell division during the day will be translated into a greater rate of root elongation, especially in the night.
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ItemProduction of heavy weight cryptorchid and wether lambs( 1996)Declining Iamb consumption remains a major problem facing the Australian prime Iamb industry. For Iamb to retain its market share and to remain a viable commodity in the future, leaner Iambs must be produced and marketed. Recent industry initiatives therefore focussed upon the production and marketing of Iambs to meet carcass specifications of 18-26 kg with a fat depth of 6 to 15 mm at the GR site. Emphasis was placed upon the production of lean carcasses weighing more than 22 kg, which are otherwise referred to as Elite Iambs. The development of production, management and marketing systems to produce and market Iamb carcasses of Elite Iamb specifications, both domestically and overseas, was the focus of this research program. The major objective of this experiment was to determine the potential of producing high and consistent quality meat from cryptorchid and wether Iambs meeting carcass specifications for Elite Iambs when slaughtered from six to eighteen months of age. One hundred and sixty three second cross Poll Dorset x Border Leicester/Merino cryptorchid and wether Iambs, born in May 1991 at Rutherglen Research Institute, Rutherglen, were grown and managed on annual pasture at an average liveweight of 50 kg prior to slaughter. Lambs were allocated to one of seven slaughter groups and finished every two months from 6 to 18 months of age and the influence of sex, age and nutrition on carcass composition, meat quality attributes and fat quality and consistency of cryptorchid and wether Iambs examined. Cryptorchid and wether Iambs slaughtered from 8 to 18 months of age produced carcasses meeting targeted specifications of greater than 22 kg with 6-15 mm of fat at the GR site. Lambs slaughtered at six months of age did not achieve the target liveweight of 50 kg at slaughter due to poor seasonal conditions which influenced the quality and availability of annual pasture in spring 1991. Cryptorchid Iambs had a lower dressing percentage and produced carcasses with a lower GR measurement than wethers at a cold carcass weight (CCW) of 23.5 kg. Cryptorchid Iambs exhibited growth rate advantages over wethers only when nutrition and seasonal conditions did not limit Iamb growth. When compared at the same fasted liveweight, cryptorchid Iambs deposited less internal fat than wethers. Cryptorchid carcasses also had a lower proportion of subcutaneous fat present on all primals, higher proportions of soft tissue and bone in the hindquarter and the EMA of the M. longissimus thoracis was significantly larger compared with wethers at a CCW of 23.5 kg. The proportion of CCW in the leg, chump and ribloin was not significantly influenced by Iamb sex. Nutritional management of lambs significantly influenced the proportion of subcutaneous fat on all primals. Meat quality was significantly influenced by Iamb sex however results obtained did not suggest that meat from cryptorchid Iamb carcasses was of inferior quality compared with wethers as only small differences in tenderness, intramuscular soluble collagen content, meat flavour, meat colour, intramuscular fat content and cooking loss were found. In addition, age and nutrition significantly influenced meat quality, however, consistent relationships were not found. The incidence of yellow soft subcutaneous fat was higher in cryptorchid Iamb carcasses than those from wethers with cryptorchids also producing subcutaneous fat with significantly higher levels of total unsaturated fatty acids and lower levels of stearic, palmitic and total saturated fatty acids compared with wethers. Age and nutrition also influenced the fatty acid composition of subcutaneous fat with Iambs fed lupins having significantly higher linoleic acid levels in subcutaneous fat. However, the magnitude of these differences did not explain differences found in fat quality and consistency between cryptorchid and wether carcasses slaughtered at different ages. Significant, though small, differences were found due to sex in both the triglyceride and phospholipid components of intramuscular fat. As higher levels of polyunsaturated fatty acids were found in the phospholipid component of intramuscular fat compared with cryptorchids, this may affect the keeping quality of meat from wether carcasses. Finally, although significant differences due to age were found in the fatty acid composition of intramuscular fat, these differences were not consistent with age and more likely reflected differences in nutritional management between slaughter groups. This study demonstrated that cryptorchid and wether Iambs turned off at heavy market weights on an improved plane of nutrition over a 12 month period produced carcasses meeting Elite Iamb specifications and yielded high quality, tender meat.
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ItemFactors influencing the efficacy of the boar effect( 1996)Maximizing reproductive efficiency is of high potential benefit to the pig industry. Mature boar contact has proven to be very effective in stimulating gilt puberty, although the results tend to be variable. The experimental work in this thesis was conducted in order to elucidate some of the factors which may be contributing to this variation. Three experiments have been carried out to examine the possibility of improving the efficacy of the boar effect. The first experiment investigated the effects of boar contact and transportation on the stimulation of early puberty attainment in gilts. Gilts were allocated to one of six treatments: control - no boar contact or transportation (C); daily boar contact (B1); daily transportation (T); three times daily boar contact (B3); daily boar contact and transportation (B1+T); and three times daily boar contact plus daily transportation (B3+T). Results show that transport alone has little influence on the timing of gilt puberty (P>0.05), although there was a significant positive interaction between transport and boar contact (P<0.01). Frequent boar contact (three-times daily) stimulated a higher proportion of gilts to attain puberty within 20 days of commencement of treatment (P<0.05) than did either once-daily boar contact or no boar contact. In addition, the proportion of gilts reaching puberty was significantly lower when gilts were exposed to daily boar contact, compared to three times daily boar contact. The second experiment studied the efficacy of the boar effect when conducted in a modified Detection and Mating Area (DMA), with or without full boar contact, or in a pen which was not the boar's own. Gilts were assigned to treatments of: no boar contact (C); once daily boar contact (B); DMA exposure (DMA); and boar exposure plus DMA exposure (B+DMA). Mean days taken from the start of the experiment to puberty attainment were significantly fewer (P<0.01) for gilts receiving daily boar contact than for gilts not exposed to boars (36.1 vs. 56.8 days), although boar exposure in addition to DMA exposure tended to produce the lowest mean days to puberty (20.6 days). However, neither days taken to reach puberty, gilt age at puberty, nor gilt weight at puberty was significantly affected by daily exposure to a DMA. The third experiment examined the stimulatory effect of the presence of cycling females on puberty attainment in gilts, and whether the effect was a direct one, or whether it operated via an increase in the stimulus value of the boar. The gilt treatment groups in this experiment were: control - no contact with boars or cycling females (C); contact with cycling females, but not with boars (F); contact with boars, but not with cycling females (B); contact with both boars and cycling females, where the boars were not exposed to the cycling females (BF-1); and contact with both boars and cycling females, where the boars were exposed to the cycling females (BF-2). The proportion of gilts reaching puberty was not significantly influenced by treatment in this study. However, the mean days taken from commencement of treatment until puberty attainment were significantly fewer for those boar exposed gilts not exposed to cycling females than for those gilts receiving neither boar contact nor being exposed to cycling females. Cycling females may exert a minor stimulatory effect on the timing of gilt puberty in non-boar exposed gilts, but this effect is not significant. Results from these studies confirm the efficacy of mature boar contact as a means of inducing early puberty attainment in the young gilt (i.e. the boar effect). Transport did not exert a significant effect on days taken to reach puberty, although transport may be slightly stimulatory to puberty attainment in gilts when used in combination with frequent boar contact. Full boar contact, when given in a DMA, may be a more effective form of gilt puberty stimulation than full boar contact in an isolated pen. Preliminary data also suggest a possible negative interaction between puberty stimuli from the boar and those from cycling females which, if substantiated, may have implications for the management of the replacement gilt.
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ItemBoar contact frequency and the timing of puberty in gilts( 1995)The literature relating to the natural attainment of puberty and those factors involved in the early induction of puberty in gilts is reviewed. The first two experiments suggested that motivated/high stimulus value boar(s) may release sufficient stimuli to overcome the seasonal change that mediates the ability of the gilt to attain natural puberty. These experiments also demonstrated that increasing the, frequency of daily boar contact provided sufficient 'cumulative stimulus' to initiate earlier pubertal development, when compared with once-daily contact (P<0.05). In fact, boar exposure three times daily significantly reduced mean gilt age at puberty relative to once-daily boar exposure (183.2 � 2.71 vs. 196.0 � 3.00 days of age respectively, P<0.01). Twice-daily boar contact resulted in an intermediate mean gilt age at puberty (190.3 � 3.01 days). There was also a trend towards a higher proportion of gilts reaching puberty earlier with increasing frequency of boar contact but this effect became less evident after day 20 of commencement of treatment. In a third experiment increasing boar contact frequency to 8 times per day failed to improve the proportion of gilts attaining precocious puberty compared with 4 times daily contact. This suggests that maximum response to boar contact is achieved using 3-4 times exposure per day. The fourth experiment again confirmed that the boar effect is enhanced when boar contact occurs several times each day. It also indicated that transport 'stress' in conjunction with frequent boar contact may further reduce gilt age at puberty. However, transport alone did not significantly advance the timing of gilt puberty attainment. A final experiment suggested that the efficacy of the boar effect can also be maximised by increasing the frequency of boar - contact when large gilt group sizes are involved. However, it may be necessary to increase exposure times (>20 minutes per exposure) to obtain this benefit. No advantage of increasing the number of boars in the exposure groups from 2 to 4 or 6 was apparent in this study.
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ItemThe digestion of ruminant feeds in vitro( 1974)Systems of feed evaluation require the determination of the digestibility of the components of the feeds. The determination of digestibility in vivo is laborious and expensive, thus considerable research effort has been directed towards the determination of techniques to measure digestibility in vitro. The development of laboratory methods for assessing the digestibility of ruminant feeds is discussed. A two- stage technique, composed of microbial and enzymatic digestion stages, has currently been applied to the analysis of herbages, mixed forages, wood, brassicas, concentrates and silages. The digestibility of substrates determined by means of the two-stage in vitro method may be affected by the method of preparation of the substrate, the pH and duration of the microbial digestion, and the ratio of the rumen fluid to the buffer solution. The source of the rumen fluid may also affect digestibility in vitro , since the activity of this. fluid varies between species, individuals within species and the diet of the donor animals. Variation due to the diet of the donor may be minimized by means of a diet with an adequate crude protein content. Regression analyses are presented relating in vivo digestibilities to values of digestibility estimated by various laboratory procedures. In vivo digestibility may be predicted more accurately by the in vitro digestibility technique than by the nylon bag technique or by chemical methods of evaluation. However, methods incorporating the digestibility of cell wall material, digestion by fungal cellulase or summative equations of chemical constituents may be used as alternatives to the two-stage digestibility technique. The review is concluded with a discussion of the relationships between in vitro digestibility values of feeds and the values of total digestible nutrients, index of nutritive value, starch equivalent, voluntary intake, digestible energy and metabolizable energy , the last two of which are currently used to formulate rations for ruminants.