| Summary: | Feed evaluation is an important prerequisite to the formulation of farm animal diets. Techniques for routine feed analysis must be rapid, simple to perform and must provide accurate, reliable results which are biologically meaningful. The aim of this thesis was to investigate the suitability and application of an in vitro gas production technique as a routine analytical tool in animal nutrition studies. Firstly, the effect of several biological, chemical and physical factors on the measurement of gas production were investigated. These factors included incubation temperature, headspace pressure, shaking movement of the in vitro cultures, the nature of the feedstuff (chemical composition, particle size and drying process), the source of the microbial inoculum and the apparatus used to measure gas production. In most studies, the manual pressure transducer technique of Theodorou et al. (1994) was used to investigate the effect ofthese factors. Incubation temperature, head-space pressure and shaking movement all had a significant effect on the amount of gas produced during the incubation of perennial ryegrass (Lolium perenne) hay. Of the four incubation temperatures investigated (25, 30, 39 and 45 °C), not surprisingly 39 °C was found to be the optimum temperature for incubation of batch cultures. Increasing the pressure in the head-space of the culture bottles (by increasing the interval at which gas was removed from the head-space from 2 to 4 or 6 h) led to a decrease in the amount of gas measured (p < 0.05), but did not alter dry matter (DM) loss or volatile fatty acid (VFA) production suggesting that the fermentation was not inhibited by the accumulation of gas. Continual shaking of the culture bottles throughout the incubation resulted in the detection of less gas than in those bottles which were not shaken throughout the incubation (p < 0.01). The nature and treatment of the substrate also affected gas production. Microwave, oven and freeze drying of perennial ryegrass resulted in higher cumulative gas production profiles than incubation of the fresh substrate (p < 0.01). Reducing feed particle size led to increases in cumulative gas production (p < 0.01). In the method of Theodorou et al. (1994) where fermentations were conducted in gas-tight sealed culture bottles which were vented at frequent intervals, gas production profiles were higher than the corresponding profiles obtained using the Menke et al. (1979) technique; where fermentations were conducted in gas-tight syringes and gas production was monitored by the ascent of the syringe plunger. Secondly, two potential applications for the technique were investigated; (1) as a routine feed analysis tool for the prediction of the digestible energy (DE) content of equine feeds and (2) as a screening method for investigating the potential use of novel feed additives. The first application was investigated by incubating sixteen feedstuffs of known DE. Gas production parameters, DM loss and VFA production were then used to derive prediction equations. The best prediction equation was DE = -0.68 + 0.01087 DML + 6.82 Z - 2.297 log LT (R2 = 0.878; RSD = 0.99; where DML is dry matter loss in vitro, Z is a rate parameter and Lj is the lag time for gas production). The second application was investigated using four antibiotics; monensin, avoparcin, penicillin G and chloramphenicol in the automated gas production system (APES; Davies et al., 1995). Differences in gas production profiles were detected between antibiotic supplemented and control fermentations suggesting that the technique can be used to screen feed additives. In vitro gas production techniques show considerable potential as routine analytical tools for animal nutrition studies. Results presented in this thesis inform the development of standardised methodologies and procedures for use during in vitro gas production studies, thus enabling this technique to be adopted as a robust, reliable and routine analytical tool in animal nutrition studies.
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