The determination of digestibility of Atriplex nummularia cv. De Kock with different techniques

Abstract

The aim of the study was to determine the digestibility of Atriplex nummularia cv. De Kock, supplemented with three levels (15%, 30%, and 45%) of either maize or barley, using different in vitro techniques. An in vivo digestibility trial was conducted, together with a number of in vitro trials. An important development has been the introduction of biological methods (Jones&Theodorou, 2000). Three digestion techniques that simulate the digestion process are currently available to determine the nutritive value of ruminant feeds: <ol> <li>Digestion with rumen micro-organisms as in the work of Tilley&Terry (1963) or gas method (Menke et al., 1979). Digestion with faeces microorganisms (El Shaer et al., 1987), as an alternative to rumen fluid inoculum.</li> <li>Cellulase methods</li> <li> In situ incubations of samples in nylon bags in the rumen.</li></ol> The results of this study showed no significant difference (P>0.05) between the rumen- and faeces inoculum in vitro techniques, but they did differ significantly from the gas production and cellulase techniques. There was also no significant difference between the gas production and cellulase techniques. Organic matter digestibility (OMD %) of the in vitro techniques differed significantly from the in vivo OMD % values. There are several possible explanations for the difference between the in vivo and in vitro OMD %. 1. Practical mistakes could have been made. 2. The simulation of the rumen motility in vitro is often difficult and it may be that all the feed particles did not have the same exposure to the micro-organisms, as it would have in the rumen of an animal. The different rumen pools are also not fully represented in vitro. 3. The fermentation characteristics and microbial constitution of the rumen inocula differ, between the animal used for the in vivo digestibility trial and the animals used for rumen inocula collection. 4. With in vivo digestibility the time of digestion is not known, and therefore the time of rumen and gastric digestion in vitro could have been too long or too short. It was found that the in vitro faeces technique of El Shaer et al., (1987) is an easier and cheaper alternative to the classic rumen fluid in vitro technique of Tilley&Terry (1963), as modified by Engels&Van der Merwe (1967). The in vitro faeces technique uses faeces as an inoculum and therefore solves the problems associated with the use of cannulated animals. The gas production in vitro technique has certain advantages, but still has the disadvantage of needing cannulated animals for rumen inocula. The cellulase-based in vitro technique in contrast eliminates the use of cannulated animals. Although the in vitro gas production technique of Pienaar (1994) and the cellulase in vitro technique could both be used to determine the OMD % of Atriplex nummularia cv. De Kock, the values will be lower than in vivo determinations.