A comparison of the digestion and reduction in particle size of lucerne hay (Medicago sativa) and Italian ryegrass hay (Loliurn italicurn) in the ovine digestive tract
B Y E. G R E N E T Station de Recherches sur la Nutrition desHerbivores, Unite' de l'lngestion, INRA, Centre de Recherches de Clermont-Ferrandl Theix, 63122 Ceyrat, France (Received 13 May 1988 -Accepted 20 March 1989 .-.~
~~ ~~Four types of hay were used: an early-cut and a late-cut lucerne (Medicago sativa) and an early-cut and a late-cut ryegrass (Loliurn italicurn). Digestibility and voluntary intake were measured on six adult sheep maintained in metabolism crates and fed 10% above the previous day's consumption, in equal portions a t 08.00 and 16.30 hours. Lucerne intake was higher than that of ryegrass at similar digestibilities. The rate of digestion of the four hays was measured using nylon bags and the rates of digestion of dry matter for late-cut lucerne were faster than those for the late-cut ryegrass. Eleven adult sheep fitted with oesophageal (four), rumen (four) or duodenal (three) fistulas were used for collecting digesta samples at the different sites. They were fed successively with the four hay types. Each was offered to appetite, once daily, in a meal taken over 4 h. The rumen of rumen-fistulated sheep was completely emptied by hand once before feeding (08.00 hours) and once after removal of refusals (12.00 hours). The number of times the ingested hay was chewed during the meal and the time spent masticating was greater for the late-cut hays and greater for the ryegrass compared with the lucerne hays. The particle size of the boluses was greater than that of the rumen contents, which in turn was greater than that of the duodenal contents and faeces. The percentage of particles > 8 mm in the boluses from the ryegrass was higher than that of lucerne, and that of the early-cut ryegrass was greater than that of the late-cut ryegrass. This was also the case for the percentage of particles > 1.6 mm in the rumen contents after the meal. The rate at which dry matter disappeared from the rumen was greatest during the meal except with late-cut ryegrass; it was particularly high with late-cut lucerne, a little less with early-cut ryegrass and much lower with the other two hays. When nylon bags containing food boluses were placed in the rumen, it was apparent that without rumination, with the exception of the very digestible earlycut ryegrass, microbial degradation alone did not reduce the particle size. It is generally held that feedstuffs can only leave the rumen after reduction of the particulate matter to less than a critical size: 1 mm in sheep (Poppi et al. 1980), and 2 mm in cattle (Grenet, 1970;Ulyatt et al. 1986). The reduction in particle size results from microbial digestion and mastication. Although ingestive mastication is important, the essential role in particle breakdown is played by rumination (Ulyatt et al. 1986): microbial digestion renders the plant material more sensitive to rumination. The ease with which plant mat...
The effect of O‐methyltransferase (OMT) cDNA modulation on cell wall composition, ultrastructure and rumen degradability was measured on transgenic tobacco (Nicotiana tabacum). The expression of OMT cDNA in antisense orientation (AS) inhibited OMT activity by 92% whereas expression of sense constructs led to plants either co‐suppressed (CS, 98% inhibition) or overexpressing OMT activity. The cell wall residues of stems were analysed for lignin content, products of nitrobenzene oxidation (NBO) and polysaccharide content. Degradability was determined by a cellulase method. Sections of stem were stained by acid phloroglucinol and Mäule reactant. Stem samples were incubated in the rumen for 8, 24 and 48 h and observed by scanning electron microscopy (SEM). Compared to controls, OMT‐depleted stems showed decreased hemicellulose content but unchanged lignin content. In contrast, syringyl units decreased by 40 and 90% in AS and CS samples respectively and NBO content followed a similar trend. Dry matter cellulase degradability was significantly improved by 3·5 and 5·6 percentage units in AS and CS samples respectively. SEM showed a greater bacterial colonisation in these samples and indicated a higher rate of rumen degradability in CS tissues than in controls. Overexpressing plants had a composition and a degradability similar to that of controls. For all the plants studied, the improvements in dry matter degradability were closely linked to the syringyl to guaiacyl ratio or to the NBO content. The modifications observed in down‐regulated tobacco were similar to those produced by bm3 maize mutation, but without lignin decrease. Genetic modifications should therefore be considered for improving forage digestibility.
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