Effect of length of ensiling on silo degradation and digestibility of structural carbohydrates of lucerne and orchardgrass

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Abstract

This study determined the extent of structural carbohydrate loss from lucerne (Medicogo sativa L.) and orchardgrass (Dactylis glomerata L.) after 5, 21 and 56 days of ensiling. About 70 and 60 kg fresh matter of lucerne and orchardgrass, respectively, were ensiled in nine silos of 120 l capacity. For each forage, the four treatments (i.e. fresh material and 5, 21 and 56 days silages) were fed to four male castrated sheep in a 4×4 Latin square design experiment to determine effects of ensiling on digestibility. The overall fermentation quality of the two forages was judged to be good and within acceptable guidelines. Dry matter (DM) and gross energy losses were small, confirming an acceptable fermentation process. Hemicellulose losses of 19.8 and 17.2% occurred in lucerne and orchardgrass, respectively, by 56 days of ensiling. Cellulose losses in both forages were small compared to those of hemicellulose. Pectin loss by 56 days of ensiling were similar to hemicellulose, being 17.3% in lucerne and 17.7% in orchardgrass. Hemicellulose digestibility decreased (P<0.05) in lucerne and orchardgrass as ensiling advanced, while cellulose digestibility was higher (P<0.05) in days 21 and 56 silages than in the harvested lucerne grass. An appreciable amount of structural carbohydrate (i.e. pectin+hemicellulose+cellulose) were degraded during fermentation.

Introduction

Information on degradation of cellulose, hemicelluose, and pectin in silage during fermentation is scarce relative to that available for protein and non-structural carbohydrates (Michael, 1984). Similarly, little is known of the utilization of these components as microbial substrates during ensiling. While previous reports indicate that the bulk of the acids produced during ensiling originate from fermentation of water-soluble carbohydrates (WSC) (McDonald and Whittenbury, 1977), more recent reports suggest that substances such as protein and structural carbohydrates can also be substrates (McDonald et al., 1991). Cellulose, hemicellulose and pectin may also be substrates for microorganisms during fermentation, and their degradation during ensiling depends on interrelated factors, such as differences in forage species, forage growth stage at harvest, moisture content at ensiling and period and length of ensiling of forage, which are not fully understood.

When forage is ensiled, 2–3 weeks are generally required for it to attain stability (McDonald et al., 1991). The length of time that silage is stable due to a low pH depends on the nature of the fermentation in the ensiled crop.

This study examined the extent of degradation of structural carbohydrates of lucerne and orchardgrass after 5, 21 and 56 days of ensiling, as well as effects of silage structural carbohydrate degradation on in vivo digestibility in sheep.

Section snippets

Silage preparation

Lucerne (Medicago sativa L.) and orchardgrass (Dactylis glomerata L.) were harvested during the early flowering and heading stage, respectively, at the Obihiro University Farm, Japan. The two species were wilted for 8 h and chopped into lengths of 3–5 cm using a mechanical forage cutter. The two species were mixed individually and representative samples obtained. About 70 kg fresh matter of lucerne and 60 kg fresh matter of orchardgrass from the remaining content of each species were ensiled in

Composition of the fresh material and silages

The WSC contents of all silages declined (P<0.05) as the length of ensiling increased (Table 1). The hemicellulose and pectin contents decreased during ensiling in both species (P<0.05), while cellulose contents in both forages increased (P<0.05).

The pH values in both forages were lower (P<0.05) at 21 and 56 days of fermentation compared to 5 days, while the lactic acid concentration was higher (Table 2). The acetic acid content decreased (P<0.05) in lucerne, but increased in ochardgrass, as

Conclusion

The DM digestibility of lucerne and orchardgrass were not influenced by ensiling, however, losses of hemicellulose and pectin increased as ensiling advanced. Hemicellulose digestibility decreased in the lucerne and orchardgrass as ensiling advanced probably due to losses of these components during fermentation. Cellulose digestibility was higher in days 21 and 56 silages as a result of extra-cellular cellulase activity from silage microflora.

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