The influence of growth stage on the nutritional value of Panicum maximum (cv. Gatton) and Digitaria eriantha spp. eriantha silage for sheep

Abstract

The objective of this study was the evaluation of grass silage made from P. maximum cv. Gatton (P. maximum) and Digitaria eriantha spp. eriantha (Smutsfinger) grass. The study was conducted in two phases. In phase one the influence of sugar (molasses) addition, wilting, growth stage and plant species, on the quality of the silages obtained, were investigated. Three growth stages were incorporated, an early (physiologically young), medium (boot) and late (full bloom) stage. Four different treatments were applied during ensiling, namely direct cut, direct cut + sugar, wilted and wilted + sugar. The second phase comprised of a partial digestibility study to evaluate the differences between plant species and growth stage, within a specific treatment. The silages used were made from the medium and late growth stages, where prior wilting and sugar addition occurred. The addition of sugar during ensiling, even with prior wilting, had a positive effect on silage quality, leading to a lower pH, ammonia-nitrogen concentration and a higher lactic acid concentration in the silage. These effects were less pronounced with the plant material ensiled at a late growth stage. These silages tended to undergo secondary fermentation between day 21 and day 120, when sugar was added. When comparing the three growth stages, within the direct cut + sugar and wilted + sugar treatments, the following was observed. The silages made from early and medium growth stages tended to be of better quality compared to the silages made from the late growth stage. In most of the comparisons the silages made from the medium growth stage were of a better quality than that of the silages made from the younger plant material. In the comparison between P. maximum and D. eriantha silages, the silages made from P. maximum grass tended to be of a better quality than that made from D. eriantha grass. The difference between the two species was smaller than the differences observed between growth stages within a specie. In the partial digestibility study, there was a difference in OMI, with sheep receiving the late growth stage silages having higher intakes. When expressed as DOMI / W0.75 no significant differences were observed between the four diets. There were no significant differences in the total digestibility of OM across the four diets. There were differences in the rumen ammonia and VFA's concentrations across the diets. There was a significant difference in the intake of nitrogen across the four diets, resulting in differences in the disappearance of nitrogen, ammonia and non-ammonia-nitrogen across the four diets. There was no difference in the total true nitrogen digestibility across the four diets.