VARIETY, HARVEST DATE AFTER PLANTING AND PLANT FRACTION OF NAPIER GRASS INFLUENCE IN VITRO GAS PRODUCTION

Abstract

An experiment was conducted to assess the relative superiority of digestibility of 4 varieties (V), 3 harvest dates (HD) and 2 plant fractions (PF) of Napier grass cultivated and harvested in the humid forest zone of Ghana. The varieties were; Local, 16798, 16786 and 16840. Except for the Local variety, all were improved varieties from ILCA (now ILRI). The HD was at 60, 90 and 120 days after planting and PF were leaf and stem fractions. Relative in-vitro gas production (IVGP) per gDM sample from in-vitro rumen fermentation was used as an indicator of superiority. The automated AnkomRF system was used to measure the IVGP at 5 minute intervals over a period of 48 hours. Gas production at 12, 24 and 48 hours was evaluated using a 4×3×2 factorial randomized design. No differences were found in ml gas production per gram DM incubated at 12 hours between stems, leaves, varieties or harvest dates. The stem fractions produced significantly more gas per gram dry matter than leaves at 24 hours (56.1 vs 64.0 ml; p=0.0376). No significant differences between varieties or harvest dates were found at 24 hours. At 48 hours differences were dependent on variety and harvest date. Total gas production for the stem fraction for all varieties was less at increasing harvest date. The leaf fraction of variety Local and 16798 followed the same trend with less gas produced at increasing harvest date. However, varieties 16786 and 16840 produced most gas from samples harvested at 90 days and 16786 increased proportionately more from 60 to 90 days than 16840 (58% vs. 7%). At 120 days, these two varieties produced only slightly less or almost the same amount of gas from samples harvested at 90 days. The leaf fraction of variety 16798 consistently produced the most gas across the harvest dates, but the same was not found for the stem fraction of this variety. The stem fraction of variety 16786, while producing 8% less gas than the best (16798), was the most stable gas producing variety when comparing samples harvested at 60 or 90 days and is therefore recommended for harvest at 90 days. If harvesting at 60 days, variety 16798 is recommended. If harvesting at 120 days variety local and 16840 will be the best option as gas production from stems at 120 days was relatively higher than the other varieties.