The effect of harvesting interval on herbage yield and nutritive value of Napier grass and hybrid Pennisetums

A 6 (accession) x 5 (cutting interval) factorial experiment was conducted over two years to investigate the effect of stage of growth on herbage production, nutritive value and water soluble carbohydrate (WSC) content of Napier grass and Napier grass x Pearl millet hybrids (hybrid Pennisetum). The purpose of the experiment was to determine the optimum stage of growth to harvest the Pennisetums for ensilage. Two Napier accessions (SDPP 8 and SDPP 19) and four hybrid Pennisetum (SDPN 3, SDPN 29, SDPN 38 and Bana grass) were compared at five harvest intervals (viz. 2, 4, 6, 8, and 10 weeks). Basal fertilizers were similar in all treatment plots, although nitrogen (N) top-dressing fertilizer was varied proportionately, depending on the harvesting interval. The application was based on a standard rate of 60 kg N/ha every six weeks. Stage of growth had significant effects on forage yield, WSC content and nutritive value of the Pennisetums. Herbage yields increased in a progressively linear manner, with age. Nutritive value declined as the harvesting interval increased. In particular, crude protein content declined rapidly (p<0.001) from 204 g kg(-1) DM at 2 weeks to 92 g kg(-1) DM at 8 weeks of growth. In vitro dry matter digestibility decreased from 728 to 636 g kg(-1) DM, whilst acid and neutral detergent fibre contents increased from 360 and 704 to 398 and 785 g kg(-1) DM, respectively. Rapid changes in nutritive value occurred after 6 weeks of growth. The concentration of WSC increased in a quadratic manner, with peaks (136-182 g kg(-1) DM) at about 6 weeks. However, the DM content of the forage was low (150-200 g DM kg(-1)) at 6 weeks. Therefore, it was concluded that Pennisetums should be harvested between 6 and 7 weeks, to increase DM content and optimize herbage production without seriously affecting nutritive value and WSC content. Accessions SDPN 29 and SDPP 19 appeared to be most suited for ensilage. It was suggested that WSC content should be incorporated as a criterion in the agronomic evaluation and screening of Pennisetum varieties.