Effect of harvesting interval in different stages of growth on yield and nutritive value of Napier grass

Increased livestock production can be achieved through the cultivation of high-quality forages with high-yielding capacity. Napier grass has been introduced to all tropical and subtropical areas of the world and is used as a forage crop with high potential productivity in terms of annual dry matter yield. However, both dry matter yield and nutritional quality of this grass have been affected by different cutting interval. This grass has been neither grown nor studied as a source of forage for ruminant nutrition in Iran.This research was conducted to identify the appropriate cutting interval for Napier grass to produce the maximum nutritive value and yield. Napier grass was cultivated in a complete randomised block design with three replicates. During the growth season, the cutting intervals were 16-20, 24-28, 43 d or 136 d after planting to corresponding cutting heights of 50, 100, 150 and 300 cm. Hence, from each plot 6, 4, 2, or 1 cuts were harvested, respectively. Cell wall fractions, organic matter, and crude protein of Napier grass from different cuts were determined. Dry matter and organic matter digestibility and in-vitro ruminal fermentation kinetics of the forages were also determined. Biomass production of Napier grass under different cutting interval programs was also assessed. The highest yield of dry matter of Napier grass (25.1 tons per hectare) was observed when only one cut was harvested at the end of vegetative stage (height of 300 cm). The digestibility of organic matter (OMD) ranged from 60 to 70 % in all treatments and the highest OMD was observed at the height of 50 cm. The maximum dry matter and digestible organic matter per hectare (15.9 and 14.2 tons, respectively) was produced when forage was harvested in one cut at the height of 300 cm. There was no difference in the yield of digestible dry matter and digestible organic matter of the forage harvested at the height of 50 and 100 cm. The concentration of crude protein in forages from different cuts ranged from 7.1 to 14.2 % and neutral detergent fiber ranged 65.7 to 76.2 %. The highest concentration of crude protein and the lowest cell wall components were observed at the height of 50 cm. The lowest and the highest gas production potential and fractional rate of gas production were observed at the cutting height of 300 cm and 50 cm, respectively. The present study showed that it is possible to obtain higher quality of Napier grass by shortening the harvesting intervals. However, this resulted in a pronounce decrease in the yield of organic matter and digestible dry matter yield per hectare. The highest organic matter and digestible dry matter per hectare was produced when Napier grass was harvested only once (136 d after planting and height of 300 cm) at the end of vegetative stage.