Dairy compost, variety, and stand age effects on kenaf forage yield, nitrogen and phosphorus concentration, and uptake
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The dairy and beef industries are looking for alternative forages with high yields that are capable of efficient manure-P recycling when soil moisture is limiting. Three varieties of kenaf (Hibiscus cannabinus L.) were grown under dryland conditions and harvested at 60, 90, and 120 days after planting (DAP) with yearly applications of 0, 10 (56 kg P), and 20 (112 kg P) Mg DM ha-1 of dairy manure compost. Kenaf variety 'India' was less productive but had as high or higher N and P concentrations than either 'Guatemala 4' or 'Everglades 41' during the second year of the trial. Yields tended to increase and N or P concentrations decrease with DAP, but were affected by rainfall patterns. In 1998, when rainfall was concentrated during the first 60 DAP and the last 30 DAP, the 120 DAP harvest averaged 5.28 Mg DM ha-1 yr-1, 2.2 times the 90 DAP average. In 1999, when rainfall fell exclusively during the first 90 DAP, the 90 DAP harvest averaged 5.06 Mg DM ha-1 yr-1, 1.6 times the 120 DAP average. Plant P concentration tended to increase and N concentration to decrease with compost application the second year. The cumulative effect of compost application increased forage DM yields as well as P and N uptake during the second year. Average P removal by kenaf in the 10 and 20 Mg compost DM ha-1 plots was 10.4 and 6.8%, respectively, of the P equivalent added by the compost. This indicates that the average forage P concentration of 2.11 g kg-1 from the plots that received compost would result in insufficient forage P uptake to avoid excessive soil P buildup with sustained yearly compost application.