ABSTRACT
Summary The acreage of stargrass and bermudagrass (Cynodon spp.) throughout tropical and subtropical regions has increased greatly. However, many Cynodons have a hydrocyanic acid potential (HCN-p) that may be toxic to livestock. The purpose of this study was to determine the HCN-p in fresh and hay samples of meristematic stem tips from nine Cynodon entries at different physiological stages of growth during the summer (19°-33°C) and autumn (11°-32°C). Two selected entries (Ona and UF-5 stargrass) were also separated into meristematic stem tips, leaves and sheaths, and nodes and internodes at different stages of maturity to monitor HCN-p, dry matter (DM), and percentage of whole plant contributed by plant components. Correlations were determined between HCN-p concentration and total solar radiation. All plant entries were staged at 7.5 cm and received 280, 25, and 93 kg/ha of nitrogen (N), phosphorus (P), and potassium (K), respectively, one week prior to commencement of summer and autumn studies. Significant differences in HCN-p concentration in meristematic stem tips were obtained among the nine Cynodon entries over a 2-year period. The HCN-p concentration basically followed either high or low levels. High-level HCN-p grasses averaged 407 and 529 ppm on a fresh-weight (FW) basis during summer and autumn, respectively; low-HCN-p grasses averaged 121 and 131 ppm FW. Average HCN-p concentrations were lowest for Callie bermudagrass (75 ppm) and highest for Puerto Rico stargrass (506 ppm). Entries decreased with maturity during summer, but during autumn HCN-p remained high until temperatures fell below freezing. After a freeze HCN-p decreased from 92 to 43 ppm and 78 to 25 ppm in high- and low-level grasses, respectively. Meristematic tips dried to 85 % DM decreased in HCN-p by 88 % and 78 % for high- and low-level grasses, when compared with fresh samples. Significant correlations (r = 0.56) were found between HCN-p and total solar radiation. Separating Ona and UF-5 grasses into plant parts for DM and HCN-p revealed similar DM levels in both grasses. However, HCN-p in UF-5 was consistently lower (P < 0.05) than HCN-p in Ona stargrass. The meristematic tips generally contained the highest (P < 0.05) HCN-p regardless of species. These data indicated differences (P < 0.05) in HCN-p concentrations between Cynodon entries that can be beneficial when managing tropical grasses.
