Influence of grazing intensity on cyanogenic toxicity in Savanna grasses in Baringo, Kenya

Abstract

The potential role of anti-herbivory mechanisms involving use of qualitative and quantitative compounds in minimizing herbivory is well known. However, synergistic responses to grazing and interactive effects on herbivores are poorly understood. The aim of this study was to quantify the interaction between cyanogenic glycosides in grasses with cattle grazing in Lake Bogoria (00°28'N, 35°59'E), Baringo County. Field experiments were carried out in ten 50×10m exclosures to quantify influence of grazing intensity and age of grasses on cyanogenic concentration. Grazing intensity was varied using simulated grazing method to determine the effect of grazing disturbance on cyanide concentration. Two grazing treatments which embraced heavy grazing (Clipping at 5cm height) and light grazing (clipping at 15cm height). Grasses were also categorized into two age classes; young (leaf blade length <2cm, no florescence and spikelet) and old (leaf blade length >2cm, florescence open and spikelet presence). Grass samples were tested for cyanogenic glycosides using impregnated picrate paper and concentration of the cyanogenic glycosides was determine by hydrolyzing the glycosides and trapping the cyanide evolved in 1M NaOH. The findings showed that five species out of 16 produced cyanogenic glycosides; Cynodon dactylon, Cynodon plectostachyus, Digitaria scalarum, Sporobolus spicatus and Cyperus laevigatus. Cyanogenic glycosides decreased with increase in age of plants. Young cuttings yielded more Hydrogen Cyanide than older cuttings of the same grasses though there were no significant difference (p-value>0.05 for all species). For example the concentration levels in young and old materials of C. dactylon 1.89 and 1.74 MgCN g-1 dw, C. plectostachyus 1.42 and 1.32, S. spicatus 1.26 and 1.17, C. laevigatus 1.24 and 1.47 and D. scalarum 1.21 and 1.13 MgCN g-1 dw. Grazing intensity had a significant effect on concentration of cyanogenic content in C. dactylon (1.14 and 1.50 MgCN g-1 dw, p-value<0.005) and C. laevigatus (1.27 and 1.58 MgCN g-1 dw) for control and second clipping respectively while the effect on others were not significant p>0.05 such as C. plectostachyus (1.23 and 1.42) S. spicatus 1.30 and 1.34) D. scalarum 1.22 and 1.28 and C. laevigatus (1.32 and 1.58 MgCN g-1 dw). On the basis on the findings, it is recommended that grazing regime of managed pastures should consider the age of pastures while allowing utilization of pastures preferably grazed on mature pastures with low levels of cyanogenic glycosides. Consequently the study recommends a response to a less grazing pressure on pastures and apparently avoid overstocking on managed pastures as the forage items concentrate relatively more defense glycosides with increase in disturbance.