Sampling methods for clover species in grazed pastures to diagnose mineral deficiencies

Abstract

Diagnosis of clover nutrient status is important for legume-based pasture systems. The protocols for sample collection and setting of nutrient guidelines are ill-defined. This research quantified how nutrient concentrations differed between laminae (leaflets) and petioles (leaf stems) of subterranean (sub) clover (Trifolium subterraneum) and white clover (T. repens) as a first step in developing appropriate sampling procedures to examine the nutrient status of clover species. Field samples of sub and white clovers from three pasture sites in the Wairarapa and Canterbury were separated into lamina and full petiole and chemically analysed for nitrogen (N), phosphorus (P), potassium (K) and sulphur (S). Lamina + petiole K concentrations were lower in sub clover (2.3%) than white clover (3.2%). Across both species, the lamina concentrations of N (4.9%), P (0.36%) and S (0.28%) were higher than in the petiole (1.8%, 0.29% and 0.12% respectively) but the reverse was found for K (1.8 % lamina, 3.7% petiole). As a consequence, increasing the proportion of petiole added with the lamina affected the nutrient concentration of lamina + petiole samples so that N, P and S% declined but K% increased. In a subset of samples, NIR analysis found the crude protein of lamina + petiole in white clover was 24% compared with 21% in sub clover. Clover laminae of both species had 33% crude protein compared with only 11% in the petiole. The diagnosis of clover nutrient status for fertiliser recommendations should be based on lamina-only samples. In contrast, mixed pasture samples for nutritive value analysis should include clover laminae and petioles, plus grass and herb pasture components, to simulate grazing intake by livestock. The lack of defined sampling protocols may explain some of the difficulties previously reported in herbage nutrient interpretation and reconciling soil test results.