progesterone radioimmunoassay (RIA) for enhancing reproductive efficiency of ruminants, and RIA based leptin and insulin like growth factor assays for assessing the nutritional status of animals; feeding of 15N enriched plant material to generate 15N-labelled excreta for research on the fate of excreta N in the environment; 15N, 13C and 34S isotopic methods for nutrient budgeting and for following the nutrient pathways in the soil–plant–animal continuum