The fate of micronutrients in excreta of sheep supplemented with organic/inorganic micronutrients in a pasture system

Micronutrients play an important role in metabolic reactions in animals. To correct for potential micronutrient deficiencies in forages, the dominant feed source for ruminants, farmers routinely administer supplements to animals prophylactically. Micronutrients that are not absorbed from supplements are excreted, providing a potential flux of micronutrients to soil for uptake by forages. However, it is unclear if the form of supplemental minerals given to animals significantly affects the flux of micronutrients in pasture systems. This study investigated the impacts of supplement form (organic or inorganic), dose, excreta type (urine and/or faeces), and soil organic matter (OM) on the flux of micronutrients in a grazing pasture system. Over two weeks, 24 sheep were supplemented with organic or inorganic forms of Zn, Cu, Mn, and Se at two industrial doses. The excreta were applied to potted soils and seeded with perennial ryegrass (Lolium perenne L.). The redistribution of micronutrients was followed from feed to excreta to soil to grass. Over 90% of the Cu, Zn and Mn, and over 80% of the Se was excreted through faeces, regardless of supplement form or dose. The concentrations of bioavailable Zn, Cu, Mn and Se in faeces were not significantly affected by the supplement form, but recalcitrant fractions of Zn, Cu and Se were, implying a potential long-term effect of supplement form on micronutrient bioavailability in soils. Uptake of Zn, Cu and Mn by ryegrass was most affected by excreta type. Although faeces contributed 20-1200 times more Zn, Cu and Mn than urine, micronutrient uptake by ryegrass was significantly higher in soils applied with urine because N, K and S in urine increased grass growth. Uptake of Se by ryegrass was limited in high OM soils regardless of excreta type, revealing antagonism between the benefits of OM to plant growth and the bioavailability of Se.