What goes around comes around is a well-worn expression, but it succinctly sums up what happens to trace minerals when composting horse manure.
University of Kentucky researchers Ashley Fowler and Mieke Brummer-Holder, and Karl Dawson, with animal nutrition company Alltech, devised an experiment to learn more about trace mineral leaching from equine compost.
The trio, writing in the journal Sustainability, said that composting is a beneficial management practice that can reduce the volume of manure, reduce pathogen load, improve the stability of the nutrients, and produce a value-added material.
However, mineral leaching from compost can be environmentally disruptive.
Little information is available regarding trace mineral leaching from equine-sourced compost, they noted, even though a 500kg horse can produce 22.7kg of manure a day.
Including bedding material, the total waste generated from a single horse can accumulate to more than 9400kg a year.
The trio set out to quantify the mineral content and leaching potential of compost produced from the manure of horses fed different amounts and forms of trace minerals (organic or inorganic).
Manure for composting was obtained from nine horses fed three different treatment pellets, with varying sources and levels of trace minerals.
The experiment was designed as a replicated 3 × 3 Latin square, with three treatments assigned to three blocks of three horses over three time periods.
By the end of the experiment, all horses had received each of the three dietary treatments in a random order, with their manure collected under each feeding regime.
The dietary treatments were provided in a soybean-meal-based pellet with either no added trace minerals, added inorganic trace minerals, or added organic trace minerals.
The inorganic and organic mineral treatments were formulated to contain the same amount of trace minerals, with the difference being source.
The added trace minerals were cobalt, copper, manganese, and zinc. The inorganic forms were provided as cobalt carbonate, copper sulfate, manganous oxide, and zinc oxide.
The organic forms were provided as metal proteinates (BioPlex®, from Alltech).
The manure was collected from each horse after a 16-day feeding period, combined with straw, composted, and then subjected to simulated rainfall to measure mineral mobility.
Concentrations of cobalt, copper, manganese, and zinc were greater in the compost made from manure from the horses fed the inorganic and organic trace minerals compared to the unsupplemented pellets — a result which the researchers expected.
Additionally, the compost from the inorganic trace minerals had greater zinc than the organic compost.
Turning to the leachate analysis, more copper leached from the trace mineral compost compared to the pellet-only compost.
The greatest quantity of zinc leached from the inorganic trace mineral compost, followed by the organic one, and the least amount from the non-supplemented compost.
A greater amount of cobalt tended to leach from the mineral composts (both inorganic and organic trace minerals) compared to the pellet-only compost.
There were no differences in manganese leaching among treatments, despite having differences in compost manganese concentration among the different dietary treatments.
“The addition of trace minerals to the diet of horses increased the concentration and the leaching potential of those minerals in compost generated from manure,” they said.
Additionally, the source of dietary trace minerals (organic or inorganic) affected the leaching potential for zinc in equine compost, with inorganic zinc resulting in more zinc leaching than organic zinc.
“Diet, including mineral concentration and mineral source, affects the composition and environmental impact of compost, so care should be taken when formulating diets for horses.
“More research is required to assess the effects of other dietary changes on compost quality and environmental risk.”
Fowler, A.L.; Brummer-Holder, M.; Dawson, K.A. Trace Mineral Leaching from Equine Compost. Sustainability 2020, 12, 7157.