Rain-induced sodium dissolution and loose mineral mixture runoff in uncovered feeders: possible effects on mineral intake, growth performance, and nutritional needs fulfilment of grazing cattle.

Abstract

The objective was to investigate the effect of rainfall on sodium (Na) dissolution and total losses by runoff of loose mineral mixtures (MM) available in uncovered feeders to grazing cattle, as well as the possible impacts on their growth performance and fulfilment of Na nutritional needs. Experiments (n = 7) were conducted to quantify the amount of Na lost due to rainfall and to evaluate the effects of different Na levels in the MM (35, 70, 105, and 140 g Na/kg) on MM assumed intake and growth performance. Cattle grazed Brachiaria sp. pastures and received free-choice MM in uncovered feeders throughout four to nine evaluation periods lasting 14 to 21 d each. MM assumed intake from the feeder was measured in each period as the difference between the supplied MM mass and the unconsumed MM mass. A new methodology was proposed to measure MM losses by runoff (exclusion feeder). The mass of runoff MM and the volume of runoff rainwater were both determined and analysed for their mineral content. The dietary Na requirement was estimated using different feeding systems. The minimum MM Na content to meet dietary Na requirements was estimated in different scenarios. Sodium chloride composed 28% of the runoff MM mass, in average. The action of rain may reduce MM Na content by 28% in a single rainfall event. Na content strongly affected MM assumed intake and, possibly, growth rate. MM losses by runoff were estimated at 25.5%, according to the exclusion feeder methodology. The minimum required MM Na content for meeting dietary Na needs may reach up to 87.3 g Na/kg. About 27% of evaluated periods may have failed to fully meet cattle Na requirements. The action of rain in unsheltered loos MM may result in unbalanced mineral consumption and unmet Na needs, leading to impaired mineral nutrition of grazing cattle, especially in tropical conditions.