Abstract
Nutrient retention within livestock systems is a critical goal for enhancing efficiency of food production and promoting sustainable livestock farming. We define nutrient retention as the sequestering of C, N, and P within biotic elements of farming systems in sinks which have minimal loss to environmentally detrimental ends. Our objective is to summarize strategies to improve nutrient retention from ruminant animal production systems using nutritional management. Nutritional strategies for nutrient retention within animals and downstream food products include dilution of maintenance; precision feeding; and nutrient synchrony. Dilution of maintenance is a phenomenon where more efficient animals partition proportionally less of their consumed nutrients to maintenance processes, allowing proportionally greater quantities to be used for productive processes. Precision feeding encompasses numerous strategies, including amino acid feeding; precision carbohydrate feeding; individualized ration formulation; and precise feed delivery; among others. These strategies all precisely match nutrients supplied within a diet to nutrients required by an animal, thereby reducing nutrient overfeeding and waste. Nutrient synchrony is a feeding strategy promoting theoretically optimal microbial growth by ensuring appropriately matched supplies of carbohydrate and N substrates to facilitate fermentation. Improved microbial fermentation also serves to reduce nutrient loss to the environment through enhanced feed digestion efficiency. Evaluating animal feeding strategies and nutrient retention requires considering nutrient sinks beyond the animal and its productive outputs. Such sinks include soil and plant nutrient stores within the ecosystem. For pastured animals in particular, grazing strategies for enhanced forage production and soil C sequestration have been broadly reviewed. Sequestration of N and P within soils and plants have been less broadly reviewed but merit further discussion in the context of attempting to design management to create non-volatile sinks for multiple nutrients simultaneously. Whole-farm strategies for enhanced nutrient sequestration should be evaluated in a multi-nutrient environment to evaluate sustainability outcomes.