349 The need for forage intake measurements

We often tell students of nutrition that “animals require quantities, not percentages, of nutrients.” Nonetheless, diets and supplements are typically formulated on a percent basis, which is convenient and works well if feed intake is known. Grazing ruminants require quantities of nutrients, but unlike their pen-fed counterparts, forage intake is unknown. Estimates of intake are essential to make management decisions relative to the quantity and timing of supplemental nutrients, which are often the largest out-of-pocket cost for ruminant production based on grazed forages. Moreover, sustainable management of grazing lands requires estimates of intake relative to available forage to set appropriate stocking rates and to assess environmental impacts (e.g. greenhouse gases). These management decisions become more critical as the physiological state of grazing animals change over time, resulting in shifts in both feed intake and nutrient requirements that affect the need for supplemental nutrients and can alter grazing management strategies. This dilemma – not knowing feed intake but needing it to make critical management and environmental decisions – necessitates the use of intake estimates as a basis for formulating supplemental nutrition plans and developing grazing management schemes. These estimates range from “thumb rules” for intake at various physiological states to more complex equations based on forage composition (also generally unknown in grazing situations), environmental variables, and various animal factors. The validity of these estimates and equations is often tested against intake values in pen-fed ruminants, as well as estimates derived from indirect methods in grazing animals (e.g. marker-based methods). Developing methods that yield accurate and precise predictions of voluntary intake by grazing ruminants has been a long-standing challenge, but the need for effective methodology and estimates of forage intake remains critical for grazing livestock.