Graduate student literature review: The development of the Brouwer equation to estimate heat production-Past, present, and future perspectives.

Most studies employ indirect calorimetry to study ruminant energetics and indirectly measure heat production (HP) using an equation published by Ede Brouwer in 1965. Initially estimates of HP used measured oxygen (O₂) consumption and carbon dioxide (CO₂) production, and this was based on the relationship developed by Antoine Lavoisier between the 2 respiratory gases and metabolic HP. These gases were subsequently integrated into the first indirect calorimetry equation by Nathan Zuntz in the late 1800s. However, based on incomplete oxidation of methane (CH₄) and HP associated with urinary N excretion, inputs into this equation now include O₂ consumption, CO₂ and CH₄ production, and urinary nitrogen losses. In most cases, little to no hydrogen (H₂) is eructated by cattle but this may increase when cattle consume some feed supplements and compounds that reduce ruminal CH₄ production and as a result, H₂ may be considered when HP is indirectly estimated.