22-Agric-A1 Applied Plant, Animal or Human Physiology · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. 04-Agric-A1 Animal or Human Physiology, National Exams May 2013 — a three-hour closed-book examination; one of two approved calculator models (Casio or Sharp) is permitted. The cover page states that five (5) questions constitute a complete exam paper and that only the first five as they appear in the answer book are marked, that each question is of equal value, and that full marks are not available for a bare correct answer — the reasoning must be communicated clearly. All six printed questions are worked here, because the set is a study resource rather than a timed attempt.
Reference texts. S.E. Curtis, Environmental Management in Animal Agriculture (thermoneutral zone, lower/upper critical temperature and the factors that shift them, group and flooring effects); D. McDonald et al., Animal Nutrition, 7th ed. (gross/digestible/metabolizable/net energy partition, heat increment of feeding); M.K. Yousef (ed.), Stress Physiology in Livestock (external stressors and energy partition, thermoregulatory heat-flow pathways); P.D. Lewis and T.R. Morris, Poultry Lighting: the Theory and Practice (photoperiod versus light-intensity reciprocity); ASABE Standards (American Society of Agricultural and Biological Engineers) (metabolic body-size scaling and sensible heat production of livestock).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Not every unit of energy an animal eats becomes growth, milk or work. Feed energy is partitioned through a fixed sequence of losses before what is left can be used for maintenance and production, and the standard animal-nutrition scheme names each stage as it strips a loss away from the previous one.
Gross energy (GE) is the total chemical energy in the feed as fed, measured by bomb calorimetry. Not all of it is absorbed: undigested feed residue leaves in the faeces as fecal energy, and what remains is digestible energy (DE). Of the DE, a further slice is lost as urinary energy (nitrogenous waste, mainly urea) and, in ruminants especially, as gaseous products of fermentation (methane, mostly); what is left is metabolizable energy (ME) — the energy actually available to the animal's metabolism. Converting and using ME is not free either: digestion, absorption and the biochemical work of assimilation generate their own waste heat, the heat increment of feeding, and subtracting it leaves net energy (NE) — the energy the animal can actually deploy. NE is partitioned LAST, and in strict priority order: maintenance (basic tissue turnover, organ function, and any thermoregulatory heat production required by cold or heat stress) is met first, and only the remainder is available for production — growth, lactation, gestation or work.
External stressor 1 — cold stress. A cold environment raises the maintenance requirement directly, because part of NE for maintenance must now cover shivering and non-shivering thermogenesis (Question 1's zones B/C) rather than just basal turnover. If voluntary feed intake rises enough to supply the extra GE this demands, NE for production can be protected; if intake cannot keep pace — common in young or poorly-fed animals — the extra maintenance cost is paid directly out of the production share, so growth rate or milk yield falls even though total feed energy intake may be unchanged or even slightly higher.
External stressor 2 — heat stress. A hot environment acts in the opposite direction on intake but the same direction on production. Animals under heat stress voluntarily reduce feed intake, because digesting feed generates the heat increment on top of an already-struggling heat budget (Question 1's zones E/F) — so GE, and every downstream energy fraction, falls. At the same time, the maintenance share of what NE remains rises, because active heat-dissipation mechanisms (panting, increased peripheral blood flow) themselves cost energy. The combination of lower intake and a larger maintenance claim on a smaller NE pool leaves markedly less for production; this is the standard explanation for the summer depression in growth rate and milk yield seen in most livestock species.
