22-Agric-A1 Applied Plant, Animal or Human Physiology · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. 04-Agric-A1 Applied Plant, Animal or Human Physiology, National Exams May 2018 — a three-hour closed-book examination; one of two approved calculator models (Casio or Sharp) is permitted. The rubric states that five (5) questions constitute a complete exam paper and that the first five questions appearing in the answer book are marked (worth 20 marks each, 100 marks total); all six (6) printed questions are worked here as a complete study resource.
Reference texts. M.K. Yousef (ed.), Stress Physiology in Livestock, Vol. I — Basic Principles, CRC Press (thermoregulation, thermoneutral zone, piloerection, endotherm/ectotherm physiology); K. Schmidt-Nielsen, Animal Physiology: Adaptation and Environment, 5th ed. (metabolic body-size scaling, Kleiber's law, thermoconformers, calorimetry); P. McDonald et al., Animal Nutrition, 7th ed. (gross/digestible/metabolizable/net energy, feed-energy partition, growth efficiency); R.L. Curtis, Environmental Management in Animal Agriculture, Iowa State University Press (thermoneutral zone, animal housing microclimate); D.M. Lewis & T.R. Morris, Poultry Lighting: the Theory and Practice (photoperiodism); ASABE Standards (American Society of Agricultural and Biological Engineers), Livestock Energetics and Thermal Environmental Management (design sensible heat production, calorimetry).
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.
General trend. A growing pig's efficiency at converting feed energy into body-tissue growth follows an inverted-U (dome-shaped) response to ambient temperature: it is highest somewhere within the thermoneutral zone (TNZ) and falls off both below the lower critical temperature (LCT) and above the upper critical temperature (UCT), because both extremes divert an increasing share of the pig's metabolizable energy (ME) away from productive tissue deposition and into non-productive thermoregulatory cost.
Variation of energy intake. Within the TNZ, voluntary feed (and hence ME) intake sits at a relatively stable near-ad-libitum baseline, set mainly by the pig's genetic growth potential and gut fill capacity rather than by thermal stress. Below the LCT, intake typically rises somewhat — the pig partially compensates for its extra thermoregulatory cost by eating more — but the rise is limited by gut capacity and cannot fully offset the added cost of cold thermogenesis. Above the UCT, intake falls sharply: this is the well-known heat-stress intake depression, driven by the pig's physiological/behavioural drive to minimize the heat increment of feeding (digesting and metabolizing feed itself generates heat, which is disadvantageous when the animal is already struggling to dissipate heat) together with a general appetite suppression under heat stress.
Variation of metabolic heat production. Heat production follows a U-shaped (not monotonic) curve. Below the LCT it rises steeply as regulatory (cold) thermogenesis — shivering, non-shivering thermogenesis, and increased voluntary activity — is engaged to defend core temperature. Across the TNZ it is essentially flat at its minimum, comprising only basal maintenance heat plus the heat increment of whatever feed is being digested. Above the UCT it rises again, but for a different reason than on the cold side: even though falling intake somewhat reduces the heat increment of feeding, this is outweighed by the added metabolic cost of active evaporative heat-loss mechanisms (panting muscle work, elevated respiratory rate) and by reduced digestive/metabolic efficiency under heat stress, so net heat production climbs again as ambient temperature continues to rise past the UCT.
Variation of feed-use efficiency for growth. Efficiency (productive tissue energy retained per unit of ME intake) is maximal within the TNZ, because essentially all of the ME above maintenance is available for tissue deposition with no added thermoregulatory tax. Below the LCT, an increasing share of ME intake is diverted into obligatory cold thermogenesis (MEm rises) rather than growth, so efficiency falls even though intake itself may be somewhat elevated. Above the UCT efficiency falls even more sharply than below the LCT, because two effects compound in the same direction: the numerator's raw material (intake) drops, while what ME does remain is increasingly consumed by the elevated cost of active heat dissipation, leaving proportionally less for growth than at either the LCT or within the TNZ.
