Energy systems for growing animals

• Energy = Most important nutrient • Ration formulated to meet energy requirements = Sets of rules relating energy supply to energy requirements. • Energy systems used to: ~ Predict performance for given energy supply. ~ Calculate energy supply required for given level of performance

• Energy requirement • Energy supply Example: An animal growing at 1.0 kg requires 15.0 MJ/day NE requirement = 15 MJ/kg NE content of feed = 5.0 MJ/kg Quantity required = 15/5 = 3.0 kg/day

• Productive process (maintenance vs production + type) • Diet quality (energy substate supply) • Level of feeding

~ most systems energy supply based on ME - Energy requirements (NE) - Energy supply (ME) - Efficiency of ME utilisation (k) Example: Feed with ME value of 10.0 MJ/kg, k = 0.5 NE value = 10.0 x 0.5 = 5.0 MJ/kg

Maintenance (ME m) Growth (ME g) - Total requirement calculated using a factorial approach: ME t = ME m + ME g

• Essential muscular activity • Active transport = end products of digestion absorbed into small intestine and blood, no excretion waste products/nutrient spread round body w/o active transport) • Protein turnover = protein constantly broken down and resynthesised (40% mointence requirment) heifers fatten before steers = lower protein mass • Enzyme & hormone synthesis If animal deprived of food, catabolises body tissue for energy (glycogen, fat, protein). Energy evolved as heat

• starve animal for 4 days • no energy from diet • require energy for vital processess • use own body tisse to provide energy • estimate net energy

• Total energy gained (NE g) • Efficiency of ME utilisation for gain (kg) • Energy value of the gain (EV g) • Daily gain (LWG)

• Liveweight • Breed type (mature weight) • Sex • Rate of gain

• Diet quality (q) • Level of feeding • Productive process (fat vs protein)