Energy Energy is the potential to do work. Energy can be converted from one form to another but can not be created or destroyed. Units (Nutrition) calorie is most often used in US = heat required to raise the temperature of 1 g of water from 16.5 to C. 1 calorie = joules 1 kcal = 1,000 calories 1 Mcal = 1,000 kcal

Energy Gross energy = Energy (E) in the feed measured in a calorimeter Digestible energy (DE) = E in the feed - E in the feces. Measured in digestion trials Metabolizable energy (ME) = E in the feed - (E in the feces + E in urine + E in combustible gases) Measured in digestion studies + collect gas loss

Energy Retained energy (RE) = ME - heat loss (HE) Measured in 1) Calorimeter study Measure energy intake and energy losses 2) Comparative slaughter experiment Measure energy intake and energy retained in gain

Energy Metabolizable energy used for: 1) Maintenance (heat loss) 2) Production (gain in body mass, milk, reproduction, wool) Feed values HE at zero feed intake is energy required for maintenance Feed intake when RE = 0 is feed required for maintenance (I m ) NE m = HE/I m NE g = RE/(I-I M )

Energy + RE, 0 Mcal _ ME intake, Mcal Maintenance NmNm NE g

Energy Value of Body Reserves Energy mobilized, Mcal = 9.4*kg mobilized fat + 5.7*kg mobilized protein During mobilization 1 Mcal of RE = 0.80 Mcal diet NEm During repletion 1 Mcal of NEm = 1 Mcal of RE

Equations to Estimate Energy Requirements of Beef Cattle Maintenance NEm, Mcal/d = 0.077SBW.75 SBW = Shrunk body wt, kg

Equations to Estimate Energy Requirements of Beef Cattle Adjustments in maintenance requirement: 20% greater for Holstein Steers and heifers similar 15% greater for bulls 20% greater for lactating cows Similar for all ages Effect of previous temperature Effect of activity on pasture

Energy Required for Maintenance Effects of environmental temperature: Zone of thermal neutrality: Animal does not have to expend energy to maintain body temperature.

Energy Required for Maintenance Lower critical temperature: Temperature at which animal begins to use energy to maintain body temperature. Affected by hair coat, hide thickness, hair coat dry or wet, body condition Upper critical temperature: Temperature at which animal begins to use energy to maintain body temperature. Affected by hair coat color, body condition

Equations to Estimate Energy Requirements of Beef Cattle For ration formulation RE is estimated from quantity of feed remaining after accounting for feed needed for maintenance.

Plant Cell PROTEIN SUGARS STARCH PECTINS FATS PRIMARY WALL SECONDARY WALL CELL CONTENTS CROSS SECTION

Inside SECONDARY WALL PRIMARY WALL CELLULOSE HEMICELLULOSE LIGNINACID DETERGENT FIBER (ADF) NEUTRAL DETERGENT FIBER (NDF) OUTSIDE

Forage ADF and Dry Matter Digestibility Adapted from Miller et al. (1991)

Inside SECONDARY WALL PRIMARY WALL CELLULOSE HEMICELLULOSE LIGNINACID DETERGENT FIBER (ADF) NEUTRAL DETERGENT FIBER (NDF) OUTSIDE

Forage NDF and Dry Matter Intake Adapted from Mertens (1985) & Burns et al. (1997)