Broiler performance has improved over the years with feed conversion ratios for 2.5 kg birds reared in “good growing conditions” falling from ~1.91 to 1.61 from 1994 to 2011. Though dietary values for nitrogen-corrected metabolizable energy are relatively unchanged, the efficiency of MEn use for tissue accretion under these conditions has improved over 19%. The theoretical limit for converting dietary substrates to tissue has been estimated at 76%. At this level of energetic efficiency, the feed conversion ratio for creating a 2.5 kg bird would be 1.01 as opposed to 1.61. The difference between birds reared under near-ideal conditions and the theoretical FCR possibility of 1.01 is the cost of the production environment.
The range of performance is further magnified when contrasted to commercial birds under varying conditions around the world. Though it is not reasonable to expect that production efficiencies will ever reach theoretical limits, it is apparent that production environments limit broiler performance and add considerable cost. Indeed, the production environment is critical as it either takes away or adds to the efficiency of MEn utilization. The purpose of Effective Caloric Value is to enable energy assignment to production environment components related to husbandry, health and nutritional technologies.
Many factors impact energy utilization. Classical descriptors found in most nutrition texts range from gross energy (GE, total energy) to net energy (NE, energy for function). The energy form most commonly utilized by the poultry industry is MEn which fails to account for heat loss. Bird heat production is influenced by myriad factors including ambient temperature (Weirnusz and Teeter, 1993), ration composition, tissue type synthesized and activity (Mckinney and Teeter, 2004). Failure to account for variations in heat production, regardless of source, eventually has the net result of creating an uncertain MEn utilization. Bird activity comprises about 18% of MEn consumption, but is influenced by management through stocking density, lighting program, feeder and waterer space; feed processing via pellet quality; ambient temperature; and diseases such as coccidiosis via elevated body temperature and malabsorption.
The sum total of the energy factors, and how they are handled, is where the science of nutrition becomes the art of nutrition. For example, many companies have seasonal adjustment (i.e. winter vs. summer programs), carcass yield and/or composition adjustment and rations to elevate the survivability or to improve FCR. But, are there additional approaches?
Feed evaluation systems
Methodology to estimate feed energy value is necessary so that we may provide rations containing the proper balance of nutrients to energy. Without an energy evaluation method, the balanced provision of nutrients would be a difficult process making carcass composition less predictable. But, what aspect should we utilize? Gross energy would be the simplest, it does tell us that roughages and grains differ from fats for energy value, but would not tell us that there is much difference between wheat-straw and a corn-and-soybean based ration. Net energy is theoretically the best as all sources of energy loss have been accounted, however, it is cumbersome and MEn is commonly utilized today. Keep in mind that the objective for any energy scheme is to provide a tool estimating ration energy value to the bird and not values for a table.
Today’s MEn and ECV systems
The MEn system has traditionally been used by the poultry industry, and poultry feeds are commonly formulated according to tables containing nutrients in proportion to MEn (NRC, 1994). Though MEn values ignore bird heat losses, the system has the advantage of being easily determined, but the disadvantage of not being strictly quantitative. Failing to adequately adjust for heat losses creates products of varying lipid content and consumer dissatisfaction. Too frequently, bird performance varies independently from MEn. Yet, for many producers, body weight, BWT, and FCR responses to diet and environment are predictable. This forms the basis of the effective caloric value system. Figure 1 illustrates the relationships existing between bird body weight, daily FCR and dietary caloric density.
Mechanistically, the ECV system utilizes producer observed BWT and FCR to estimate the amount of dietary MEn required to mimic bird performance under the reference standard conditions. Though the reference standard may be removed from the strict producer environment, the ECV difference between two producer environment inputs is well correlated with producer benefit. Consequently, the energy value of nonnutritive components may be estimated as caloric density and capitalized.
Historically, managerial, nutritional, environmental and health aspects of broiler production have been viewed as separate entities. However, these multifaceted components overlap, and the final production outcome is contingent upon this interface. Fundamentally, energy metabolism is the ideal denominator among these seemingly disjointed factors. For example, facility and/or managerial improvements may decrease broiler energy expenditure associated with bird activity, and, consequently, additional energy becomes available to the bird for growth. Assuming the diet provides sufficient indispensable nutrients for lean accretion, the additional calories would be shunted to lean growth. If insufficient nutrients are available, the added energy would be deposited as fat. Managerial and environmental factors potentially impacting bird energy expenditure in the production environment include:
• Stocking density (to to ~225 kcal/kg)
• Lighting regime (to ~125 kcal/kg)
• Litter quality regime (to ~135 kcal/kg)
• Feed form regime (to ~187kcal/kg)
• Temperature regime (to ~325 kcal/kg)
• Human interaction
• Immunity development (to 15 kca/kg)
• Disease cost (to ~1,500 at extreme)
The overall cost to the producer may be viewed by contrasting the germ-free bird versus those exposed to varying microbial-protozoa loads. Lev and Forbes (1959) were the first to report that chickens reared in axenic (germ-free) environments grew more than 10% faster than those exposed to conventional flora.
Coccidiosis, mediated by protozoa of the genus Eimeria, is among the major disease challenges facing the poultry industries. Though a variety of therapeutics is available to minimize coccidiosis incidence and severity, birds will normally develop immunity during the production cycle. Vaccination at the time of hatching speeds immunity, and immunity timing can be critical to performance as late challenges have higher costs for birds lacking immunity. Coccidiosis mediated lesions are associated with elevated maintenance energy need, malabsorption, depressed appetite, reduced BWT and worsened ECV and FCR.
Reductions in bird performance (Figure 2) due to coccidiosis intestinal lesions are also related to bird energy metabolism. Maintenance increases (Figure 3) in proportion to metabolic body size for the unchallenged lesion-0 birds as they mature. Bird maintenance needs are elevated slightly for immunity development and markedly so for coccidiosis, thereby impacting FCR (Figure 4). Indeed, energy needed for immunity development was just 5% higher compared to 28% for coccidiosis. This cost becomes disproportionately elevated as birds age.
An additional cost of coccidiosis is feed passage. Figure 5 displays bird calories lost in the excreta with the control birds averaging just 3.6% loss while birds with full coccidiosis averaged over a 50 Kcal additional loss each day in excreta and reached values exceeding 110 kcal per day with age. Considering that the coccidiosis period is thought to run about six days, this would total 660 kcal loss in the excreta alone.
The impact of intestinal lesion score upon ECV throughout the growth curve (Figure 6) indicates that low lesion scores have a pronounced impact upon ration energy loss with consequences becoming more pronounced late in the growth curve. It is therefore critical that intestinal health be optimized. Immunity to the cocci challenge is the best way to ensure this.