Factors that influence energy requirements of broiler breeders

Managers of broiler parent stock face the challenge of intense feed restriction (Renema et al., 2007) because of over 4-fold increases in body weight (BW) over the last 50 years (Zuidhof et al., 2014b). Body weight control is of primary importance for broiler hatching egg operators. Therefore, feed allocation decisions have become a major focus. How much should you feed the birds? It depends on their age, their BW, rate of gain, and level of production. How much you feed them also depends on their body composition, their immune status, the temperature and lighting conditions in the barn, and the diet formulation. Knowing how much birds weigh is important, but knowing how much they are gaining is even more important. BW gain essentially provides a summary of how well nutrient supply is matching nutrient requirements in real time.
Rapid and accurate BW feedback on a breeder flock is like receiving feedback from your eyes while driving. Closing your eyes while driving is foolish because you will quickly drift off course, leading to potentially tragic consequences. Managing a broiler breeder flock without frequent BW measurements is like closing your eyes. Without constant, precise information about where your flock is headed, you can quickly wander off course, with high production and economic consequences. Ideally, frequent and accurate BW measurements would be linked to an immediate feeding decision. In this paper, I will discuss energy requirements of broiler breeders. They are important to understand because they are the factors that contribute to variation in response to feed.
Nutrient Requirements
A nutritionist’s goal is to match nutrient supply to the nutrient requirements of the flock. Therefore, feed allocation decisions for broiler breeders are driven by nutrient requirements. Ideally, feed would be allocated according to the needs of every individual bird. In reality, flock managers must still make feed allocation decisions according to the average bird, and mange feed delivery to ensure equal consumption by as many birds as possible.
Classically, a nutrient requirement is defined as the intake or dietary concentration of a nutrient that results in a maximum desired response. For broiler breeder hens, the desired output is high quality chicks. However, this response to nutrient intake is complex, and confounded because of the time lag of 7 to 10 days between nutrient intake and the fully formed egg. It is further confounded by the fact that eggs are removed from the hen, stored for a variable period of time, transported, stored again, and finally incubated for 21 days before the chick is ready to tell its story. Recruitment and growth of ovarian follicles, mating success, embryo development, and hatching all separate the performance indicator (chicks) from the input (feed) by at least 4 to 5 weeks. During this time, many environmental factors influence the success of the chick. Rearing feeding programs are even further confounded, since pullets and cockerels consume feed for over six months before the first chick is produced.
Energy partitioning
The metabolizable energy (ME) a broiler breeder consumes is partitioned to 3 main areas: maintenance, growth, and reproduction. Energy partitioning can be thought of as a flow of nutrients to a variety of potential outcomes, analogous to water flowing downward in response to gravity (Figure 1). The highest priority uses of energy are represented as the lowest points in the body of the hen. The first priority is maintenance of the vital organs. Once this energy requirement is met, growth of the body can occur. At some level of intake above the maintenance requirement, reproduction and fat storage occurs. At optimal feeding levels, there is a balance between growth and reproductive outputs. In the breeder hen, overfeeding results in excess skeletal and muscle growth and development, excess fat storage, and over-development of the ovary.

Maintenance. Kleiber’s law (Kleiber, 1932) relates maintenance energy to metabolic body weight, with typically accepted values within the range of 78 to 125 kcal/kg^0.75 (Lasiewski and Dawson, 1967; Leeson and Summers, 2001). The coefficient and the exponent can vary, and are often debated, and the values depend primarily on the metabolic state. This is partially a function of how fast an animal is growing, or how many eggs are being produced. The maintenance ME requirement (ME_m) also depends on environmental temperature, light intensity and daylength, activity level, and immune status (Figure 2). Body composition, specifically the proportion of fat tissue (low ME_m) and lean tissue (higher ME_m) also affects the total amount of energy required for maintenance.

 

Production. Growth and reproduction have specific energy costs. The composition of gain (lean or fat) contributes to the energy requirement for growth, since fat has a much higher retained energy (approx. 9 kcal/g of fat) than lean tissue (protein contains approximately 4 kcal/g on a dry matter basis). Because lean tissue is 67% water, the retained energy of lean tissue on a whole body basis is closer to 1.5 kcal/g. Energy requirements for egg production are approximately 3.01, 0.47, and 0.08 kcal/g of yolk, albumen, and shell, respectively (McLeod et al., 2014). A typical value for egg production is 1.75 kcal/g of egg produced (Chwalibog, 1992). Production levels are dynamic, and vary with age and level of production. Romero et al. (2009) developed empirical equations to predict energy requirements for maintenance, egg production and growth that varied with age. These variations are due to composition of production and of the body (body and egg components change with age).

Strain- and sex-relateddifferences in growth and reproductive outputs also lead to variation in maintenance and production energy requirements. Differences in target BW curves primarily affect the energy requirement for growth. Modern breeders are feed restricted to a high degree, such that most of the energy requirement for maintenance and growth are related to lean tissue. Around puberty, hormonal changes in response to photostimulation of the hypothalamus cause an increase in fat deposition during this time in females. Biologically speaking, within-sex differences between strains can be explained well by differences in body composition and the composition of gain and reproduction.

Immune response. The environment contributes primarily to differences in maintenance ME requirements (ME_m). Inflammation, which is part of the innate immune response, can influence appetite. There is also an energetic cost associated with mounting a fever. Vaccinations constitute a considerable energy requirement, and feed allocation decisions should be more generous following vaccinations. Disease situations and even response to commensal bacteria have an energy cost, as well. Although there is a solid theory around energy costs associated with immune response, there are few estimates of the actual energy cost of the immune response.

Nutrition. Dietary energy level itself contributes to maintenance ME requirements. Broiler breeders fed a low ME diet had a higher core body temperature, and subsequently lost more heat to their environment (Paul, 2013; Figure 3). This increased ME_m. Feed allocation (kcal/bird) should be slightly higher with lower nutrient density diets. Romero et al. (2009) estimated that ME_m increased 0.34 kcal per kcal of ME intake. The implication was that fully 1/3 of each additional kcal of energy consumed was lost as heat as dietary ME decreased.

Light. Light intensity and daylength can influence bird behaviour and physiology, and therefore nutrient requirements. High light intensity increases bird activity levels, and therefore increases the amount of activity-related heat production. Activity increases heat loss to the environment, and therefore increases ME_m. Feed allocation should therefore be higher in environments with higher light intensity. Daylength influences the relative amount of time birds spend in a low or high activity state during night and day, respectively. Birds on a longer daylength have an elevated activity level for a longer period of time, and this increases the core body temperature (Figure 3 and Figure 4), heat loss to the environment, and therefore feed allocation needs to be higher when daylength is longer.

Temperature. Environmental temperature also affects ME_m. In adult breeders, the optimal environmental temperature (thermoneutral zone) is around 24°C (Paul, 2013). It is likely higher for pullets because the degree of feed restriction relative to growth and production potential is greater during rearing. Breeder pullets and hens grown in warmer environments (23 and 27°C) had higher core body temperatures during the night. Regulation of core body temperature in homoeothermic broiler breeders likely prevented a drop below those observed at 19°C and below (Figure 4). Effectively, this increases the temperature differential between bird and environment, increasing heat loss and increasing ME_m and required feed allocation in an increasing rate below 19°C. 

How can you apply this knowledge to broiler breeder feed allocation decisions?

Shorten your feedback loop. Feed allocation accuracy is improved when the time between BW measurements decreases. Weigh birds frequently (at least twice per week). In any system, shortening the feedback loop increases the precision of the decision. A shorter time between BW measurements will improve your feed allocation precision. Think about how your eyes providing feedback to your brain. Driving a car well is only possible when your eyes provide rapid feedback. Your eyes process many images every second. Think about how imprecise driving might be if you opened your eyes once per second, or once every 10 seconds. It would not be long before the car would drift out of your lane, or collide with objects around you. Weighing birds infrequently is like driving with your eyes closed. It allows you to adjust your feed allocation by a smaller amount more often, and the resulting BW curve is smoother. Weigh birds accurately. Duplicate your technique and protocol every time you weigh birds so that the consistency of your sample is good.

Consider key variables. There are a few variables that are really important for feed allocation decisions. Ironically, BW is not one of them. BW is important so that you can estimate the rate at which birds have been growing since the last time you measured BW. BW gain is much more important than BW per se.

Qualitative restriction (diet dilution). It is very important to give all birds a great chance of eating their share of the feed allocation. Try to distribute the feed to the birds as uniformly as possible. Low dietary nutrient density is a good strategy, but it is of decreasing value as the gap between broiler growth potential and breeder BW targets increases. Feed dilution did little to improve BW uniformity in a recent trial. Holm (2010) decreased nutrient density by 25% by diluting a standard broiler breeder diet with ground oat hulls. In my laboratory, I increased the volume of a breeder diet by 400% by adding ground expanded polystyrene. Instead of consuming the daily feed allotment in 15 minutes, the birds consumed it in 20 minutes (Zuidhof, unpublished data). Even this radical attempt to increase the degree of qualitative restriction treatment did not improve uniformity.

Spin- or scatter-feeding. Pelleting a breeder diet and distributing it into the litter to increase the space and time required for birds to find their daily feed resulted in a modest but significant increase in flock uniformity (Holm, 2010). Other work (van Middelkoop, personal communication) has shown similar results. Interestingly, daily scatter-feeding reduced the recovery rate of Salmonella from the environment of broiler breeders relative to birds fed daily or every other day in a trough (Wilson et al., 2013).

Grading. Many regions currently grade broiler breeders periodically during rearing to increase uniformity. This is an effective way to manage feed allocations according to the BW of groups of birds. By sorting birds into small, medium, and larger BW groups, multiple feed allocations can be applied within the same cohort to increase the growth rate of smaller birds, while decreasing the growth rate of larger birds. This has resulted in improved uniformity (Holm, 2010) and anecdotal reports of increased egg and chick production from the global hatching egg industry.

In my laboratory, we have been focusing our attention on the responses of individual broiler breeders to their environment and nutritional inputs. This has yielded insights that will be of great value for managing broiler breeder flocks in the future. One of the consoling trends we observed infers that reproduction is a firmly entrenched biological priority. Figure 5 shows that the rate of BW gain drops when egg production begins. At puberty, there is a biological shift in nutrient partitioning from growth to reproduction. This effect was highly persistent across experimental treatments, including BW profile, photostimulation age, strain, and environmental temperature (Zuidhof, unpublished data). This key biological priority suggests that as long as birds remain in a positive energy balance, BW profiles can be substantially altered with minimal impact on lifetime productivity. More research is needed to determine whether long-term increases in the BW profile (i.e. BW profiles that do not return to the current recommended target profile) are feasible.

There is an increasing gap between the genetic potential of broilers for growth and the target BW of broiler breeders. The former has increased at a compounded rate of over 3% per year (Zuidhof et al., 2014b), and the latter has hardly changed since the late 1970s (Renema et al., 2007). As a result, competition for feed is intense in the pullet and cockerel barns, and flock uniformity is one of the biggest technical challenges faced by breeder technicians. This problem is not going to go away in the foreseeable future. Rather, it will only increase in intensity as long as breeders select for growth rate, yield, and efficiency in broilers. 

Precision feeding may be the way to manage broiler breeders in the future. The ultimate way to manage BW is to weigh every bird at the time a feed allocation decision is to be made. This has become reality with a new precision feeding system developed at the University of Alberta (Zuidhof et al., 2013; 2014a). A simple algorithm moves the focus of breeder management from the feeding decision to BW (Figure 6). Having rapid and frequent BW feedback on every bird every time it seeks a meal provides perfect clarity to the challenge of BW control. Precision feeding actually lets you forget about what affects nutrient requirements because the system allocates feed to individual broiler breeders based on immediate feedback from individual BW. This is an ideal way to control BW gain, and to provide nutrients to precisely match nutrient requirements. If the temperature drops, and ME_m increases, every bird will receive just enough extra feed to compensate for the changing environment, and grow according to the target BW profile. If a bird lays an egg, it loses weight, and receives extra feed that effectively replenishes the nutrients invested in the egg within hours of oviposition. If the nutrient density of the feed changes, birds will adjust their feed intake perfectly, maintaining a positive energy balance, and maintaining the target BW precisely.

In 2 studies thus far, precision feeding reduced the CV in BW to less than 3%. This kind of uniformity is likely to result in higher egg production, fertility, and chick numbers. The system is yielding novel insights into the performance and behaviour of individual birds in a manner never before achieved. If scale up is successful, precision feeding may revolutionize the management of broiler breeders. 

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