When assessing egg production, most people focus on body weights and body weight gains and compare this to a standard body weight curve. This is largely an ineffective technique because pullet body weight is a crude indicator of subsequent hen performance.

If it were that simple, over time, every flock would follow the same body weight programme. Excellent egg production is achieved across a host of body weights; therefore body weight must not be as significant as some experts believe. Simply examining body weights in a best versus worst egg production comparison is of minimum value (Figure 1).

Body weight measurements should be used as a check and balance of the breeder program and has value in two distinct areas. First, birds must have a minimum amount of muscle content at light stimulation and body weight is one means of evaluating this process. Secondly, all breeder studies comparing high and low body weights that have been completed through Cobb research indicate that body weight at light stimulation is positively correlated with egg production (Figure 2). From an egg production standpoint, it is beneficial to err on the side of heavier rather than lower body weights.

Figure 1. Analysis of 925,000 hens comparing pullet body weights of good and poor producing flocks within the same complex and time period.
There is a 21 TE/HH difference between these two groups (disaster flocks have been removed).
Very little differences in body weights are evident, signifying that there are more important variables that influence egg production.

Figure 2. Body weights at light stimulation effects on egg production
(adapted from Jeanna Wilson, University of Georgia).

The core factor influencing flock performance is nutrient intake, not body weight. It appears that our current recommendations have been correct but have somewhat overlooked the process of attaining adequate energy intakes during specific periods of a breeder’s life. We have been letting body weight determine how much to feed but in reality it should be, within reason, the opposite. Supplying adequate nutrients at appropriate times provides a flock with positive well-being signals to produce the correct building blocks for good production. Timing is critical. Increased nutrient consumption at the wrong time will elevate mortality, reduce peak egg production and overall production. The following recommendations are created from breeder studies conducted by Cobb-Vantress and a review of worldwide customer data.

Peak Egg Production is Critical

The analysis needs to begin in the hen house since this is where the results are generated. The complete egg production cycle is critical but peak egg production determines the sold flock performance 75 per cent of the time. In other words, if a flock peaks poorly it has a 75 per cent chance of having poor life of flock results. For every one per cent increase in peak production, an addition of 1.6 TE/HH will be achieved. Factors occurring after peak are important but less critical as it relates to egg production. The intake of energy from light stimulation to first egg can have a dramatic effect on survivability and subsequent egg production.

Aggressive feeding of a flock that is not ready for light stimulation will negatively affect flock performance. The same aggressive feeding of a flock that is ready will have no negative consequences. Light stimulation is one input that a successful breeder flock must have but from this time until peak production, the hen must receive controlled well-being signals via energy intake.

The response of the hen to light stimulation is determined by its well-being or general condition. The well-being signal is defined as all variables (defined and as of yet undefined) that positively impact the hen’s performance. After light stimulation, controlling nutrient intake is the primary driver defining the well-being of the bird.

Today’s breeder diets oversupply the pullet and hen with all nutrients except for metabolizable energy, therefore energy is the limiting nutrient. Energy must be elevated during the flock’s climb to peak egg production but high amounts of body weight coupled with increases in egg production will elevate mortality and lower total egg production. The amount of energy supplied can exceed the hen’s oxidative capacity to convert feed energy into tissue and egg mass, causing elevated mortality and unneeded stress.

Muscle Mass Key Component

Body weight is a crude measure of hen well-being. The hen’s lean mass content (fleshing) is a critical component of well-being. The minimum lean (muscle) mass required at light stimulation is 1.9 and 2.1kg (4.2 and 4.6lb) for the C500 and C700 hens, respectively. Although muscle mass is the key component, a typical body weight to reach the critical muscle mass is 2.2 and 2.3kg (4.85 and 5.10lb) for the C500 and C700 hens, respectively.

If a flock contains a small number of underweight birds at light stimulation, these body weight targets may be slightly lowered. The C700 product requires an increased muscle mass and will have a higher lean mass/body weight ratio compared to the C500 hen.

The total amount of fat required for hen well-being is insignificant under today’s management system. Fat is required for proper reproductive development but, like many things, more is not always better. Studies have suggested that a minimum amount of carcass fat is required for proper sexual maturity. The period of accelerated feed intake and body weight gain from weeks 15 weeks of age to light stimulation allows hens to have enough fat reserve for proper reproductive system development.

Light Stimulation

During the time-frame between light stimulation and five per cent egg production, energy intake is negatively correlated with hen survivability (Figure 3). Assuming the correct muscle mass was achieved at light stimulation, hens need relatively low energy increases during this period. This is not the right time to try and correct an error associated with improper pullet management.

If a flock is not ready for light stimulation, it is much more beneficial to delay light stimulation than aggressively to feed the flock after light stimulation. The focus during this period should be on reducing stress related to transfer, along with emphasis on reproductive system development rather than body weight gain. An average weekly energy increase should be no more than 9kcal per bird during this period.

Figure 3. The effects of weekly feed increases post-light stimulation on hen mortality (21-30 weeks of age).

The Cobb hen is very efficient at producing an egg. Not including the energy content of the egg, the hen only requires 11kcal per egg. Adding the energy content of the egg, the hen requires 2.11kcal per gram of egg weight (Table 1). Thus, for a hen laying a 60-gram egg, the energy needed to produce and lay this egg is 127 kcal.

As egg production and egg size increases, it is logical to assume that the energy needs of the flock increases. For every five per cent egg production, a flock requires an increase of 7.5kcal per hen per day of metabolizable energy. The 7.5kcal per day increase already includes the energy needed for body weight gain. If the egg production responds to more aggressive energy intakes, then the flock’s well-being has been compromised and undue stress and elevated mortality will occur. The error most likely occurred during pullet rearing and the pullet programme must be adjusted to eliminate this stress.

Post-peak management is important, but not as critical as pre-peak management. Body weights must be monitored, as excessive body weights may decrease fertility and hatchability. For every five per cent drop in egg production, the flock will require a decrease of 7kcal per day of energy intake. There is a slight difference in pre- and post-peak energy amounts for egg production due to the fact that egg size and body weight are larger and body weight gain is decreased.

When withdrawing feed, it may be necessary to prevent flocks from perceiving a large reduction in energy intake. It is better to be less aggressive in feed withdrawal than too aggressive as long as peak feed is not excessively high. If aggressive feed withdrawal does not affect egg production, peak feed amounts must be adjusted lower.

Absolute feed or energy intakes have not been mentioned. The energy requirement – keep in mind that energy is the limiting variable – for body weight gain and egg production is constant under most environments. Energy required for maintenance is highly variable and determined via body weight, ambient temperature exposure, immune system status, etc.

From a practical view point, energy increases required for egg production should be added to the energy intake at the start of egg production. This accounts for maintenance energy differences as well as miscalculations in dietary metabolizable energy. In other words, peak energy amounts must float on an individual flock basis and never be a fixed amount.

Correlated Egg Production

The most critical time of flock’s life is the time prior to light stimulation. Unfortunately, this period covers five months (i.e. the complete time in rearing) and many factors can influence the well-being of the flock. It can be overwhelming to analyze this period as a whole. Breaking this period into segments is a much more manageable proposition and reveals that there are certain periods during the pullet rearing cycle when certain events must occur and other periods which are less critical. Energy intake and muscle accretion are highly correlated to egg production and have the most impact on performance. For the most part, body weights are of minimal consequence during this period.

The only time that crude protein and amino acid intake are truly limiting is during the first few weeks of a chick’s life. Increased crude protein intake will improve flock uniformity but also increase body weights (Table 2). Heavy body weights early will require a higher degree of feed restriction later. From an individual bird perspective, it is better to be underweight early and allow for less feed restriction later. From a flock standpoint, there is a fine line between having good flock uniformity and heavy body weights.

One misconception is that a chick with high amounts of skeletal mass early will cause a need for a larger bird at light simulation. In fact, the pullet only has five per cent of its total mature skeletal mass produced at four weeks of age and subsequent growth has a much greater impact on frame size.

Energy intake from 10 to 14 weeks of age is an important factor affecting breeder flock performance. Flock analysis has revealed that increasing energy intakes during this time boosts egg production regardless of body weight gains. Obviously, body weight and energy intake are positively correlated but the body weight gains are less critical than energy intake. The energy bump required during this period is possibly associated with some type of metabolic/catabolic learning for future energy partitioning during egg product ion.

An average weekly energy increase of at least 7kcal per bird per day during weeks 10 to 14 is required. If body weights are below target, weekly energy increases of greater than 7kcal per day are required. When body weights are at or above standard, the 7kcal per day weekly energy increases still need to take place. The correction of heavy body weights needs to take place prior to 10 weeks of age.

In the past, Cobb has recommended feeding pullets on a sigmoid type of curve even though the body weight curve is somewhat linear. This reasoning is sound but less emphasis should be placed on body weight. From 14 weeks of age to light stimulation, the weekly increase of energy should average 18kcal per bird per day. Body weights must still be monitored, since they provide us with a indicator of maintenance needs and environmental stress. Metabolizable energy increases may be adjusted higher to achieve the minimum body weight standard but never lowered. Muscle accretion during this period is important but energy intake is probably the most important event. With practical diets, muscle accretion is limited by energy intake.

Energy Intake is Primary Factor

Most breeder experts have focused on body weight and body weight gain to improve egg production. The main focus should be placed on managing the bird according to the environmental conditions and always keeping in mind the objectives. One should not attempt to manage or troubleshoot flock performance by only comparing body weights to a standard body weight which is printed in a manual. Each flock is different but each requires similar well-being signals. Achieving consistent flock performance requires the magnitude of the well-being signals to change based on the situation. There are numerous variables impacting breeder performance but the primary factor to successful broiler breeder performance is energy intake.

Managing flocks based on accumulated energy intakes over long periods of time is inadequate. There are certain time frames (four to six weeks in length) of a breeder’s life when energy intake should be controlled tightly and other periods when energy intake should be less restricted. Every week of a pullet’s life is important.

However, flock wellbeing is validated during the ages of 10 weeks to first egg. As our knowledge of the bird increases, breeder recommendations must be based on the environmental conditions. Modeling will provide a greater understanding of how the breeder hen functions and will allow us to make breeder recommendations based on the environment, housing and management conditions. One breeder program will not fit all flocks and what works for one customer will not necessarily work for another.

This paper does not refute current recommendations, as demonstrated by annual improvement in breeder performance. However, focusing solely on body weights and to a certain extent on fleshing has restricted our progress in improving flock performance and achieving consistent results.