Feeding Programs for Egg-strain Pullets up to Maturity

The major concern today with feeding and managing laying hens, is achieving desired weight for- age in the pullet, and especially during the early growth period. Genetic selection has been for reduced body weight in these birds, in order to improve feed efficiency, but a consequence of this is reduced feed intake. Ensuring that birds consume sufficient nutrients daily is made even more difficult when ingredient quality is poor and birds are subjected to heat stress or disease situations. The pullet manager must be skilful in managing diets and the environment, such that pullets consume an adequate level of nutrients up to maturity. While such nutrient intake varies with strain and environment, goals are around 800g crude protein and 18 Mcal ME consumed up to 18 weeks of age.

Diet specifications for pullets are shown in Table 1.

Feeding Management Of Growing Pullets

A. General Considerations
It is generally agreed that most Leghorn and brown egg strains have changed over the last five to 10 years, and because of this, nutritional management is becoming more critical. In essence, these changes relate to age at maturity, although it is questionable that this has changed suddenly in just a few years. In fact, what has been happening is that age of maturity has slowly been decreasing by about one day per year. Unfortunately, many producers are just now becoming aware of earlier maturity because their conventional programs are no longer working, and this is especially true for many strains of brown egg pullets. Moving birds to laying cages at 21-22 weeks is no longer feasible, and this now invariably results in management problems. Similarly, first egg appearing at 16-18 weeks means that we must critically review our rearing programs. The key to successful nutritional management today is through maximizing body weight of the pullet. Pullets that are on target or slightly above target weight at maturity will inevitably be the best producing birds for the shell-egg market.

The traditional concern with early maturity has been that it results in small egg size. Results from our early studies indicate the somewhat classical effect of early maturity in Leghorns without regard to body weight (Table 2).

There seems little doubt that body weight and/or body composition are the major factors influencing egg size both at maturity and throughout the remainder of the laying period. Summers and Leeson (1983) concluded that body weight is the main factor controlling early egg size (Table 3).

We concluded that although there is evidence to indicate that nutrients such as protein, methionine and linoleic acid can influence egg size throughout the laying cycle, these nutrients have little effect on early egg size. This is probably related to the pullet producing at maximum capacity at least up to the time of peak egg mass.

Although it is fairly well established that body weight is an important criterion for adequate early production, there is still insufficient evidence regarding optimum body structure and composition. Frame size is being discussed, and is now most frequently included in breeder management guides as a form of monitoring. It is known that most (90%) of the frame size is developed early, and so by 12-16 weeks of age, the so-called "size" of the pullet is fixed. While this parameter is useful as another monitoring tool and its measurement should be encouraged, we have had little success in affecting frame size without also affecting body weight. It therefore seems very difficult to produce, by nutritional modification, pullets that are below target weight yet above average frame size and vice versa. The relationship between body weight and shank size is further complicated by the fact that environmental temperature also affects bone length independent of nutrition.

It would appear that early maturing chickens reach sexual maturity at significantly younger ages, but at similar body weights compared to later maturing birds. It seems as though early maturing birds achieve a threshold level of body mass and commence production when the minimum physiological age is reached, while late maturing birds at the same age do not have the body mass required for production. Recent reports have indicated the requirement of a certain lean body mass prior to onset of maturity. With most mammals, attainment of minimum fat reserves are essential for puberty, and so it seems likely that body composition is as important as total body mass in influencing the onset of egg production. In studies involving a relatively small number of birds, we have seen no correlation between age at first egg and either percentage or absolute levels of body fat. While no clear picture has yet emerged with respect to body composition and maturity, it seems likely that birds having some energy reserve as they approach peak egg production are less prone to subsequent problems. Too frequently, a production curve as shown in Figure 1 is observed with commercial flocks.

Our experience suggests that if this type of production loss is not due to an identifiable disease and/or management problem, then it most likely relates to birds being deficient in energy. It is perhaps not too surprising that birds are in such a precarious situation with respect to energy balance. Most mammals such as cows and sows must lose body weight during peak lactation in order to meet energy requirements. Perhaps the most classical case of energy deficiency at this time is seen with the turkey breeder. Due to a decline in feed intake from time of first lighting through to peak egg production, the turkey breeder necessarily loses considerable body mass in an attempt to maintain energy balance. It is likely that the same situation applies to both the Leghorn and modern brown egg type pullet. Obviously, the effect is most pronounced for underweight flocks with small appetites where energy intake is minimal. In fact, with many flocks exhibiting production characteristics as shown in Figure 1, it is body weight at housing that deserves immediate investigation rather than factors occurring later at the time of the production loss.

The key to solving many of our present industry problems would therefore seem to be attainment of "heavy" pullets at desired age of maturity. In this instance, "heavy" refers to the weight and condition which will allow the bird to progress through maturity with optimum energy balance. It is likely that such conditions will be a factor of the flock in question, being influenced by stocking density, environmental temperature, feather cover, etc. Unfortunately, attainment of desired weight for age has not always proven easy, especially where earlier maturity is desired or when adverse environmental conditions prevail. Leeson and Summers (1981) suggested that energy intake of the pullet is the limiting factor to growth rate, since regardless of diet specifications; pullets seem to consume similar quantities of energy (Table 4).

All of these birds were of comparable weight even though diet specifications were dramatically variable. As seen in Table 4, birds consumed similar quantities of energy, even though protein intake varied by 85%. These data suggest that if adequate protein intake is achieved, additional diet protein does little to stimulate growth rate.

In more recent studies, we have reared Leghorn pullets on diets varying in protein or energy, and again, energy intake seems to be the major factor influencing body weight (Tables 5 and 6).

These studies indicate the growth rate is more highly correlated with energy intake than with protein intake. This does not mean to say that protein (amino acid) intake is not important to the growing pullet. Protein intake is very important, but there does not seem to be any measurable return from feeding more than 800g of protein to the pullet through 18 weeks of age. On the other hand, it seems as though the more energy consumed by the pullet, the larger the body weight at maturity. Obviously, there must be a fine line between maximizing energy intake and creating an obese pullet.

B. Maximizing Nutrient Intake
If one calculates expected energy output in terms of egg mass and increase in body weight, and relates this to feed intake, then it becomes readily apparent that the Leghorn must consume at least 90g/bird/day and the brown egg bird close to 100g/bird/day at peak production. With egg-type stock, feeding is related to appetite and so our management programs must be geared to stimulating appetite. The practical long-term solution is to rear birds with optimum body weight and body reserves as they begin production. This situation has been aggravated in recent years, with the industry trend of attempting to rear pullets on minimal quantities of feed. Unfortunately, this move has coincided with genetically smaller body weights and hence smaller appetites, together with earlier sexual maturity.

In order to maximize nutrient intake, one must consider relatively high nutrient dense diets, although these alone do not always ensure optimum growth. Relatively high protein (16-18% CP) with adequate methionine (2% CP) and lysine (5% CP) levels together with high energy levels (2800-3000 kcal/kg) are usually given to Leghorn pullets, especially in hot weather situations. However, there is some evidence to suggest that high energy diets are not always helpful under such warm conditions. This situation may relate to stimulation of nutrient intake when lower energy diets are fed at high temperatures (Table 7). In this recent study, Leghorn pullets were heavier at 126 days when fed the high energy diet in the cool environment, but diet had no effect at 30°C. As expected, pullets ate less of the high energy diet, and because all other nutrient levels were fixed, this results in reduced intake of all nutrients except energy. Pullets therefore ate less protein and amino acids when fed 3000 vs 2500 kcal ME/kg, and this can be critical where intake per se is less at 30°C. The pullets fed 3000 kcal/kg are borderline in intake of balanced protein at 870g versus our requirement for 800 g to this age. High energy diets may therefore not always be beneficial under heat stress conditions, and intake of other nutrients such as protein and amino acids must be given priority during formulation.

The Leghorn pullet eats for energy requirement, albeit with some imprecision, and so energy:protein balance is critical. All too often, we see inadequate amino acid intake when high energy corn-based diets are used, the result of which is pullets that are both small and fat at maturity.

One of the most important concepts today in pullet feeding, is to offer diets according to body weight and condition of the flock, rather than according to age. For example, traditional systems involve feeding starter diets for about six weeks followed by grower and then perhaps developer diets. This approach does not take into account individual flock variation, and today this can be most damaging to underweight flocks. It is becoming more difficult to attain early weight for age. This means that flocks are often underweight at four to six weeks of age. This can be for a variety of reasons such as sub-optimal nutrition, heat stress, disease, etc. The worse thing that can happen to these flocks is an arbitrary introduction of a grower diet, merely because the flock has reached some set age. Today, we must feed the higher nutrient dense starter until the target weight is reached. For example, Figure 2 shows an underweight flock at six weeks.

To change this flock to a grower at six weeks of age will cause problems because the flock will likely stay small until maturity, then be late maturing and produce a sub-optimal number of eggs that will also be small. This type of flock can most effectively be "corrected" by prolonged feeding of the starter diet. In this situation, the birds reach the low end of the guide weight at almost 10 weeks of age (Fig. 2). At this time, a grower diet could be introduced. Since the flock is showing a growth spurt, then feeding to almost 12 weeks could be economical – we now have a flock that is "heavy". We have therefore converted the flock from being underweight and a potential problem, to one that is slightly over weight and so ideally suited to realizing maximum genetic potential during peak production. Some producers, and especially contract pullet growers, are sometimes reluctant to accept this type of program, since they correctly argue that feeding a high protein diet for 10-12 weeks will be more expensive. Depending upon local economic conditions, feeding an 18% protein starter diet for 12 versus six weeks of age, will cost the equivalent of two eggs. A bird in ideal condition at maturity will produce far in excess of these two eggs relative to a small underweight bird at maturity.

C. Suggested Feeding Program
The following schedule is recommended for growing pullets to maturity:

Starter 18-19%CP; 2750-2900 kcal ME/kg
Day old _____________Target body weight

Grower 15-16%CP; 2750-2900 kcal ME/kg
Target wt ____________Mature body size

Pre-lay or layer 16-18%CP; 2750-2900 kcal ME/kg
Mature body size __________1st egg

As previously indicated, we are not making recommendations regarding age or even dictating the body weight at which diet changes should occur. Rather, the recommendations dictate the need for flexibility and the treatment of each flock as an individual case. For example, the starter diet is to be used until target weight for age is achieved. Hopefully, this will be at around 450g when the Leghorn bird is six to eight weeks of age. However, each flock will be subjected to varying environmental conditions, and so this may vary. The time of change to a lower-nutrient dense diet is when a desired weight-for-age is achieved, which we suggest is a weight that will be towards the top side of the breeder’s growth curve. Changing at a specific weight, or a specific age in isolation can lead to disastrously underweight flocks.

In our recommendations, we suggest the lower-nutrient dense grower diets to be fed from this target weight-for-age up until the desired mature body size is achieved. Again, we are not dictating a specific mature body weight, since this may be varied at the desire of the pullet grower (see following section). Pre-lay diets should only be used in an attempt at conditioning the calcium metabolism of the bird (see following section) and not as a means of initiating catch-up growth. Such growth spurts rarely occur at this age, and as such, pre-lay diets are being used as a "crutch" for poor rearing management.

An argument that is often heard about the role of body weight at maturity is that it is not, in fact, too important, because the pullet will show catch up growth prior to first egg ie: if the pullet is small, it will take a few days longer to mature, and start production at the "same weight". This does not seem to happen, as small birds at 18 weeks are smaller at first egg (Table 8).

These data suggest that the smaller pullet does show some compensatory growth to the time of the first egg, although this is insufficient to allow for total "catch-up" growth. It is also interesting to note the relationship between body weight and age at first egg, and also between body weight and size of first egg. In other studies, we have followed up on the growth of the pullet through a production cycle in relation to 18-week (immature) body weight. Again, there is a remarkably similar pattern of growth for all weight groups indicating that immature weight seems to "set" the weight of the bird throughout lay (Figure 3).

Most importantly from a production viewpoint, is the performance of birds shown in Figure 3. When the lightest weight birds were fed diets of very high nutrient density (20% CP, 3000 kcal ME/kg) they failed to match egg production and egg size of the largest weight pullets that were fed very low nutrient dense diets (14% CP, 2600 kcal ME/kg). These results emphasize the importance of body weight in attaining maximum egg mass output.

The actual body weights to be achieved during rearing will obviously vary with breed and strain. Most Leghorn strains should weigh around 400g, 900g and 1300g at six, 12 and 18 weeks respectively. Similarly, the brown egg birds should weigh around 500g, 1000g and 1500g at these ages. As a rule of thumb, these weights for age can be used as guidelines for anticipated diet change.

Discussion to date has focused on the role of body weight and appetite of the growing pullet. While rearing programs such as reverse-protein (Leeson and Summers, 1979) have application where delay in maturity is required (usually due to inadequate light control) nutritional management programs today must allow for maximum early growth so as to attain breeder’s recommended weight goals as soon as possible. This type of nutritional management obviously entails accurate monitoring of body weight, a task that has too often been neglected with Leghorn pullets.

D. Manipulation Of Mature Body Size
In the preceding discussion, we have outlined the importance of maximizing body weight at sexual maturity, and the reasons for this have been explained. Under certain conditions, it is realized that some tempering of mature body size may be economically advantageous. Because body size has a dramatic effect on egg side, large birds at maturity can be expected to produce large eggs throughout their laying cycle. Depending upon the pricing of various egg grades, a very large egg may be uneconomical to produce, and in most instances, tempering of egg size of birds at 40-65 weeks of age is often difficult to do without some accompanying loss in egg numbers. Because body weight controls feed intake and egg size, an easier way of manipulating life-cycle egg size is through the manipulation of mature body size. If the maximum possible egg size is desired, then efforts must be made to realize the largest possible mature weight. However, where a smaller overall egg size is economical, then a smaller pullet is desirable. Such light weight pullets can be achieved by growing birds slower through the growth cycle, or more economically by light-stimulating pullets at an earlier age.

E. Pre-lay Nutrition
Pre-lay diets are often used to try and manipulate body size or to bring about a transitional change in the birds calcium metabolism prior to maturity.

1. Pre-lay calcium
There is still considerable confusion and variation practised in the levels of calcium given to birds prior to egg production. During the laying cycle, the bird utilizes its medullary bone reserves, in the long bones of the leg, to augment its diet supply when a shell is being formed. Because egg production is an "all or none" event, the production of the first egg obviously places a major strain on the bird’s metabolism, when it has to contend with a sudden 2g loss of calcium from the body. Some of this calcium will come from the medullary bone, and so the concept has arisen of building up this bone reserve prior to first egg. This obviously means higher levels of calcium in pre-lay diets. There are basically three options for calcium feeding around the time of maturity.

i. Use of 1% calcium grower diets until around 5% egg production

As previously mentioned, the largest weight pullets in a flock will likely mature earlier, and so it is these birds that may be disadvantaged with inadequate levels of calcium at this time. If such birds receive a 1% calcium grower diet at the time they are producing their first few eggs, they will only have a sufficient calcium reserve to produce two to three eggs. At this time, they will likely stop laying, or less frequently continue to lay and exhibit cage layer fatigue. If these earlier maturing birds stop laying, they do so for four to five days, and then try to start the process again. The bird goes through very short clutches, when at this time she is capable of a very prolonged 30-40 egg first clutch. Advocates of prolonged feeding of grower diets suggest that it makes the bird more efficient in the utilization or absorption of calcium, such that when she is eventually changed to a layer diet, improved efficiency continues for some time, and so the bird has large quantities of calcium available for shell synthesis. Figure 4 indicates that percentage calcium absorption from the diet does decline with an increased level of calcium in the diet.

However, there is no scientific evidence to suggest that efficiency of utilization is affected, and in fact calculations from Figure 4 indicate that as the calcium level in the diet is increased, calcium retention increases even though percentage retention has declined.

If 1% calcium grower diets are used as pullets mature, these diets should not be used after appearance of first egg, and to 0.5% production at the very latest. It must be remembered that under commercial conditions, it is very difficult to precisely schedule diet changes, and so decisions for diet change need to precede actual time of diet change, such that production does not reach 5-10% before birds physically receive the calcium enriched diets.

ii. Use of 2% calcium pre-lay diets
Specialized pre-lay diets are a compromise, in that they provide more calcium than do most grower diets, but still not enough for sustained production. The concept of using so-called pre-lay diets is to build up the medullary reserves without adversely influencing general mineral metabolism. However, as previously discussed with grower diets, 2% calcium pre-lay diets are inadequate for sustained egg production, and should not be fed past 1% egg production. The main disadvantage of pre-lay diets is that they are used for a short period of time, and many producers do not want the bother of handling an extra diet at the layer farm. There is also reluctance by some producers with multiage flocks at one site to use pre-lay diets, where delivery of diets with 2% calcium to the wrong flock on site can have disastrous effects on production.

iii. Early introduction of 3.5 - 4.0% calcium layer diets
In terms of calcium metabolism, the most effective program is early introduction of the layer diet. Such high calcium diets allow sustained production of even the earliest maturing birds. As previously mentioned, higher calcium diets fed to immature birds, lead to reduced percentage retention, although absolute retention is slightly increased (Table 9).

Feeding layer diets containing 3.5% calcium prior to first egg, therefore results in a slight increase in calcium retention of about 0.16 g/day relative to birds fed 0.9% calcium grower diets at this time. Over a 10-day period, however, this increased accumulation is equivalent to the output in one egg.

Early introduction of layer diets is therefore beneficial in terms of optimizing the calcium balance of the bird. However, there has been some criticism leveled at this practice. There is the argument that feeding excess calcium prior to lay imposes undue stress on the bird’s kidneys, since this calcium is in excess of the immediate requirement and must be excreted. In the study detailed in Table 9, we do show increased excreta calcium. However, kidney histology from these birds throughout early lay revealed no changes due to pre-lay calcium feeding. Recent evidence suggests that pullets must be fed a layer diet from as early as six to eight weeks of age before any adverse effect on kidney structure is seen (see following section on urolithiasis). It seems likely that the high levels of excreta calcium shown in Table 9 reflect fecal calcium, suggesting that all excess calcium may not even be absorbed into the body, merely passing through the bird with the undigested feed. This is perhaps too simplistic a view, since there is other evidence to suggest that excess calcium may be absorbed by the immature bird at this time. Such evidence is seen in the increased water intake and excreta water content of birds fed layer diets prior to maturity.

Early introduction of a layer diet seems to result in increased water intake, and a resultant increase in excreta moisture. Unfortunately, this increased water intake and wetter manure seems to persist throughout the laying cycle of the bird (Table 10).

These data suggest that birds fed high calcium layer diets during the pre-lay period will produce manure that contains 4-5% more moisture than birds fed 1% calcium grower or 2% calcium pre-lay diets. There are reports of this problem being most pronounced under heat stress conditions. A 4 to 5% increase in manure moisture may not be problematic under some conditions, although for those farms with a chronic history of wet layer manure, this effect may be enough to tip the balance and produce a problem.

In summary, the calcium metabolism of the earliest maturing birds in a flock should be the criterion for selection of calcium levels during the pre-lay period. Prolonged feeding of low-calcium diets is not recommended. Early introduction of layer diets is ideal, although where wet manure may be a problem, a 2% calcium prelay diet is recommended. There seems to be no problem with the use of 2% calcium prelay diets, as long as birds are consuming a high calcium layer diet not later than 1% egg production.

2. Pre-lay Body Weight And Composition
Pre-lay diets are often formulated, and used, on the assumption that they will improve body weight and/or composition, and so correct problems arising with the previous growing program. Body weight and body condition should not really be considered in isolation, although at this time we do not have a good method of readily assessing body condition in the live pullet. For this reason, our main emphasis at this time is directed towards body weight.

The most important criterion used during rearing is pullet body weight as described previously. Each strain of bird has a characteristic mature body weight that must be reached or surpassed for adequate egg production and egg mass output. In general, pre-lay diets should not be used in an attempt to manipulate mature body size. The reason for this is that for most flocks, it is too late at this stage of rearing to meaningfully influence body weight - all too often, pre-lay diets are used as a crutch for poor rearing management.

However, if underweight birds are necessarily moved to a layer house, then there is perhaps a need to manipulate body weight prior to maturity. With black-out housing, this can sometimes be achieved by delaying photostimulation - this option is becoming less useful in that Leghorns and brown egg strains seem now to be maturing early without any light stimulation. If pre-lay diets are then necessarily used in an attempt to correct rearing mismanagement, then it seems as though the bird is most responsive to energy. This fact likely fits in with the effect of estrogen on fat metabolism, and the significance of fat used for liver and ovary development at this time. While such high nutrient density pre-lay diets may be useful in manipulating body weight, it must be remembered that this late growth spurt (if it occurs) will not be accompanied by any meaningful change in skeletal growth. This means that in extreme cases, where birds are very small in weight and stature at approximately 16 -18 weeks of age, then the end result of using high-nutrient dense pre-lay diets may well be pullets of correct body weight, but of small stature. These short shank length pullets seem more prone to prolapse/pickout, and so this is another example of the limitations in use of classical pre-lay diets.

While body composition at maturity may well be as important as body weight at this age, it is obviously a parameter that is difficult to quantitate. There is no doubt that energy is likely the limiting nutrient for egg production for all strains of bird, and around peak production, feed may not be the sole source of energy. Labile fat reserves at this time are therefore, essential to augment feed sources that are inherently limited by low feed intake. These labile fat reserves become critical during situations of heat stress or general hot weather conditions. Once the bird starts to produce eggs, then its ability to deposit fat reserves is greatly limited. Obviously if labile fat reserves are to be of significance, then they must be deposited prior to maturity. As with most classes of bird, the fat content of the pullet can best be manipulated through changing the energy:protein balance of the diet. If labile fat reserves are thought necessary, then high energy, high fat pre-lay diets should be considered. As previously stated, this scenario could well be beneficial if peak production is to coincide with periods of high environmental temperature.

The requirements for a specific body composition at the onset of maturity have not been adequately established. With mammals, onset and function of normal estrus activity is dependent on the attainment of a certain body fat content. In humans for example, onset of puberty will not occur if body fat content is less than around 14%. No such clear cut relationship has emerged with egg layers. Work conducted with broiler breeders, in fact indicate a more definite relationship between lean body mass and maturity, rather than fat content and maturity.

3. Subsequent Egg Weight And Egg Composition
It seems as though egg size is ultimately controlled by the size of the yolk that enters the oviduct. In large part this is influenced by body weight of the bird, and so factors described previously for body weight can also be applied to concerns with egg size. There is a general need for as large an early egg size as is possible, especially with breeder hens. Most attempts at manipulating early egg size have met with limited success. Increased levels of linoleic acid in prelay diets may be of some use, although levels in excess of the usual 1% found in most diets produce only marginal effects on early egg size. From a nutritional standpoint, egg size can best be manipulated with diet protein, and especially methionine concentration. It is logical, therefore to consider increasing the methionine levels in pre-lay diets.

For breeder hens, one also has to consider egg composition in relation to successful embryo development. It is well known that hatchability of eggs from young breeders is lower. The reasoning for this suboptimal hatch seems to relate to "maturity" of embryonic membranes, and movement of nutrients from the yolk and albumen to the embryo. However, part of this problem may also relate to inadequate passage of certain nutrients from the breeder hen into the egg. For example, it is known that young breeders do not deposit normal quantities of biotin into the egg - normal biotin concentration in the egg is apparently not achieved until production of the 8 - 10th egg. If this is a general effect with a number of key nutrients, then it would seem worthwhile to study the effect of pre-lay nutrient intake on egg composition in relation to embryonic needs.

4. Pre-pause
In recent years, there has been interest in some countries of so-called pre-pause feeding programs. The idea behind these programs is to withdraw feed, or feed a very low nutrient dense diet at time of sexual maturity. This somewhat unorthodox program is designed to "pause" the normal maturation procedure, and at the same time to stimulate greater egg size when production resumes after about 10-14 days. This type of pre-lay program is therefore most beneficial where early small egg size is undesirable.

Pre-pause can be induced by simply withdrawing feed, usually at around 1% egg production. Under these conditions, pullets immediately lose weight, and fail to realize normal weight-for-age when refed. Egg production and feed intake normalize after about 22 weeks, although there is 1-1.5g increase in egg size. Figure 5 shows the production response of Leghorn pullets fed only wheat bran from 18 weeks (1% egg production) through to 20 weeks of age. This data is presented on an equalized physiological basis, rather than equal age basis.

The most noticeable effects of a pre-pause diet such as wheat-bran, are very rapid attainment of peak egg production and an increase in egg size once re-feeding commences. These effects (Figure 5) are undoubtedly due to increased feed intake. This management system could therefore be used to better synchronize onset of production (due to variance in body weight), to improve early egg size or to delay production for various management related decisions. The use of such pre-pause management will undoubtedly be affected by local economic considerations.

5. Urolithiasis
Kidney dysfunction often leads to problems such as urolithiasis, and this most commonly occurs during the late growing or early egg production phase of the pullet. While infectious bronchitis can be a confounding factor, urolithiasis is most often induced by diet mineral imbalance in the late growing period. At post-mortem, often one kidney is found to be enlarged and contain mineral deposits known as uroliths. Some outbreaks are correlated with a large increase in diet calcium and protein in layer versus grower diets, coupled with the stress of physically moving pullets at this time, and being subjected to a change in the watering system (usually onto nipples in the laying cages). The uroliths are most often composed of calcium-sodium-urate.

The occurrence is always more severe when growing pullets are fed high calcium diets for an extended period prior to maturity. For example, urolithiasis causing 0.5% weekly mortality, often occurs under experimental conditions when pullets are fed layer diets after 10-20 weeks of age (relative to maturity at 18-19 weeks). However there is no indication that early introduction of a layer diet for just two to three weeks prior to maturity is a causative factor.

Because diet electrolytes can influence water balance and renal function, it is often assumed that an electrolyte excess or deficiency may be predisposing factors in urolithiasis or gout. Because salts of uric acid are very insoluble, then the excretion of precipitated urate salts could serve as a water conversation mechanism, especially when cations are excreted during salt loading or when water is in short supply. When roosters are given saline water (1% NaC1) and fed high-protein diets, uric acid excretion rates are doubled relative to birds offered the high protein diet al.,ong with non-saline drinking water. Because uric acid colloids are negative charges, they attract cations such as Na, and so when these are in excess, there is an increased excretion via urates, presumably at the expense of conventional NH4 compounds. There is some evidence of an imbalance of Na+K-C1 levels influencing kidney function. When excess Na+K relative to C1 is fed, a small percentage of the birds develop urolithiasis. It is likely that such birds are excreting a more alkaline urine, a condition which encourages mineral precipitation and urate formation.

Urolithiasis therefore seems to be most problematic in laying hens fed high levels of calcium well in advance of sexual maturity. Although the situation is often confounded with IBV infection, it seems obvious that no more than 1% calcium should be fed to Leghorn birds prior to maturity. Feeding prelay (2% Ca) or layer diets containing 3% calcium for two to three weeks prior to first egg is not problematic, and surprisingly, uroliths rarely form in adult male breeders fed high calcium diets. High levels of crude protein will increase plasma uric acid levels, and potentially provide conditions conducive to urate formation. Certainly numerous mycotoxins influence kidney function, and so general mill management regarding quality control and/or use of feed additives to suppress their harmful effects would likely be beneficial.

In humans at least, urolith formation can be controlled by adding urine acidifiers to the diet. Studies with pullets show similar advantages. Adding 1% NH4C1 to the diet results in a more acidified urine, and uroliths rarely form under these conditions. Unfortunately, this treatment results in increased water intake and associated wet manure. One of the potential problems in using NH4C1 in laying hens, is that it induces a metabolic acidosis and this is detrimental to egg shell quality especially under conditions of heat stress. Such treatment also assumes the kidney can clear the increased load of H+, and for a damaged kidney, this may not always occur. As a potential urine acidifier without such undesirable side effects, several researchers have studied the role of Alimet® a methionine analogue. From five to17 weeks of age, pullets were fed diets containing 1 or 3% calcium in combination with 0, 0.3 or 0.6% Alimet®. Birds fed the untreated high calcium diet excreted alkaline urine containing elevated calcium concentrations together with urolith formation and some kidney damage. Feeding 0.6% Alimet® acidified the urine, but did not cause a general metabolic acidosis. Alimet® therefore reduced kidney damage and urolith formation without causing acidosis or increased water consumption.

It is concluded that urine acidification can be used as a prevention or treatment of urolithiasis, and that this can be accommodated without necessarily inducing a generalized metabolic acidosis. From a nutritional viewpoint, kidney dysfunction can be minimized by not oversupplying nutrients such as calcium, crude protein and electrolytes for too long a period prior to maturity.

F. Brown-egg Pullets
There is very little information available on specific nutrient needs of brown egg pullets, and whether or not they need diets any different to those used for white egg birds. It is generally assumed that white and brown egg pullets are similar in their nutrient needs relative to body weight. Brown egg pullets are usually heavier than white egg birds, although this difference seems to be decreasing over the last few years. For example, in the past it has been fairly common practice to start physical feed restriction after 10-12 weeks of age, in order to control growth rate. Today, with many strains of pullet, this feed restriction is unnecessary, and in fact may be detrimental in hot weather conditions. The principle of feeding management of brown egg pullets is essentially the same as described for the slightly smaller white egg bird as outlined in the previous sections. Achieving target weight-for-age must be the major criterion of the growing program, because this ensures the best chance of realizing the bird’s genetic potential as a layer.

If physical feed restriction is necessary, due to birds becoming overweight, then bird uniformity becomes a major concern. With a mild restriction program, birds can be allowed to "run-out" of feed one day per week and, usually this will do little harm to uniformity. If it is necessary to impose a greater degree of feed restriction, on a daily basis, then it is important to ensure rapid and even feed distribution, much as subsequently discussed for broiler breeders. Feed restriction should be relaxed if birds are subjected to any stresses such as beaktrimming, vaccination, general disease challenge or substantial reduction in environmental temperature. An alternative management procedure for overweight birds, is to schedule an earlier light stimulation and move to layer cages (see Fig. 3.4). There is an indication that young brown egg pullets may not adjust feed intake too precisely in response to adjusting diet energy level (Table 11).

As energy level is increased at a fixed protein level (Table 11), a reduction in growth rate is sometimes seen because protein and amino acid intake are limited. Brown egg pullets seems to change their feed intake very little under these conditions, and consequently there is improvement in growth rate. In another study, pullets were fed diets at 2750 or 3000 kcal ME/kg. Over the 126 days growing period, brown egg pullets consumed 6% more energy when fed the high energy diet (20.6 versus 19.4 Mcal). Contrary to this increased energy intake, white-egg pullets consumed about 18 Mcal ME regardless of energy level in the diet.

An alternative scenario in explaining these results is that the heavier brown-egg pullet has reduced amino acid needs, and so when fed high energy diets there is less effect on amino acid intake relative to needs. In a series of studies, we have shown the brown egg pullet to grow quite well on very low levels of lysine relative to that recommended by most breeders (Table 12).

Up to 42 days of age, the lysine requirement of the pullet seems to be 0.58 - 0.68% of the diet, which is substantially less than values of 0.9-1.0% as recommended by most breeders. From 84- 126 days during the later phases of growth, there was no response to growth rate with more than 0.46% diet lysine. These experimental results suggest that under moderate environmental temperatures, it may be inadvisable to use high energy diets for growing brown egg pullets. On the other hand, assuming their response to diet energy is independent of temperature, then it may be easier to stimulate growth of these pullets under heat stress conditions, simply by increasing the nutrient density of the diet.

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