Effects of broiler breeder phosphorus nutrition on breeders and broiler progeny

JOHN T. BRAKE, Ph.D., PAS^* 
Prestage Department of Poultry Science, North Carolina State University, Raleigh, NC 27695-7608 USA

One cannot discuss phosphorus (P) in broiler breeder diets without discussing calcium and phytase given the intimate linkage that has developed among these dietary components while poultry nutritionists have strived to improve environmental sustainability and keep feed costs low. Further, an optimum dietary level of P has not universally been agreed upon given that information obtained from AgriStats indicated that the average available phosphorous (AvP) level in the commercial USA breeding diet has recently been 0.37% with a range from 0.27% to 0.43%. Presumably, the lower level was facilitated by phytase addition. Unlike the commercial laying hen where low AvP diets have been managed successfully for many years as a means to improve egg shell quality and reduce breakage, broiler breeders must produce quality broiler chicks that do not exhibit live performance problems. This additional factor has complicated the development of optimum diets in broiler breeders. In fact, there has been relatively little research that investigated the vertical effects of P nutrition in broilers and their parents.

______________________	Our initial review of the research literature found few references specifically related to broiler breeders and their progeny. The most disturbing was a review by van Tuijl (1998) that recounted the results of P regulations in the Netherlands.In the case of broiler breeders, the initial results were increased mortality and morbidity that resulted in restoration of former dietary AvP levels in many cases. Most of the problems were due to insufficient AvP in both the grower and breeder diets that led to femoral head necrosis, leg weakness, and associated bacterial infections but no study of the broiler progeny was conducted. It was concluded by van Tuijl (1998) that no conclusive recommendations could be made at that time as to the minimum safe dietary AvP level for broiler breeders. Much has been resolved since that time and there were some data from which estimates can be made about P requirement for broiler breeder females. Wilson et al. (1980) reported that a total dietary P level of 0.41% produced optimum hatchability and egg production from females during a 28-wk production period. Dietary total P levels of 0.36% and 0.31% gave slightly lower performance. Older birds have been reported to exhibit intestinal phytase (Edwards, 1993; Maenz and Classen, 1998) and low dietary inorganic P has induced intestinal phytase in rats (Moore and Veum, 1983). These latter effects may serve to facilitate lowered dietary AvP in adult broiler breeders when typical diets have been fed. It should also be noted that most broiler breeder diets have contained low energy wheat by-products that have been shown to contain phytase that would have remained active in typical mash type diets fed in the USA. Berry et al. (2003) suggested that an AvP level of 0.1% could be reached in slat-litter pens with the use of supplemental phytase. This was accomplished by the complete removal of inorganic P. Although this study encompassed a full production cycle, the effects on progeny were not studied. Harms et al. (1964) had reported that maternal P supplementation had little effect on progeny tibia ash but Triyuwanta et al. (1992) studied the progeny of dwarf broiler breeder hens fed 2, 6, or 10 g AvP per kg diet (0.2, 0.6, or 1.0%) in cages and reported that initial tibial breaking strength of the progeny improved markedly with increasing maternal AvP. However, this effect was not evident at 7 wk of age after subsequently receiving normal broiler diets. However, Ekmay et al.(2012) reported that progeny tibia ash was not affected by a caged breeder NPP level of 0.15% versus 0.40% even though egg weight and production were decreased and hen mortality was increased. It was interesting to surmise that greater mortality may have eliminated certain hens with altered P requirement and thus altered the progeny genetics as well. The role of incubation and hatching egg management in this scenario was suggested by the work of Shim et al. (2008) who reported an increased incidence of phosphorus-rickets when eggs from older broiler breeders were stored for 10 versus 1 d. Egg storage has delayed yolk sac absorption and yolk utilization, which has been shown to be site of P storage in the egg, has been found to be affected by a number of incubation factors (personal observations).
	Our studies over the past dozen years at NC State University have involved several broiler breeder flocks and their progeny broiler flocks. Typical industry management has been employed as broiler genetics and phytase efficacy have improved. Thus, variation in results over time has been observed. We have typically fed corn-soy mash diets with wheat midds or wheat bran in the breeder formula as filler and broiler diets have been largely corn-soy based. We have fed standard diets to 8 wk of age with approximately 0.90% calcium and 0.45% AvP to provide some assurance of leg health and then began our dietary manipulations. Phytase source has changed during the course of our studies as improved products have become available. In our initial study, both males and females received diets containing 0.1%, 0.2%, 0.3%, and 0.4% AvP with and without phytate from housing in the slat-litter laying quarters from 21 to 64 wk of age. The lowest level contained no added inorganic P. The laying diets contained 2.7% calcium as standard in our laboratory and as used by Berry et al. (2003). At 32 and 61 wk of age all eggs laid on two consecutive days were incubated together and male chicks were placed in a litter floor broiler house to 7 wk of age. The final design for the broiler males was a 4 x 2 design with four breeder P levels and two breeder phytase levels. The broiler calculated dietary AvP level chosen was 0.25% before the addition of phytase to mimic broiler feed formulations in low P feed environments. Even though the breeders were fed quite minimal levels of feed there were no effects whatsoever of dietary P or phytase. This was probably due to the fact that the summer conditions were not very hot and that the diets contained 1% wheat bran, which could have contained sufficient phytase activity to meet the needs of the breeders. Nevertheless, these data suggested that there would be no great damage to the breeders of using no added inorganic P during the breeding period if the breeders had been grown in such a manner as to optimize their skeletal development and metabolic efficiency. There were no effects on the broilers.
	In the second breeder flock, we removed the wheat bran from the diet and, by chance, encountered very hot summer weather. The laying diet contained 0.1% or 0.4% AvP with or without phytase. This particular summer produced some of the highest temperatures in history and heat stress was evident. Unfortunately, the house burned when the flock was 50 wk of age and all of the breeder data were truncated but broiler flocks at 35 and 43 wk of age had been grown. There was significantly higher female breeder mortality during the hottest part of the summer from the 0.1% AvP without phytase group. The males were not similarly affected. This high female mortality resulted in less eggs laid per hen housed but no effect on hen-day egg production, fertility, or hatchability of fertile eggs. Egg weight and egg shell quality were not affected. The data suggest that if the broiler breeder hens did not die they performed normally and were not affected by the severe heat stress and supposed P deficiency. Two broiler levels of AvP, 0.25% and 0.18%, before the inclusion of phytase, were used at 35 wk of age. Four broiler levels of 0.25%, 0.21%, 0.16%, and 0.12% AvP, before the inclusion of phytase, were used at 43 wk. On an overall basis the breeder AvP level had no effect on broiler performance from 35 wk and 43 wk breeders. Breeder phytase had no effect on broiler mortality and feed efficiency but 42-d BW was reduced at 35 wk (2167 versus 2114 g; P < 0.07) and 43 wk (2210 versus 2149 g; P < 0.05). These initial data demonstrated that the dietary AvP required for minimal performance of broiler breeders increased from 0.10% to 0.22% when birds were exposed to heat stress. However, this revised estimate of the AvP requirement was still considerably below the level of 0.40% AvP that had been commonly used in standard industry diets. Importantly, the reduced AvP level of 0.22% could be achieved by removing all added inorganic P from the feed and replacing this with phytase enzyme. However, while these studies suggested that as little as 0.22% AvP with phytase was adequate to maintain egg production, there appeared to be some detrimental vertical effects on performance of the broiler progeny when progeny were reared on low AvP diets with phytase.

We then reared broiler breeders on four dietary rearing treatments from 10-21 wk of age and applied four dietary laying treatments from 22-64 wk of age with treatment identity maintained from rearing to laying. Breeder pullets received 0.45% AvP, 0.35% AvP plus phytase (+0.1% AvP), 0.35% AvP, and 0.25% AvP (+0.1% AvP) to 21 wk of age. From 22 wk of age breeder males and females received 0.45% AvP, 0.35% AvP plus phytase (+0.1% AvP), 0.26% AvP, and 0.16% AvP (+0.1% AvP) to 64 wk of age. A reduction in AvP from 0.40% to 0.22% with phytase had no adverse effect on egg production but evidence emerged in this experiment that only the highest level of AvP without phytase supported maximum fertility. Eggs were collected and incubated at 38 and 54 wk in two broiler experiments with breeder treatment identity maintained. Broiler chicks were reared on a starter diet containing adequate AvP to 14 d of age and 9 d of age in the two experiments, respectively. Thereafter a number of AvP combinations were fed. An interesting vertical effect observed in both broiler experiments was a significant reduction in day-old BW of the broiler progeny when the dietary AvP of the broiler breeder layer diet was reduced below 0.40%. This effect was transient and had disappeared by 21 d of age. However, it was found that feed consumption of broilers from breeders receiving phytase was reduced in an increasing manner as the broilers aged. This resulted in a numerical decrease in BW at 42 d of age. This seemed to mimic earlier effects of reduced BW but the fact that a normal AvP starter diet was fed may have muted the effects.

In two additional studies involving individual cages and a two-thirds slat floor house where all eggs laid for a number of days were analyzed it was confirmed that reduced breeder AvP reduced egg weight and egg shell weight. A reduction in egg number and therefore total egg mass was also observed in the slat floor study. This was as expected as Derick Balnave had demonstrated such effects in commercial layers in Australia some years ago. These were restored by phytase in a dose-related manner in these two experiments. This was interesting in that reduced AvP in commercial layer diets has been routinely used to improve egg shell quality in terms of reduced breakage. This suggested an important difference between meat-type layers and egg-type layers, which may simply be related to breeders having access to litter.

The broiler breeder data suggested that these birds on slat-letter floors can lay perfectly well with no added inorganic P in the breeder laying diet as long as there was some added phytase during hot weather. The hens that did not die during heat stress performed perfectly well in terms of egg production, fertility, and hatchability. This suggested that there was a population of broiler breeders that did have sufficient intestinal phytase to obtain their daily P requirements from a corn-soy diet. This population can be estimated to be about 60-70% of the flock. This was consistent with the knowledge that broilers can be selected over a period of years to have a lower AvP requirement.

There was some “vertical” effect of phytase from parent to progeny demonstrated that although small, was real as evidenced by the fact that the effect repeated although manifested somewhat differently depending upon experiment. It seems to the present author that the probable cause for this effect was that some alteration of metabolic function in the broilers has been caused by the use of phytase in the breeder hen under the conditions of this study. This was probably modified by incubation conditions that affected yolk utilization. Personal observations suggested that when incubation was optimum that no effects of the breeder diet on hatchability and broiler chicks was apparent but less than optimum incubation could elicit some perturbations.

Our research confirmed that the dietary AvP broiler breeders in slat-litter housing can be reduced to approximately 0.22% in the layer diet without significant detrimental effects on broiler breeder egg production or broiler performance provided that the broiler progeny were fed starter diets containing adequate levels of dietary AvP without phytase. A greater AvP safety margin would be advisable during the breeder rearing period.

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References

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