Limiting Ovarian Development to Maximize Chick Production in Broiler Breeders

There are several important instances in the management of broiler breeders where "more" is not "better". In simple terms, "more feed" allotted to hens to the extreme degree of ad libitum feeding reduces settable egg production by as much as 40 eggs per hen.

Breeder hens use dietary energy differently than egg-type hens. Young broiler breeder hen respond to extra dietary energy by develping extra ovarian follicles. Several research projects have been carried out at the University of Alberta, that have shown that "more ovarian development" does not mean "better reproductive efficiency". Clearly, a high priority during the early laying period is to promote ovarian development at a rate that will result in the highest number of settable eggs. This paper will look at the process of sexual maturation of the breeder pullet to improve egg production, fertility and hatchability.

Why is the prepubertal period so important?

The critical time in broiler breeder management is the period from photostimulation (lighting) to peak production. This period is characterized by relatively fast weight gains, and the changes brought about by the hormones being produced by a newly active ovary. Photostimulation is generally considered the cue to initiate puberty, although the response to light is readily modified by level of feeding.

Light energy passes through the skull and illuminates the hypothalamus. When the hypothalamus receives a photostimulatory signal (long day length above a threshold of intensity), hormones involved in ovarian function are produced. One of the first responses you can see by looking at the ovary of the bird after lighting is that the very small ovarian follicles begin to increase in size. The small follicles produce large quantities of estrogen, the hormone that causes most of the reproductive transformation associated with puberty.

Firstly, estrogen influences the production of yolk precursors (building blocks of yolk production )in the liver of the bird. The liver visibly enlarges and becomes paler as its fat content increases for the production of egg yolk lipids.

Secondly, the oviduct increases in size so it is ready to receive ovulated follicles. This step appears to be a limiting one for heavy turkeys, as the ovary in these birds develops faster than the oviduct does, so there is a high incidence of follicles that are ovulated that are not picked up (internal ovulation) and hence, not formed into normal eggs.

Thirdly, estrogen influences changes to bone composition that allow calcium to be mobilized daily to make egg shells.

Finally, estrogen combines with male sex hormones (androgens) which results in changes to plumage, comb size and color and sexual receptivity to males.

How we measured the interaction of photoperiod and feed allocation from 20-25 weeks

Shaver Starbro pullets were reared to 20 weeks of age to determine the influences of lighting program and feed allocation from 20-25 weeks on ovarian development and production traits.

The experiment used two lighting programs and two levels of feeding. A "slow photoperiod" treatment (SP) was gradually brought from 8L:16D with weekly increases in day length. A "fast photoperiod" treatment (FP) was increased from 8L:16D to 15L:9D in a single step at 20 weeks of age. The "slow feed" treatment group (SF) received very moderate increases in feed allocation from 20 to 25 wk of age . Increases in feed allocation of greater than 5 g, were divided up into two smaller increases per week. The "fast feed" treatment group (FF) received a more generous feed increase between 20-25 weeks of age. It was not the original intent to decrease feed allocation to these birds; however, at 22 wk of age we discovered that the increases at 20 and 21 weeks of age had a greater effect on body weight that planned. The decision to decrease the feed allocation at 22 weeks of age was made to prevent the fast feed hens from becoming too heavy. It should be noted that the withdrawal of feed did not result in a decrease in body weight in subsequent weeks. From 25 to 64 weeks of age all of the birds were exposed to the same photoperiod (15L:9D) and a common feed allocation.

A group of birds were killed on the day following sexual maturity to examine development of the reproductive tract and carcass composition. Individual egg production records were kept and eggs were incubated at weekly intervals to assess treatment effects on chick production. Feed allocation was decreased after peak egg production to keep the birds at proper body weight targets (for age).

How did the different photoperiods and feeding programs affect the birds?

Body weight - There were no significant differences in age at sexual maturity between any of the groups. Body weight and fat pad weights at sexual maturity were not different in terms of main effects. Within the group of slow photoperiod hens, the fast feed program resulted in heavier body weights compared to the slow feed program.

Ovary - Ovary morphology was highly influenced by both the photoperiod and feed allocation treatments. The slow photoperiod treatment had a greater ovary weight than did the fast photoperiod treatment. The fast feed treatment resulted in significantly heavier ovary weight and one more large follicle than the slow feed treatment. To our knowledge this is the first report of a relatively minor feeding difference affecting large follicle recruitment in commercial breeders.

Egg production - All treatment groups had very high rates of total egg production when expressed in terms of main effects or the interactions of the main effects. There was a 10.9 egg advantage to use of the slow feed feeding program compared to the fast feed program. This fact is presumably related to improved control of the follicular recruitment process, as fast feed birds also had fewer large follicles at sexual maturity.

Fertility and hatchability - Hatchability was reduced in the fast photoperiod program compared to the slow photoperiod program. Embryonic mortality was examined on day 7, 14, and 21 of incubation. The greatest embryonic mortality was found in the fast photoperiod treatment in the later stages of incubation.

Eggs from the fast fed hens had higher embryonic mortality than eggs from the slow fed hens. This indicates that developmental problems may be associated with the eggs from birds with excessive follicle development. These data suggest that minor differences in feed allocation have an influence on ovarian form and function. Proper management of feed allocation during the pre-lay phase is critical to optimize the number of saleable chicks produced.

The above research data support the hypothesis that chick production in broiler breeder hens can be compromised by over-feeding early in lay. Future management programs for breeder hens should focus on limiting excessive follicle production so that there is an orderly cooperation between the ovary and the oviduct in the production of hatching eggs. Such a program would minimize egg shell problems, reduce the incidence of multiple-yolked eggs and result in long egg-laying sequences. Long egg-laying sequences lead to high peak production and excellent persistency of lay. Further studies on limiting follicular growth by controlling energy intake and where nutrients are partitioned or distributed through the body and the role of light in sexual maturation are underway at the University of Alberta.

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