R.A. RENEMA
F.E. ROBINSON
M.J. ZUIDHOF
Alberta Poultry Research Centre,
University of Alberta, Edmonton,
Canada
Broiler breeders must have the genetic potential for efficient growth as well as the ability to effectively reproduce. However, the interaction between nutritional and reproductive traits is complex and continually changing with the introduction of new genetic lines. These studies were designed to gain a better understanding of how selection for growth traits and variability among strains affects growth traits, reproductive morphology and production traits of broiler breeders. Breeder strain influenced sensitivity to photostimulation, to excess feeding and to pullet growth profile. By understanding how sensitivity to changes in nutritional status differs among strains differing in muscling, then feeding and management strategies can be refined to maximise the production efficiency of the hen.
Introduction
Broiler breeders are a moving target. While broiler 42-day body weight is increasing each year, the target body weight for mature male and female broiler breeders has changed little (Rustad and Robinson, 2002). In 1979, Hubbard male and female breeders were approximately 50% of the 42-day broiler weight. In 2001, this percentage had decreased to 36.1 for males and 30.3 for females. The situation for knowing what nutrients these birds need is compounded by the development of “yield” lines, carrying increased amounts of breast muscle yield, often on a smaller carcass frame. This increased growth efficiency is expressed partly in the greater capacity for muscle growth. Pym et al. (2004) indicated that differential fractional rates of protein deposition, breakdown and synthesis have resulted in increased protein retention in a high compared to a low efficiency line.
The reproductive efficiency of broiler parents is increasingly dependant on very specific feed restriction and lighting programs to optimise reproduction. Broiler breeders must have the genetic potential for efficient growth as well as the ability to effectively reproduce (Robinson et al., 1993). Excess body weight can result in reduced egg production, hatchability, liveability, egg weight, feed efficiency and increased shell porosity (Wilson and Harms, 1986; Robinson et al., 1993). Overfeeding can accelerate the sexual maturation process and elevate ovarian large yellow follicle (LYF) numbers in birds of similar BW (Renema et al., 1999). Furthermore, feeding programs during rearing and early lay can change frame size and breast muscle fleshing in the birds (Wilson et al., 1995).
These studies were designed to gain a better understanding of how selection for growth traits and variability among strains affects growth, conformation, composition, reproductive morphology and production traits of broiler breeders.
Strain variation in the acceleration of sexual maturation
Broiler breeder strains vary in the extent to which sexual maturation is influenced by nutrient intake (Robinson et al., 1998). Four commercial strains were reared on a common body weight target, and fed one of three feeding programs from photostimulation (22 wk): ad libitum; Fast-Feed (weekly feed adjustments based on a 5 g increase for every 5% increase in production); and Slow-Feed (daily adjustments of 1 g/d between 22 and 26 weeks of age, and 0.5 g/d until 31 weeks of age).
There was a one-week range in the mean age at first egg among the strains. Ad-libitum feeding did not accelerate sexual maturation in two of the strains, suggesting these birds have a later maturation of the hypothalamo-pituitary axis. This is a significant finding that serves to show a basis for genetic differences in photo-sexual response among commercial stocks. In these late-maturing strains, it would seem to be pointless to subject these birds to increasing day lengths and feed allocations as soon as the operator would more traditional early ¬maturing strains. In the strains responding to overfeeding, maturation was accelerated by 6 to 7 d, on average.
The number of large yolky follicles varied among strains, with the two strains that reached sexual maturity first having the fewest large follicles. These data strongly suggest that it is essential to follow management recommendations specific to a breeder genotype.
Impact of growth selection on sensitivity to overfeeding
A study was designed to show the effects of degree of selection for yield traits on the ability to cope with a feeding challenge. The strains were: Random-bred (unselected since 1977), Ross 308 (a high-yield bird suited for the whole-bird market), and Ross 508 (a very high-yield bird suited for the cut-up and further processing market). Each strain was raised to the same target body weight at 20 wk of age, when they were individually caged. Beginning at photostimulation (22 wk of age), pullets were fed 100% (control), 120%, and 140% of the feed needed to maintain the Ross 508 growth curve. A total of 90 birds were dissected at sexual maturity and 144 were kept to 58 wk of age for measurement of production traits.
The timing of sexual maturity was affected by strain, with the RB20, Ross 508 and Ross 308 birds laying eggs 16.5, 20.2, and 27.4 d after photostimulation (Table 1). Birds of all strains appear to have acquired the appropriate level of growth and composition to support rapid sexual maturation. At sexual maturity (onset of lay), the Ross 508 birds had the highest proportion of breast muscle. Conversely, the Random-bred hens were the fattest – reflecting the less efficient growth of their older genetics. The 120 and 140% treatments only added an additional 5.3% and 9.7% to BW at sexual maturity, respectively (Table 1).
Feeding regimen had a big impact on egg production, with 166, 159, and 137 settable eggs produced by the 100, 120, and 140% groups, respectively. Interestingly, the modern, high breast-yield Ross 508 birds were the most sensitive to overfeeding, producing 177 eggs with the 100% feed allocation compared to only 123 eggs on the 140% feed allocation. When coupled with decreased rates of fertility and hatchability in the 140% treatment, overfeeding had a devastating effect on chick numbers. The hatchability of the 140% Ross 508 hens near the end of the study ranged between 20 and 30%. At the end of the trial, 94% of the 100% feed allocation birds were still in active lay compared to 86% in the 120% allocation group and only 63% in the 140% allocation group. The productivity of the Ross 308 hens was least impacted by feed allocation, demonstrating a better tolerance to a range of feeding profiles than either the Random-bred or the Ross 508 hens in this study.
Effect of strain and growth profile on production traits
An experiment was performed to test how the interactions between genetic strain, age at photostimulation and target body weight profile impact growth rate and efficiency, nutrient partitioning, sexual maturation and reproductive efficiency.
The strains used were: Hubbard Hi-Y, Ross 508, and Ross 708. The four body weight profiles separated at 5 wk and converged at 32 wk of age as follows: STANDARD (control); LOW (12 wk body weight target = 25% lower than STANDARD followed by rapid gain to 32 wk); MODERATE (12 wk body weight target = 150% of STANDARD followed by lower rate of gain to 32 wk); and HIGH (12 wk BW target = 200% of STANDARD followed by minimal growth to 32 wk).
One of the primary effects of the growth profiles was on frame size. The long-term concern would be that feeding a small-framed to the same body weight target, as a larger framed bird will result in increased fatness and the triggering of reproductive disorders associated with overfed hens. During the period immediately after photostimulation at 18 wk, the LOW birds had a very high feed allocation relative to that of the other growth curve treatments to allow their weight profile to converge with the others by 32 wk. Despite what would normally be considered excess feed, sexual maturation was still delayed in the LOW birds. In contrast, the HIGH birds had a very low feed allocation during this period, which delayed sexual maturation the Ross 708 hens and suggesting that these birds do not tolerate nutrient shortages well. These birds carried a greater proportion of breast muscle and less fat than the other strains, which may contribute to their inability to cope with reduced feed at this critical time. Photostimulating birds at 22 wk of age alleviated most of these problems.
Body weight at sexual maturity was 3.40, 3.21, 3.01, and 2.84 kg for HIGH, MODERATE, STANDARD, and LOW birds, respectively. The body weight differences impacted shank and keel length, indicating differences in frame size. Interestingly, ovary weight in the later maturing LOW birds (55 g) was 6 g heavier than in the other groups. The number of large yellow follicles on the ovary did not change with photostimulation age (average of 7.5 follicles), except in the HIGH birds, where it dropped from 8.1 in birds photostimulated at 18 wk to 6.5 in those photostimulated at 22 wk. Feed allocation to the HIGH birds was quite low during this period to keep the body weight on target, which likely impacted ovary development.
The reduced feed on the MODERATE and HIGH profile also reduced early egg size and stunted the length of the prime sequence (the characteristically long daily egg laying sequence occurring early in lay) (Table 2). Although these birds were larger, their early production traits were similar to that of a much smaller bird. This illustrates how recent feeding level may have a greater impact on production traits than body or growth pattern does.
Ultimately, the 18 wk PS-age birds yielded 8 more eggs (170) than 22 wk PS-age birds to 58 wk of age, with no affect on unsettable egg production. On average, total egg production was similar among growth profile treatments. However, there was variability in the productivity of specific strains grown on some profiles. The Ross 708-HIGH hens, for example, under-performed (138 eggs) compared to the other profiles (mean = 166.3). Alternatively, Ross 508-HIGH birds laid the same number of eggs as Ross 508-STANDARD birds (mean = 178.7) (Table 2).
Examination of individual growth profiles revealed strain-based strategies for managing reproduction. The Ross 708 tied up nutrients deposited during the pullet phase tightly, and was unable to mobilise nutrients from storage, as they were needed under conditions of dietary deficiency. This may be partly due to their increased breast muscle mass. Under more normal feeding conditions these birds performed very well. The Hubbard Hy-Y hens appeared much more able to mobilise nutrient stores, and were not hindered by the very low feed allocations provided to the HIGH birds during sexual maturation. Economic analysis of production traits revealed that using a STANDARD feeding profile and photostimulating pullets at 22 wk of age was most often the best management practice on a cost/chick basis. Photostimulating birds at 18 wk of age resulted in higher total egg production, although much of this advantage was lost in small, unsettable eggs (<52 g) and a higher degree of production variation among hens. Ultimately feeding profiles affected egg production traits differently among strains, with little effect of photostimulation age.
Conclusions
The newer broiler breeder genetic strains are becoming more specialised and appear to have more specific management methods associated with them. Both genetic strain and feeding treatment affected how the birds came into production and had some influence on carcass fleshing traits. However, this did not have a consistent effect on egg production traits. The negative effects of overfeeding were more pronounced in the highest breast-yield strain. These studies indicate some of the complexity in the interaction between nutritional and reproductive parameters and demonstrate the need for strain-specific management strategies.
References
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Robinson, F.E., Wilson, J.L., Yu, M.W., Fasenko, G.M. and Hardin, R.T. (1993). Poultry Science, 72: 912-922.
Robinson. F.E., Renema, R.A., Bouvier, L., Feddes, J.J.R., Zuidhof, M.J., Wilson, J.L., Newcombe, M. and McKay, R.I. (1998). Canadian Journal of Animal Science, 78: 615¬623.
Rustad M.E. and Robinson, F.E. (2002). Poultry Science, 82(Suppl. 1.): 52.
Pym, R.A.E., Leclercq, B., Tomas, F.M. and Tesseraud, S. (2004). British Poultry Science, 45: 775-786.
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Wilson, J.L., Robinson, F.E., Robinson, N.A. and Hardin, R.T. (1995). Journal of Applied Poultry Research, 4: 193-202.
From Proceedings of the “18th Australian Poultry Science Symposium”, New South Wales, Australia.
