The value of digestibility coefficients and protein

Ariane Helmbrecht - Evonik Nutrition & Care GmbH, Hanau- Wolfgang, Germany

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At all times and for all human cultures animal protein took an important part of nutrient supply and has been always of a high value.
This got even more obvious during the last century when an increase in wealth was positively correlated with the increase in consumption of meat products. Poultry became globally more and more important as a supplier of animal meat, the main reason for this fact is the high efficiency for meat production in comparison to feed consumption. This advantage was produced by a strict genetic selection, which was favored by the genetic potential of poultry species. Within the poultry species improved for efficient meat production turkey fills a special position: turkeys provide the positive qualities of being lean by nature and therefore turkey meat is meeting the demand for a more healthy nutrition, but also turkeys are big enough to get parts for a roast serving for a family dish.

Improvement by genetic selection

Examining the development of turkey breeds of the last 70 years reveals the tremendous potential for protein deposition and therefore, for muscle growth. Research on the optimal particle size of turkey diets by Ziegenhagen et al. (1947) resulted in body weights of 1.77 kg and a feed conversion ratio of 2.57 for the best group at 8 weeks of age (mixed sex of Broad Breasted Bronze poults). Nowadays, male BUT 6 toms achieve 4.7 kg at 8 weeks of age with a feed conversion ratio of 1.66. As a consequence of selection, it was possible to increase not only the growth in time but also altering the muscle and fat deposition towards leaner birds. In Germany BUT 6 turkeys are grown up to 24 weeks of age with a final body weight of 25.82 kg and a feed conversion ratio of 2.77. Objectives for body composition at 22 weeks of age predict 34.0% breast meat and 13.6% thigh meat yield of 23.36 kg live weight. Johnson and Asmundson (1957) reported 10% breast meat and 4% tibial muscle weight for a bird of 10.01 kg live weight at 24 weeks of age. Although the objectives for the modern strain calculated inclusive skin, the difference is huge and the progress in the efficiency of protein deposition remarkable. Havenstein et al. (2007) compared a randombred turkey strain known as RBC2, which was developed in 1966 at the Ohio Agricultural Research and Development Center (OARDC; Wooster, OH), with a commercial turkey strain of 2003.

Summarized results showed a doubling in growth rate to market age (13.99 vs. 7.26 kg) and feed efficiency was 20% better at 20 weeks of age (2.638 vs. 3.278 g/g) and 50% better at 11 kg body weight (2.132 g/g at day 98 vs. 4.208 g/g at day 196). This improvement was partly dedicated to the change in nutrition and nutritional management. For instance, the introduction of 2-3 week feeding periods instead of only three phases overall. But also the nutrient composition of diets had changed. Crude protein content in 1966 was 29.0 % for the period week 0-8, in 2003 the 2-week-period phases provided 27.5, 26.5, 25.5, and 23.5 %. Due to the possibility of using crystalline amino acids, crude protein quality changed. While in 1966 birds received 1.72% of lysine (Lys) and 0.92% of methionine+cystine (Met+Cys), in 2003 birds of the first two weeks were served with 1.80% Lys and 1.20% Met+Cys, which was reduced stepwise to levels of 1.45% Lys and 1.00% Met+Cys at weeks 7 to 8. The work of Havenstein et al. (2007) reveals not only the tremendous progress in breeding, but also the impact of an improved nutritional management. During the last 60 years research regarding the need for nutrients resulted in recommendations for the optimum nutrient support closer and closer to the real need for maintenance plus maximum growth in alignment with genetic improvement.

Ideal protein profile

In turkeys data for amino acid requirement is scarce, especially data dedicated to the different growing stages. Another approach to estimate the requirement for amino acids is the Ideal Protein concept. For this, the ratio of every amino acid to Lys is determined either in a dose response study for a single amino acid or in a dilution study for a batch of amino acids. The team of Firman conducted several trials to improve the amino acid profile for turkeys and challenged the so-called Missouri Ideal Turkey Protein. They found the scientifically determined profile not meeting the genetic potential but already a 5% increase did, which is also close to current recommendations by breeders companies. In least cost formulations the next limiting amino acid not supplemented in a free form is always determining the crude protein content and therefore, the cost of the diet. Right now the use of free methionine, lysine, threonine and in some regions tryptophan has been established. Valine and isoleucine are introduced more and more for the very last years. The next amino acid is arginine (Arg), which is also available on the market but still expensive. Due to high prices Arg is not yet considered in turkey diets. When considering recommended levels of Arg, diet optimization is asking for protein-bound Arg, which is of course increasing crude protein and therefore costs. In practice, feed formulators tend to ignore Arg requirement to keep the crude protein level as low as possible. These lower Arg levels are contrary to currently available scientific data as in a first test the current recommendation of 105% Arg to Lys was challenged. This test showed no significant difference in growth performance and breast meat yield between recommended and reduced levels. Only the numerical difference in breast meat yield (30.03 vs. 28.59% of carcass) resulted in discussions for the profitability of a reduced Arg level.

Arg:Lys ratio and its effect on growth performance and breast meat yield

Therefore, a trial was conducted testing the effect of four increments of Arg:Lys ratio adapted by varying the CP level in male BUT 6 turkeys during growing phase P5 and P6 on growth performance, breast meat yield and foot pad lesions. The trial was run in cooperation of Aviagen Turkeys Ltd., Moorgut Kartzfehn von Kameke GmbH & Co. KG and Evonik Nutrition & Care GmbH. At the research facility of Moorgut Kartzfehn in Germany 1,408 male turkeys distributed to 16 floor pens were fed with commercial diets from P1 to P4. Pens were randomly allocated to one of the four feeding treatments: 90, 95, 100, and 105% Arg:Lys ratio adjusted by protein-bound Arg. For none of the measured performance parameters a significant difference could be detected. However, analysis of diets showed deviation of actual Lys and Arg content from diet composition resulting in even lower Arg:Lys ratios (90, 94, 96, and 100%). Calculating the intake of Arg:Lys and sorting it in descending order, revealed a ratio of 94-96% Arg:Lys being the most beneficial for maximum growth and breast meat yield.

As a conclusion of this second trial to the same topic, a ratio lower than the recommended 105% Arg:Lys seems not to result in reduced growth performance or breast meat yield. From economic point of view such a diet would be more profitable only due to the reduction in crude protein.

From the Proceedings of the 11th Turkey Science and Production Conference