عنوان مقاله [English]
Introduction Modern layer strains have high genetic potential for egg production, and may produce eggs for more than one laying cycle by inducing molting. Good egg production is obtained when adequate nutrition is provided, one of the most important nutrients is protein, particularly limited nutritional essential amino acid. Additionally, in layer flocks feed represents more than 70% of the cost of production. In this context, protein is one of the most costly nutrients and therefore there is a benefit in reducing any excess protein in the feed by making use of synthetic amino acids wherever feasible to reduce both the excess waste of nitrogen and the production cost. Synthetic amino acids such as methionine, lysine and threonine are commonly used in the poultry industry because those are limiting in diets based on cereal and soybean meal and are now readily available at a competitive cost. The essential amino acid, threonine, is the third limiting amino acid. Most of the previous research has been done on determining the amino acids requirements of laying hens in the first production cycle, and information on laying hens requirements in the second production cycle is very limited. As a result, the nutrient requirements of second-cycle hens are assumed to be similar to those observed in hens during the first laying cycle in the strain booklet guide and or NRC, 1994. The objective of this study was to determine the digestible Threonine (dig Thr) requirement of laying hen during the second production cycle fed wheat-soy diet as measured by laying performance.
Materials and methods The birds that used for this experiment were selected from a flock that were molted at the 80wk of age on a non-fasting feeding program according to the molting recommendation by Hy-line W36 laying hens guide and had 103wk age. Hens were selected according to body weight and egg production. The pre-experimental period was two weeks for acclimatization. Basal diet were formulated according to the guidelines in the Hy-Line W-36 Commercial Management Guide (2016) according to the average pre-experimental period data; egg production less than of 80% and feed intake 98 g/b/d. Three hundred and eighty four layers were distributed to the 6 increasing levels of dig Thr 0.44, 0.46, 0.48, 0.50, 0.52 and 0.54%, with Thr-to-Lys ratio of 60, 63, 66, 69, 72 and 75%, respectively in a completely randomized design, with 8 replicates and 8 birds per experimental unit. The experiment lasted from 105 to 116 weeks of ages. All hens were weighed at the beginning and at the end of the experimental period. Birds were given artificial light (16L: 8D). Feed and water were provided ad libitum. Egg production (number and weight) and mortality were recorded daily, whereas feed consumption was measured every 4wks as feed disappearance. The daily intakes of Thr were calculated by multiplying the concentration of each in the experimental diet by feed conception. Digestible threonine requirement for optimal laying performance response parameters such as egg production (EP), egg mass (EM) and feed conversion ratio (FCR) to the daily dig Thr consumption by using NLIN procedure, through linear and quadratic broken-lines and quadratic polynomial regression fit models.
Results and discussion The increasing dig Thr levels showed linear effect on egg weight, and quadratic effect on egg production (EP), egg mass (EM), and feed conversion ratio (FCR). The EP and EM increased and FCR improved linearly as dietary dig Thr levels increasing up to 0.48% (Thr-to-Lys ratio of 66%), and then EP and EM decreased and FCR deteriorate. The dig Thr requirements for the optimization of laying performance was depends on what parameter is taken into consideration for optimization and it is possible to compare models by their R2 and MAE. In this study, the model that provided the best fit was the quadratic polynomial regression model for EP, EM and FCR. The predicted dig Thr requirements with quadratic polynomial regression model for optimal EP, EM and FCR were 502, 505 and 517 mg/b/d, respectively. The dig Thr requirement to produce 1 g of EM was 9.57 mg. The optimum values estimated for FCR was slightly higher compared to those estimated for EP and EM.
Conclusion The outcomes of the present study showed that, in the wheat-soy based diet formulated for second cycle of laying hen the threonine is a limited amino acid. The dig Thr requirements vary according on what parameter is taken into consideration for optimization. The dig Thr requirement for optimized FCR is higher than EP and EM. In the wheat-soy based diet formulated for second cycle of laying hen the level of digestible Thr 0.48% (Thr-to-Lys ratio of 66%) were recommended.