An increase in body mass per unit time is considered as a growth index (Nikkhah et al., 2010). If the growth rate can be predicted at different stages of development, it will be possible to determine the amount of nutrients needed and provide the most economical nutrition management plan (Naghos et al., 2013). A growth prediction method is based on the use of growth models. These models are mathematical functions and describe the pattern of body weight growth (Goliomytis et al., 2003). Poultry growth models are non-linear regression functions such as Gompertz, Richards, and logistic (Chaji at al., 2015). When the Gompertz model is used for data, it is expected that the chicks with lower initial growth rate, reach to maximum growth age sooner and exhibit greater exponential weight loss than those with higher growth rates (Novak et al., 2004). The point at which the growth rate changes (from increase to decrease) is called the turning point, and the age at which the maximum growth rate is achieved is called the age at the turning point. In fact, it can be said that the turning point divides the growth curve into two phases of increasing growth rate and decreasing growth rate (Marcato et al., 2008). The use of antibiotics in poultry nutrition as an antimicrobial growth promoter is undoubtedly beneficial for improving broilers growth performance and preventing diseases (Barton 2000). Despite the positive effects of antibiotics, researches have shown that the remains of antibiotics in poultry carcasses lead to the development of resistant bacteria strains in the human body and may prevent treatment of many diseases. So, prebiotics, probiotics, organic acids, medicinal herbs, spices, extracts and essential oils have been suggested as antibiotic substitutes in poultry nutrition (Azeke and Ekpo 2009). Antioxidant compounds in olive leaf as bioactive compounds, have high potential for enhancing animal health and performance. Adding natural antioxidants to feed is one of the most effective ways to delay lipid oxidation and improve growth performance parameters (Botsoglou et al., 2010). The aim of this study was to estimate the growth parameters of broiler chickens fed diets containing olive leaf powder and α-tocopheryl acetate using Gompertz model.
A total of 300 one-day-old broiler chicks (mixed sex) of Cobb 500 strain with similar mean body weight were assigned to a completely randomized design with five treatments, three replicates and 20 chicks per replicate. The experiment lasted up to 42 days of age. The dried olive leaves in the shade, were milled using a mechanical mill with a diameter of 0.5 mm and were added to the experimental diets. The experimental diets consisted of: corn-soybean basal diet (negative control), basal diet supplemented with 250 mg/kg of α-tocopheryl acetate (positive control) and basal diet containing three levels of olive powder (2, 2.5 and 3%). All diets were homogenized in terms of metabolizable energy, crude protein, and other nutrients. Feed and water were supplied for ad libitum</em> consumption throughout the experiment. House temperature was initially set at 32. During the experiment, the lighting program consisted of 23L: 1D. At the end of each rearing period (starter, grower and finisher), the weight of all chicks in each unit of experiment was measured by digital scale at an accuracy of one gram. The number of mortality and weight of dead and culled chicks were recorded separately to adjust feed conversion ratio. Four hours prior to weighing, the birds were deprived from the feed for uniformity of digestive tract.Growth curves were plotted and fitted using the Gompertz model.
The highest predicted mean body weight (BW) of the chicks during the first and second weeks of life were observed in negative control followed by 2.5% olive leaf powder treatment. But, this trend was changed from the third week; so that the highest predicted mean weight at this week was related to the positive control treatment and then in the 2.5% olive leaf powder treatment. In the fourth week, the highest predicted BW was related to 2.5 and then 3% of olive leaf powder treatments. In the 5th and 6th weeks of the age, positive control and 2.5% olive leaf powder showed the highest predicted BW in compare with other treatments. These changes in BW at different weeks of age are probably due to various factors such as tolerance of dietary fiber, growth, health and adaptation of the digestive tract with the applied treatments. The time of growth rate change (increase to decrease) for negative control treatment was at 33.9 days of age. At this time, the BW reached to 1432 g which was lower than the positive control and olive leaf powder treatments. This means that chicks need more time to reach their desired weight. Therefore, as the time of growth rate change in the negative control treatment arrived sooner, it had a lower final weight (4251.8 g). On the other hand, the time of growth rate change (increase to decrease) in positive control treatment was at 39.8 days, which was the longest time between the negative control and other treatments. In this case, the highest final BW was predicted in this group (6115.8 g). The time of change of growth rate for olive leaf powder treatments was predicted with slight differences compared to the positive control treatment and thus the differences in final predicted BW of these treatments were negligible. In a study which soybean meal was replaced with 10, 20, and 30% of raw or heated chickpeas, the results showed that the time of change of growth rate (increase to decrease) in the diet containing 30% crude chickpeas was predicted to be in later ages (47 day) than the other treatments and this was probably due to the presence of more anti-nutrients in this treatment (Amirabadi et al., 2011). In another study, levels of 0, 0.75, 1.5, 2.25, 3 and 3.75% sodium bentonite were supplemented to the diet. The time of growth rate change for the sodium bentonite treatments was increased compared to the control and it was concluded that more time is needed to reach the broilers to desired weight (Chaji et al., 2014). As the age of the bird increases, the growth rate increases, but this increase is limited and, after reaching to its maximum value, decreases, causing the growth curve to change. The turning point in the Gompertz equation is constant and is approximately 37% of maturity weight. In the study of Mignon-Grasteau et al. (1999), body weight of male birds at the turning point (1191 g) was estimated to be higher than body weight of female birds at this point (862 g). This trend has been observed in another study (Ricklefs 1985). It seems that, selection for growth performance in broilers has changed their growth curve.
The Gompertz growth function can be used as a part of the main statistical models to fit weight trait data in broilers. All olive leaf powder and positive control treatments increased the age to reach the turning point in compare with negative control. The effect of 2.5% olive leaf powder and positive control treatments on predicted mean body weight and improvement of growth rate parameters were more effective than other treatments. Also, 2.5 and 3% olive leaf powder and positive control treatments were more effective at reaching the turning point of growth curve and final body weight compared to other treatments. Due to the improvement or increase of more broilers traits by 2.5% olive leaf powder and positive control treatments, application of these two treatments is recommended.
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