The effects of particle size and energy levels in pellet diets on nutrient digestibility in growing broilers

The benefits of feed processing are well recognized by the poultry industry. Pelleting is often used by feed mills due to enhanced feed usage and improved animal performance. In addition, pelleting results in increased nutrient digestibility due to the mechanical action of heat (López & Baião, 2004).

Feed physical traits can be easily evaluated. Understanding poultry anatomical and physiological changes caused by feed particle size/physical form can be instrumental in the selection of optimum levels regarding these two parameters for each individual company. Grain grinding typically results in low levels of investment, decreased power expenses, and increased feed mill output, factors that can be economically crucial. In agreement with Amerah et al. (2007), smaller particle sizes result in improved nutrient digestibility due to increased surface area available for the action of digestive enzymes. Nevertheless, controversial results have been reported from studies correlating particle size with nutrient digestibility.

The purpose of this research was to evaluate the effects of particle size and energy levels in pelleted feeds on nutrient digestibility and energy values in growing broilers.

Three hundred (300) Ross, male broilers weighing 1.038±0.030 kg were housed at the rate of 10 birds per each of 30 experimental units, from 30 to 33 days of age. Birds were raised up to 22 days of age using a common feed, then allocated to the different treatment groups. Water and feed were given ad libitum.

Feed nutritional levels differed only with regard to energy values. Feed compositions and calculated nutritional values are shown in Table 1. Feeds were formulated based on the nutritional values of ingredients, as per the Brazilian Tables for Poultry and Swine (Tabelas Brasileiras de Aves e Suínos, Rostagno et al.) (2005). Feed nutritional levels were established in agreement with Lara et al. (2008). In order to measure corn particle sizes, 3 different screens (fine, < 1.0 mm; medium, 2.5 mm; coarse, 6.0 mm) were used. Treatments were defined by one fo the 3 corn particle sizes (fine, medium or coarse) and one of the 2 energy levels (3,100 or 3,200 kcal/kg ME, by the inclusion of 2% or 4% oil, respectively.)

Nutrient digestibility was determined using the traditional (i.e., total excreta collection) method. Feed offered was weighed at the beginning and again at completion of the experimental period. At experiment start, birds were weighed in order to obtain homogeneous treatment groups. Prior to starting excreta collection, birds were fasted for 12 hours to empty the gastrointestinal tract. Excreta were collected twice per day during the 4 day experimental period. Excreta were collected in plastic bags, weighed, and stored in the freezer until the end of collection period, then weighed and pre-dried in an incubator @ 60°C with forced air for 72 hours (h). Excreta were then exposed for 2 h at ambient temperature, weighed, homogenized, and sampled, in order to determine dry matter (DM), ether extract (EE), and nitrogen (N) contents.

Experimental feed samples were also subjected to the same analyses, as described by the Brazilian Compendium of Animal Nutrition (Compêndio Brasileiro de Nutrição Animal) (2005). The analyses were carried out by the Nutrition Laboratory, EV, UFMG (Laboratório de Nutrição da Escola de Veterinária da UFMG). Excreta samples were previously subjected to an acid hydrolysis process for fat to become available.

Using both lab analytical results and feed intake/excreta production data, all apparent dry matter digestibility coefficient (CDMS), crude protein digestibility coefficient (CDPB), and ether extract digestibility coefficient (CDEE) were calculated.

The experimental design was completely at random, using a 3 x 2 factorial arrangement (3 particle sizes x 2 energy levels) for a total of 6 treatments with 5 10-bird repetitions. Results were subjected to analysis of variance and the means were compared using Tukey's test with the SAEG (2007) software.

Results of % DM digestibility (CDMS as abbreviated in Portuguese), CP digestibility (CDPB), and EE digstibility (CDEE), apparent metabolizable energy (EMA) and nitrogen balance-corrected apparent metabolizable energy (EMAn), in kcal/kg, of feeds as a function of both particle size and energy levels for broilers during the period of 30 to 33 days of age are shown in Tables 2, 3, 4, 5 and 6, respectively.

Table 2. Per-treatment dry matter digestibility coefficient

Particle size x dietary energy level interaction was observed on CDMS (P≤0.05). For the level of 3,100 kcal ME/kg feed, no effect of particle size existed on CDMS. At the level of 3,200 kcal ME/kg, the intermediate particle size had an adverse effect on CDMS. Feeds with both fine and coarse particle sizes resulted in improved CDMS as dietary energy levels increased from 3,100 kcal ME to 3,200 kcal ME, while medium particle size had no effect on energy levels.

This improvement can be related with the extra-caloric effect of oil addition, which typically results in improved dietary nutrient utilization, thus improved animal performance.

Table 3. Per-treatment crude protein digestibility coefficient

Particle size x dietary energy level interaction existed on CDPB (P≤0.05). CDPB improved as particle size increased (from fine to intermediate to coarse) with the lowest energy level, and from medium to coarse particle size at the highest energy level. When energy level results are compared, improved crude protein digestibility can be seen in the face of coarse particle size at a metabolizable energy level of 3,200 kcal.

Table 4. Per-treatment ether extract digestibility coefficient

No significant (P> 0.05) particle size x energy level interaction existed on CDEE. Regardless of particle size, the higher energy level resulted in increased (P≤0.05) CDEE. This result can be explained by the effect of the higher oil inclusion levels in the diets, which enhances nutrient digestibility thus improving the feed conversion rate at the higher energy level. Regardless of the energy level, particle size had no impact (P> 0.05) on this response.

Particle size x dietary energy level interaction was seen on EMA (P≤0.05). As far as the energy level of diet is concerned, increased EMA occurred with the higher dietary energy level. Regarding corn particle size, at the level of 3,100 kcal/kg, EMA was adversely affected (P≤0.05) by coarse particle size.

Table 5. Per-treatment apparent metabolizable energy values

For the level of 3,200 kcal/kg, the EMA values determined in the feeds were similar (P> 0.05) between the fine and intermediate particle sizes, but for the coarse particle size an increased (P≤0.05) EMA value was found. This is due to a slower feed passage from the gizzard to the small intestine (Ravindran et al., 2006) and to the numbers of smaller particles in the gizzard, which enhance digestion and enzyme action (Lentle, 2005).

A significant (P≤0.05) particle size x energy level interaction was observed on EMAn. Improved (P≤0.05) EMAn vales occurred as dietary energy levels increased.

 

Table 6. Per-treatment nitrogen balance-corrected metabolizable energy values

For the 3,100 kcal/kg energy level, reduced (P≤0.05) EMAn values were observed as particle size increased. At the level of 3,200 kcal/kg higher (P≤0.05) EMAn values were also observed with coarse vs. fine particle sizes, while the medium particle size resulted statistically equal to both fine and coarse particle sizes. Regarding dietary energy levels and EMA, increased EMAn values were observed with the higher dietary energy level.

Feeds containing 3,200 kcal/kg ME with coarser particle size result in enhanced nutrient digestibility in 30-33-day-old broilers.

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