**Background:** Dietary protein is essential in canine nutrition, and animal by-products are a major protein source in the pet food industry. In the EU, approximately 18 million metric tons of animal by-products (category 3) are generated yearly. The particle size of raw materials can affect nutrient digestibility in monogastric animals, but little information is available on the impact of particle size of animal by-product meals in complete dog diets. This study aimed to evaluate the effects of different particle sizes of poultry by-product meal on apparent nutrient digestibility and fecal characteristics in Beagle dogs.
**Methods:** Six healthy intact female Beagle dogs (median age 6–10 years, mean BW 16.6 ± 2.03 kg, median BCS 4.98/9) were used in a crossover design. Broiler carcasses (including deep breast muscles, neck, wingtips, poultry fat, and skull) were milled to three degrees: coarse, fine, and very fine. The coarse diet had 45.2% of particles >1.00 mm, while the fine and very fine diets had 82.3% and 86.1% of particles <0.20 mm, respectively. Each trial lasted 10 days (5-day adaptation, 5-day total fecal collection). Dogs were fed 170 g DM/day (0.5 MJ ME/kg BW^0.75/d). Apparent digestibility was determined using the total fecal collection method per AAFCO recommendations. Fecal consistency was scored on a 5-point scale (1 = extremely firm, 5 = watery diarrhea). Fecal short-chain fatty acids (SCFA) were analyzed by gas chromatography. Statistical analysis used SAS version 7.3, with Shapiro-Wilk normality test, Ryan-Einot-Gabriel-Welsch test for parametric data, and Kruskal-Wallis test for non-parametric data (p < 0.05 considered significant).
**Key Results:** No significant differences were found among the three grinding treatments for any measured parameter. Apparent digestibility of organic matter (coarse: 95.7 ± 0.42%, fine: 95.1 ± 0.85%, very fine: 95.8 ± 0.89%; p = 0.306), crude protein (coarse: 95.1 ± 0.57%, fine: 92.8 ± 2.32%, very fine: 93.9 ± 1.73%; p = 0.207), and crude fat (coarse: 98.3 ± 0.57%, fine: 98.4 ± 0.80%, very fine: 99.0 ± 0.22%; p = 0.971) did not differ between groups. Fecal output (range: 14.3–15.7 g DM/d; p = 0.080), fecal score (range: 1.50–1.70; p = 0.599), fecal DM content (range: 40.9–43.4%; p = 0.878), and fecal pH (range: 6.64–6.83; p = 0.986) were not significantly different. Fecal SCFA concentrations (acetic, propionic, iso-butyric, n-butyric, iso-valeric, n-valeric acids) also showed no significant differences between groups. All dogs remained healthy, with high food acceptance (score 3.00), no food refusals, vomiting, or diarrhea.
**Clinical Implications:** This study demonstrates that the particle size of poultry by-product meal (coarse, fine, or very fine) does not negatively affect nutrient digestibility or fecal quality in healthy adult dogs. All diets resulted in protein digestibility well above the normal range of approximately 80% recommended by FEDIAF, with values between 92.8% and 95.1%. Fecal scores remained near the optimal value of 2 across all treatments. These findings support the use of poultry by-product meals in dog food formulations regardless of grinding degree, which has positive implications for sustainable pet food production by enabling utilization of animal by-products that might otherwise go to waste. However, the study was limited by a small sample size (n = 6) and short duration, and further research is needed to investigate the effects of full substitution of poultry carcasses as a complete feed for companion animals in terms of nutritional balance.