Effects of different chlorine to sodium ratios in the starter diet on performance, yolk sac absorption, carcass characteristics, crop filling, and behavioral observations in broiler chicks

Introduction: Sodium and chlorine deficiencies lead to a significant reduction in osmotic pressure, resulting in water loss, dehydration, and ultimately, a severe decrease in bird performance (Chen et al., 2023). Increased levels of chlorine and sodium also cause reduced voluntary feed intake, wet litter, interference with normal functions, and toxicity (Yu et al., 2022). The National Research Council recommends sodium and chlorine levels for optimal growth during the starter (0-21 days), grower (22-43 days), and finisher (43-56 days) phases as 0.2%, 0.15%, and 0.12%, respectively (NRC, 1994). The Ross 308 strain guide also suggests sodium and chlorine levels of 0.18%-0.23% during different rearing phases (Aviagen, 2022). While various reports address the effects of different chlorine and sodium levels in broiler chickens, the effects of the chlorine to sodium ratio in broilers have not been examined. The Ross 308 strain recommends a chlorine to sodium ratio between 1:1 and 1.3:1 (Aviagen, 2022). The Hy-Line strain recommends a chlorine to sodium ratio of 1:1 to 1.1:1 in hot seasons (Hy-Line, 2016). Aviagen's technical report also states that chlorine levels in the diet should not exceed 10% above the sodium levels (Kretzschmar-McCluskey et al., 2014). The Hubbard strain nutritional guide recommends a chlorine to sodium ratio range of 1.1:1 to 1.3:1 (Hubbard, 2022). The stimulatory role of sodium and chlorine on feed and water intake has been observed in previous studies (Wang et al., 2020; Yu et al., 2022; Chen et al., 2022). However, the potential role of the chlorine to sodium ratio on performance during the starter phase of broiler chicks has not yet been investigated. Therefore, this study aims to examine the effects of different chlorine to sodium ratios in the starter diet on performance, yolk sac absorption, carcass characteristics, crop filling, and behavioral observations in young broiler chicks.
Material and methods: A total of 420 one-day-old Ross 308 male broiler chicks were used in a completely randomized design with 5 treatments, 6 replicates, and 14 chicks per replicate. The experimental diets including: 1) starter diet with a chlorine to sodium ratio of 1:1, 2) starter diet with a chlorine to sodium ratio of 1.1:1, 3) starter diet with a chlorine to sodium ratio of 1.2:1, 4) starter diet with a chlorine to sodium ratio of 1.3:1, and 5) starter diet with a chlorine to sodium ratio of 1.4:1. The experimental diets were fed only from days 1 to 10. Average body weight gain, average feed intake, and feed conversion ratio (adjusted for mortality if observed) were calculated for the starter phase (1-10 days) and the entire period (1-30 days). Water consumption was monitored from days 1 to 7, and the water-to-feed consumption ratio was calculated for this period. Feed and water consumption behaviors were recorded at 3, 6, 12, 24, 48 and 72 hours after chick placement in each pen. Crop filling tests were conducted at 2, 4, 8, 24, and 48 hours after chick placement. On days 3 and 5 of the experiment, 2 chicks from each replicate were randomly selected and euthanized using carbon dioxide gas. They were then transported to the carcass separation facility for the dissection of carcass components and internal organs. Additionally, on days 3 and 5 of the experiment, the yolk sacs were separated and weighed using a high-precision scale with an accuracy of 0.001 grams. All data were analyzed using the ANOVA option of the general linear model of SAS software. Significant differences between treatment means were determined by Tukey's multiple range test. Differences in means were regarded as significant at P < 0.05. A polynomial regression analysis was also employed to predict the effect of the different chlorine to sodium ratios on various parameters tested.
Results and discussion: The results of the present study showed that increasing the chloride to sodium ratio in the diet led to a linear increase in body weight gain during days 1-10 (P<0.01) and 1-30 (P<0.05). In line with the results of the present study, Chen et al. (2020) reported that chloride levels of 0.15%, 0.2%, and 0.25% resulted in linear and quadratic improvements in body weight gain. The observed increase in body weight with the higher chloride to sodium ratio in the present study can be attributed to increased feed and water intake (Yu et al., 2022). Additionally, increasing the chloride to sodium ratio also led to a linear increase in feed intake during the 1-10-day period (P<0.05). Wang et al. (2020) also observed that increasing the chloride level in the diet from 0.06% to 0.1%, 0.15%, 0.2%, and 0.25% led to linear and quadratic increases in feed intake in laying hens from 43 to 54 weeks of age. These researchers attributed the observed increase in feed intake following the increase in dietary chloride levels to the inhibition of the effect of metabolic alkalosis on feed intake (Wang et al., 2020). Carcass yield, relative weights of the liver, small intestine, spleen, pancreas (P<0.05), bursa (P<0.05), and cecum also increased linearly with an increase in the dietary chloride to sodium ratio at 5 days of age (P<0.01). Moreover, the residual yolk sac decreased significantly in a linear manner with an increase in the dietary chloride to sodium ratio during 3 (P<0.01) and 5 (P<0.05) days of age. One of the primary factors influencing yolk sac absorption is the motility and peristaltic movements in the small intestine (van Der Wagt et al., 2020). Yolk sac absorption depends on feed intake, as increased feed consumption accelerates peristaltic movements, stimulating the discharge and absorption of the yolk sac in the small intestine (Mikec et al., 2006). At 10 days of age, plasma concentrations of sodium and chloride increased linearly with an increase in the dietary chloride to sodium ratio (P<0.01), while the concentration of potassium decreased linearly (P<0.05).
Conclusion: In general, it can be concluded that increasing the chloride to sodium ratio in the starter diet stimulates feed intake, leading to faster yolk sac absorption, improved weight gain, and better organ development in young broiler chicks.