The Impact of Dietary Electrolyte Balance on Male Broiler Performance Offered Reduced Crude Protein Diets

Dietary electrolyte balance (DEB) is defined by the interrelationship between sodium (Na⁺), potassium (K⁺) and chloride (Cl^-) where DEB = Na⁺⁺K⁺ -Cl^- as mEq/kg and it plays an influential role in homeostasis of the body fluids. Borges et al. (2004) investigated DEB in two experiments and concluded that there was a quadratic effect on weight gain and feed conversion efficiency (FCE) when the DEB was increased by the supplementation of Na⁺ alone. Feed intake was maximised at a DEB of 264 mEq/kg, when Na⁺ level was increased in the diet and, 213 mEq/kg when both K⁺ and Na⁺ levels were increased in the diet. These authors reported that the ideal DEB, obtained by the manipulation of both the Na⁺ and Cl^- levels, was between 202 and 235 mEq/kg. In more recent work, Borges et al. (2011) reported a wide range of acceptable DEB’s in the literature; from 200 to 350 mEq/kg, depending on environmental temperature, humidity and other factors and have suggested that the optimum DEB is around 250 mEq/kg of feed.

Teeter and Belay, (1996) reported that the maintenance of blood pH and CO2 levels were critical to growth rate in broilers and noted that the buffering systems within the bird ensure that pH is usually maintained at optimal physiological levels. However, these authors added that in extreme conditions, when demand for electrolytes is elevated, maintaining buffering capacity may have an adverse effect on other physiological conditions. In broiler chickens, an electrolyte imbalance is known to affect the metabolism of a number of the essential amino acids, particularly arginine and lysine (Kim et al., 1989; Riley and Austic, 1989). In earlier work, Austic and Calvert (1981) concluded that lysine, arginine, glutamic acid and glutamine play an important role in the regulation of acid-base balance.

In reduced crude protein (CP) broiler diets, DEB is often overlooked or ignored. A reduction of CP in diets based on maize (or wheat) and soyabean meal, reduces dietary levels of K⁺ due to a reduction of soyabean meal inclusion and may also increase Cl^- level from synthetic sources of AA’s (Borges et al., 2011; Lambert and Corrent, 2018). The combination of lower CP and lower K⁺ reduced water intake by 1.4%, decreased the water to feed intake ratio and lead to a reduction of litter moisture by 2.2% per CP percentage point (Lambert and Corrent, 2018). In recent work by Belloir et al. (2017), DEB was not considered but, by calculating DEB from the ingredients used in this study, the DEB declined in a linear manner from 209 mEq/kg, in the 190 g/kg CP diet, to 127 mEq/kg in the 150 g/kg CP diet and may partly explain why the diets below 170 g/kg CP had a significantly worse FCR (P < 0.01).

In Murakami et al. (2003), the effect of DEB on chick performance in reduced CP diets supplemented with free amino acids was investigated. A non-significant linear response in weight gain (g; r = 0.96) and feed intake (g; r = 0.98) was observed in broilers aged from 1 to 21 days post-hatch when DEB was increased from 200 to 320 mEq/kg. However, a significant (P < 0.05) negative linear relationship (r = -0.89) was observed for FCR over the range of DEB tested in these broilers. In broilers aged 21 to 42 days post-hatch, these authors observed significant (P < 0.05) quadratic responses to DEB for body weight gain (g; r = 0.89) feed intake (g; r = 0.64) and FCR (r = -0.95). The reported optimum DEB levels for weight gain and feed intake were 230 and 270 mEq/kg and the best FCR was achieved at 168.9 g/kg CP and 245 mEq/kg. Therefore, the purpose of this paper is to report on the effect of DEB using sodium chloride, sodium bicarbonate and potassium carbonate to either maintain DEB at a set level or allow it to decline in reduced CP diets.

As part of a larger feeding study, a total of 210 male, off-sex, Ross 308 chickens were offered maize-soy diets formulated to contain 200, 188, 172 and 156 g/kg CP and DEB of 230 mEq/kg from 14 to 35 days post-hatch with an additional diet of 156 g/kg CP allowing DEB to reduce to 120 mEq/kg (Table 1). Diets were formulated to iso-energetic levels of 12.85 MJ/kg (apparent metabolisable energy) and standardised ileal digestible (SID) lysine of 11 g/kg. Essential SID amino acid (AA) ratios were maintained across all diets using recommendations from AMINODat 5.0, Platinum (Evonik Industries, Germany). Each dietary treatment was offered to 7 replicate cages of 6 birds per cage. Broiler growth performance, feed intake, relative abdominal fat-pad weights and water to feed ratios were recorded. The experimental data was analysed via the JMP 13 Statistics program (SAS Institute). The conduct of the feeding study fully complied with specific guidelines (2016/973) approved by the Animal Ethics Committee of the University of Sydney.

Overall weight gains from 14 to 35 days were 1902 g with an average feed intake of 2951 g and an average FCR of 1.552. The Ross 308 standards (2014) for male broilers for the same ages are 1795 g/bird for weight gain, 2965 g/bird for feed intake and an FCR of 1.652. The results achieved therefore exceeded the breed standards by 6.0% for weight gain and 6.1 % for FCR (Table 2). Reducing dietary CP had no effect on weight gain whilst feed intake increased in a linear manner (r = -0.156; P = 0.018). Consequently FCR worsened as dietary CP reduced with significantly (P < 0.001) inferior FCR at the lowest dietary CP levels (treatments 4D and 5E). Water intake and feed intake were recorded between days 31 to 33 post-hatch (data not shown) and there was a non-significant (P = 0.160) trend to decreasing water intake as CP declined. However, combined with a significant increase in feed intake over this period (P = 0.034) the water to feed intake ratio (water:feed) decreased significantly as protein declined, in a linear manner (r = 0.444; P < 0.001). Excreta dry matter content ranged from a minimum of 184.9 g/kg in treatment 2B to a maximum of 201.2 in treatment 3C and was not significant (P = 0.390) suggesting that the volume of excreta output declines as dietary CP reduces. A reduction in DEB between the lowest CP diets (treatments 4D and 5E) had no impact on all of the performance parameters and relative fat-pad weights measured (Table 2).

Whilst, in the context of normal CP diets, the range of acceptable DEB levels reported in the literature are wide, few studies have investigated the effect of DEB in reduced CP diets. In the present study, a reduced CP diet with a low DEB was included as a single treatment to determine if there was any effect of DEB on broiler performance in broilers offered reduced dietary CP. In reduced CP diets with large amounts of supplemented crystalline AA’s, the current study was unable to demonstrate any effect of DEB and this may be partly due to better AA balance in modern broiler diets and the source of supplemental Na⁺ and K⁺ as salts rather than organic acids, which is in agreement with Austic and Calvert (1981), who also observed no response when salts of these ions were used (as chloride and/or sulphate). This suggests that the molecular structure of the electrolyte should also be considered in DEB studies and that the source of electrolyte ions plays an important role. The confounding effect of changing dietary pH with the use of metabolisable organic acids and the use of hydrochloric acid to adjust Cllevels on broiler performance was not considered in the report from these authors but cannot be ignored.

In Murakami et al. (2003), the lack of significant responses for weight gain and feed intake in broilers aged 1 to 21 days post-hatch is at odds with the excellent regression coefficients for these parameters in this study. In addition, potassium chloride (KCl) was used to adjust the DEB as soyabean meal was reduced and, replacing the K⁺ in this manner leads to high Cl^- levels and concomitantly low Na⁺ levels in many of the test diets. An excess of Cl^- and/or diets deficient in Na⁺ may lead to erroneous conclusions regarding the influence of DEB in reduced CP diets.

Belloir et al. (2017), concluded that a reduction of the dietary CP content by several CP percentage points is possible in growing-finishing broilers, without consideration of DEB with positive implications for the sustainability of broiler production. From the present study, it can be concluded that a reduction in DEB below the reported optimum levels is not a limiting factor in reduced CP diets supplemented with crystalline EAA’s. However, it is prudent to maintain DEB levels at accepted optimum levels of 225 to 250 mEq/kg for future broiler trials with reduced CP diets.