A Summary of the Effects of Feeding and Daily Variation on Acid-Base Status in Resting Horses

Diurnal Review: by Kerri-Jo Smithurst, University of Guelph

Plasma acid-base state affects, and may also be a reflection of, the health of equine athletes. The physicochemical model, as developed by Stewart (1981), defines the blood constituents that effect or determine acid-base state. These constituents, the partial pressure of carbon dioxide (PCO2), the strong ion difference ([SID]), and the total concentration of weak acids and bases ([Atot]), are the independent variables in the physicochemical equation. These independent variables are affected by external and internal influences throughout the day. The acid-base state in equine plasma can be completely described by the equilibrium between the independent variables and quantified using the physicochemical approach to acid-base balance.

By monitoring plasma acid-base parameters the occurrence and origins of daily variations can be understood. Variations in plasma result from diurnal influences such as activity and feeding, nocturnal influences such as sleeping, and the underlying circadian rhythm of an organism. Results from blood analysis affect the interpretation of a horse’s biochemistry, which leads to conclusions about health and affect drug-testing results.

The purposes of this literature review are to provide an introduction to acid-base assessment in clinically normal horses at rest and outline changes observed with feeding over a 24-h period. The literature on diet, feeding and daily variation of blood acid-base status of horses at rest is summarized. The purpose of the research described in this thesis is to investigate effects of feeding and daily variation of equine blood parameters on plasma acid-base status. Therefore, the presented physiological information should improve our basic knowledge of the daily changes in plasma acid-base status and set the stage for further research on acid-base status in horses.

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Usually a combination diet of forage and grain rations is fed to performance horses. However, when and how much the animal is fed is also an issue; it can vary from complete access to pasture forage with a small amount of grain supplement to two feedings a day with a high grain to forage ratio. The effect of diet depends both on the type of feed itself and the timing of feeding of horses; individual large meals have an immediate impact over periods of 6-8 h via fluid shifts, while metabolic/respiratory effects appear to be the main influence over 24 h (Kronfeld 2001, Mongin 1981).


The acid-base state can be described by the equilibrium between the independent variables, [SID], [Atot], and PCO2, and quantified using the physicochemical approach to acid-base balance. Acid-base status is affected by daily variations due to feeding factors (including DCAD, amount, composition and timing of meals), confounding the ability to establish baseline values for plasma constituents. The influence of feeding and exercise as well as incomplete sampling over a full 24-h period have confounded research looking at daily variations in equine plasma constituents.

Besides the importance of establishing baseline values for plasma constituents, acid-base variables are important to the horse racing industry for drug testing. Alkalizing agents are used to enhance performance. Drugs can be used to manipulate plasma [TCO2]. A TCO2 blood test is performed in Ontario prior to both Standardbred and Thoroughbred races to determine whether an alkalizing substance (usually in bicarbonate form) has been administered (colloquially known as “milkshaking”). A [TCO2] greater than or equal to 37.0 mmol/L in venous blood plasma is considered a positive test. [TCO2] is a measure of the total carbon dioxide concentration in blood, which is primarily made up of HCO3- and CO2 in solution. However, CO2 occurs naturally in the blood, therefore controversy exists over the reliability of the TCO2 test. TCO2 status is also affected by Hct, Hb, total [PP], Na+, K+, Cl-, Ca2+, Lac- and Pi concentrations. By quantifying the plasma acid-base variables under minimal outside influences the daily variation of [TCO2] can be assessed.

We hypothesized that equine plasma acid-base parameters exhibit daily variation independent of feeding and exercise. We examined variation in plasma [TCO2] and other plasma constituents throughout the day, without the effects of feeding, to identify the main factors in blood that determine the daily acid-base state of the horse. The purpose of the first trial was to identify the main electrolyte and acid-base constituents in blood plasma that exhibit daily variation. The second trials purpose was to determine the effect of feeding on plasma TCO2 and 19 blood constituents describing the acid-base and electrolyte state of horses. Blood constituents were assessed to allow definitive determination of factors affecting [TCO2] and other acid-base variables.