An evaluation of naturally occurring markers to measure digestibility and intake in equines

Feed intake and digestibility are important measurements to enable horse owners to determine energy and nutrient availability for various physiological and behavioural processes and performance demands. Accurate measurements of the quantity of feed and concentrate consumed and evaluation of feed utilisation enable adjustments in feeding programs to achieve desired weight goals, maintain optimal body condition and optimise performance. Measurements of intake and digestibility are of paramount importance for good feeding management, yet these measurements are not conducted by horse owners. Intake and digestibility measurements are routinely used in the dairy industry to optimise efficiency. Indigestible markers such as acid insoluble ash (AIA) and acid detergent insoluble ash (ADIA) in forages and concentrates can be used to estimate the digestibility and intake of forages in animal studies. The indigestible markers are components of the diet that pass through the digestive system without being broken down or absorbed. In this study three internal indigestible markers were assessed for suitability to measure dry matter digestibility (DMD) and dry matter intake (DMI) in equines. Ten horses were stabled, exercised and provided ad libitum access to haylage. Feed intake of the horses was measured daily, and freshly voided faecal samples were collected for a period of ten consecutive days. Acid insoluble ash (AIA), acid detergent insoluble ash (ADIA) and crude protein (CP) were determined in all forage, concentrate and faecal samples. The precision of the laboratory assays used to measure the quantity of internal markers in feed and faecal samples was evaluated by comparing duplicates using Lin’s Concordance Correlation Coefficient (CCC). All three markers showed excellent repeatability with CCC of 0.99 (95% CI: 0.98-0.99), 0.98 (95% CI: 0.97-0.99) and 0.94 (95% CI: 0.92-0.96) for AIA, ADIA and CP, respectively. Digestibility was estimated using five different formulas based on forage acid detergent fibre (ADF), forage neutral detergent fibre (NDF), forage CP, AIA and ADIA markers and faecal crude protein (fCP). All formulas showed differences in estimated digestibility (p<0.05) except for the AIA marker formula and the fCP formula. Intake was estimated using two formulas, first based on a digestibility estimate and second using a linear regression model created from the correlation between calculated value of total fCP (TfCP) and measured intake. There was no significant difference between measured intake and intake estimated using the regression model. The intake formula based on digestibility significantly underestimated intake (p<0.05). Total fCP shows potential for intake estimations for horses on haylage based diets. Estimation of DMD with various different formulas was inaccurate and further work is needed to establish a reliable DMD prediction model for equines.