Digestibility and Metabolism in Icelandic Horses Fed Forage-only Diets

The objective of this thesis was to investigate digestibility and metabolism in Icelandic horses fed forage differing in stage of maturity/harvesting date and at two feeding levels. Additional aims were to compare digestibility and metabolic plasma profile in Icelandic and Standardbred horses. Finally, one aim was to estimate the digestible energy (DE) and digestible crude protein (DCP) requirements of Icelandic horses and compare them to widely used recommendations based on estimates from other breeds. Four studies were performed as change-over arrangements and total collection of faeces was performed for 6 days. In addition, urine was collected in two studies to estimate energy and nitrogen losses. In studies I and II the effects of maturity and harvesting date on the nutritional value of different forages was estimated. Study I was performed on timothy haylage and study II on meadow haylage. In each study 4 horses were used. There was a negative relationship between harvesting date and the coefficient of total tract apparent digestibility (CTTAD) of all measured components. The lignin and acid detergent fibre contents were the main components affecting the CTTAD of organic matter (OM), while the content of neutral detergent fibre (NDF) had less influence. In study III, 8 horses were used and fed high-energy haylage at two feeding levels. The lower feeding level was set at 1.0 and the higher at 1.5 times maintenance energy intake. For all measured dietary components except crude protein (CP), the CTTAD was higher at the lower feeding level. In study IV, 6 Icelandic and 6 Standardbred horses were fed 2 different haylages. No overall breed differences were detected for the CTTAD of energy, CP and NDF. However, the Standarbred horses had higher CTTAD of OM. There were also differences between the breeds in total plasma protein, urea and insulin concentrations. The estimated DE and DCP requirements in sedentary maintenance fed Icelandic horses were comparable to minimum requirements found in general recommendations for horses.