Drying procedures affect non-structural carbohydrates and other nutritive value attributes in forage samples

Abstract

Forage non-structural carbohydrates (NSC) are an important source of energy readily available to rumen microbes. Sampling procedures that quickly reduce plant metabolic activity after cutting and preserve macromolecular structures are required for accurate estimation of forage NSC concentration and other nutritive value attributes. Although several drying procedures exist, their efficiency to reflect composition of fresh samples is unclear. We compared five drying procedures on fresh samples of timothy ( Phleum pratense L.) and alfalfa ( Medicago sativa L.): (1) oven-drying at 55 °C for 48 h; (2) high-temperature pretreatment at 100 °C for 1 h followed by oven-drying at 55 °C for 48 h; (3) freezing pretreatment at −20 °C for 1 mo followed by oven-drying at 55 °C for 48 h; (4) microwave pretreatment for 1 min followed by oven-drying at 55 °C for 48 h; and (5) freezing at −20 °C for 1 mo followed by freeze-drying. Starch, sucrose, glucose, fructose, fructans (timothy) or pinitol (alfalfa), N, neutral detergent fibre assayed with a heat-stable amylase (aNDF), acid detergent fibre (ADF), neutral detergent insoluble N (NDIN), and acid detergent insoluble N (ADIN) concentrations, and in vitro true digestibility of DM (IVTD) and digestibility of neutral detergent fibre (dNDF) were determined in the spring growth and summer regrowth of two production years. Soluble carbohydrate (SC) concentration was estimated using the sum of sucrose, glucose, fructose, and fructans or pinitol. The NSC concentration was obtained by adding SC and starch. Averaged across forage species and growth periods, NSC concentrations were similar and generally highest ( P<0.001) in freeze-dried samples (89.8 mg/g DM), the reference procedure, and the microwave-pretreated samples (86.7 mg/g DM); oven-drying at 55 °C resulted in the lowest NSC concentration (57.6 mg/g DM). Values of other nutritive value attributes with the microwave pretreatment were comparable to those of the other drying procedures, except that it tended to increase forage NDIN. The NDIN concentration was lowest ( P<0.001) in freeze-dried samples (2.1 mg/g DM) and generally highest in microwave-pretreated samples (7.5 mg/g DM). The effect of drying procedure on aNDF, ADF, IVTD, dNDF, and ADIN concentrations varied slightly with forage species and growth period. Principal component analysis (PCA) highlighted the closeness between freeze-drying and microwave pretreatment for the global assessment of the nutritive value of forages, including NSC concentration. Microwave pretreatment should be given greater consideration as an effective and easy-to-use sample preparation procedure for the characterization of the nutritive value of fresh forages.