چكيده لاتين :
Introduction: Alfalfa (Medicago sativa L.) is commonly used as high-protein forage in diets of lactating cows. Cultivated alfalfa is the main forage ingredient for dairy rations in Iran with 5.5 million metric tonnes of hay harvested during season 2009–2010 (Yari et al., 2012). Alfalfa contains high nutrients levels, high digestibility, and unique proportion of structural to non-structural components (Yu et al., 2003). Chemical composition, nutritive value and crude protein (CP) fractions of alfalfa are influenced by cultivar, stage of maturity (SM; Coblentz et al., 2008; Yari et al., 2012), climate condition (Van Soest, 1994) and cutting time (CT; Brito et al., 2008, 2009; Yari et al., 2012) due to accumulation of non-structural carbohydrate during the day (Brito et al., 2008, 2009). Various CP fractions present in feed differ in rate and extent of ruminal degradation. These fractions influence the amount of CP degraded in the rumen and escaping to the lower digestive tract (Van Soest, 1994). Furthermore, the knowledge of these CP fractions and degradation is used in modern diet formulation programs such as NRC (2001) to formulate ruminant diets. Therefore, information about CP fractions and degradability (NRC, 2001) should be taken in to consideration when formulating diets for ruminants.
Material and methods: This experiment was conducted in a field (10 m × 10 m) at Tropical Research Centre of Ilam University (Mehran, Iran; 33º07´N, 46º10´E). Total field was divided into 4 main plots (4 m × 5 m). Each main plot was then divided into 4 subplots (5 m2), so that the hay of each one harvested at different stage of maturity (pre-bloom vs. early bloom) and cutting time (06:00 am vs. 06:00 pm). No mineral-N fertilizer was applied on the plots during the growing period. The field was irrigated three times weekly during experiment. Stage of pre-bloom is approximately one week before the point at which the hay would be commercially harvested and the stage of early bloom is at the point of commercial cutting. Experimental treatments consisted of 1: alfalfa hay harvested pre-bloom in the morning, 2: alfalfa harvested pre-bloom in the afternoon, 3: alfalfa harvested early bloom in the morning and 4: alfalfa harvested early bloom in the afternoon. At the time of sample collection, a quadrate (250 cm2) was randomly thrown in each plot (one time) and all stems were harvested 5 cm above ground using a small scythe to calculate fresh yield. Immediately after cutting, the weight of fresh sample of each quadrate was recorded. Alfalfa samples dry matter (DM) was determined by oven drying for 48 h at 60º C. Before chemical analysis, samples were ground to pass through a 1-mm screen. The samples were analysed for DM, ash, ether extract (EE), CP, calcium and magnesium according to the standard methods (AOAC, 1990). The neutral detergent fibre (NDF), acid detergent fibre (ADF) and acid detergent lignin (ADL) were determined according to the procedures of Van Soest et al. (1991), without heat stable amylase and sodium sulphite. Non-fiber carbohydrate (NFC) was calculated according to NRC (2001). The CNCPS was used to divide CP into five fractions including A, B1, B2, B3, C (Sniffen et al., 1992; Licitera et al., 1996). Ruminal degradability of DM, organic matter (OM), CP and NDF were measured by in situ method. Rumen undegradable protein (RUP) content of samples were estimated based on chemical composition and CP fractions according Kirchhof (2007), Sniffen et al. (1992) and NRC (2001) equations Results and discussion: Alfalfa hay harvested at early bloom had higher fiber, lignin, RUP, EE and protein B3 and C fractions and lower NFC and ruminal DM and OM degradability compared to alfalfa hay harvested at pre-bloom (P<0.05). These results were in agreement to other studies (Yu et al., 2003; Yari et al., 2012). Alfalfa leaves have a higher CP, higher digestibility and lower fiber content compared with alfalfa stems (Van Soest, 1994). Therefore, the decreased leaf:stem ratio with advancing SM found in this study likely caused reduced CP, reduced in situ degradability and increased structural carbohydrates (e.g. NDF and ADF) similar to published results (Yari et al., 2012; Yu et al., 2003). Protein A fraction, crude protein (CP), calcium and magnesium of alfalfa hay not affected by MS. Harvesting alfalfa in the morning increased CP, neutral and acid detergent insoluble CP and RUP contents and decreased NFC content and ruminal DM and OM degradability compare to alfalfa harvested in the afternoon (P<0.05). Content of protein B3 fraction, OM, EE, lignin and calcium were not affected by alfalfa hay cutting time (P>0.05). These results were consistent to findings of previous experiment (Yari et al., 2012; Kirchhof et al., 2010). During the day, alfalfa leaves did likely use of photosynthesis end-products to convert fraction A components into true protein (fraction B; Van Soest, 1994). The higher NFC content in alfalfa hay harvested in the afternoon compared to that harvested in the morning can be attributed to accumulation of photosynthetic sugars during the day (Van soest, 1994). Factors such as time of cutting, day length, temperature, sunshine intensity and mode of forage conversation can influence CHO fractionation during day (Van Soest, 1994; Brito et al., 2008, 2009) and might therefore be the cause of differences among studies.
Conclusion: Potential nutrient supply to the animal reduced with advancing alfalfa maturity. With advancing maturity, protein content and digestibility decreased, while fiber fractions increased. Cutting alfalfa in the afternoon increased NFC content compared with cutting alfalfa in the morning. Collectively, alfalfa harvested at pre-bloom and in the afternoon had higher nutritional value.
