the effect of growth stage and cutting time on chemical composition in vitro digestibility and fermentative gas production of alfalfa

introduction cultivated alfalfa (medicago sativa l.) is the main, most of the time the only, forage for ruminants in iran as well as many other parts of the world. harvesting high quality forage is critical for iranian dairy industry, because it affects the performance of animals and reduces amount of concentrates utilization in dairy diets. unfortunately the most parts of the concentrates ingredients are imported. growth stage and cutting time in the day are known as the main factors affecting alfalfa hay quality. fiber content increases and protein fraction decrease with increasing the alfalfa maturity (31). leaf: stem ratio, digestive behavior and availability of alfalfa hay are mainly influenced by stage of maturity and cutting time. however, there is limited published information on the effect of growth stage and cutting time on nutrient characteristics of alfalfa hay. therefore, the objective of this study was to evaluate the chemical composition, in vitro digestibility and fermentative gas production of alfalfa hay harvested at different growth stage and cutting time through in vitro experiments. materials and method this study was done at the research farm of ferdowsi university of mashhad. iran. the experimental farm was divided into 3 equal parts. each part was allocated to one of the three growth stages (early bud, late bud and early bloom). every main plot then was divided into 2 equal parts for allocation to two cutting times (at 6.00 and 18.00 hrs.). therefore, the six plots were randomly assigned to six treatments in a factorial arrangement of 3x2. oven dried (65◦c for 48 h) chopped alfalfa hay samples were ground to pass through a 1-mm screen. the samples were analyzed according to the standard procedures for chemical composition (2 , 27). procedure of in vitro gas production was performed according to menke and steingass (1988). rumen fluid was obtained from three fistulated baluchi male sheep before morning feeding. the dm degradation data were fitted to the exponential equation p = a + b (1 - e-ct) (21). the in vitro dry matter, ndf and organic matter digestibility were determined according to the arroquy et al. (2005) procedure at 0, 2, 4, 6, 8, 12, 24, 36, 48, 72, 96 hrs ofter incubation.. test samples were incubated for different hrs and then filtered through the nylon cloth with the pore size of 44 microns. the remaining materials were dried at 60 °c for 72 hrs and utilized for the subsequent analysis according to the procedure. the digestibility data were fitted to the exponential equation d(t) = d(i). e(-k. t) + i (3). results and discussion morphological characteristics and chemical composition of the alfalfa samples are shown in table 1. with advancing the growth stage dm content was also increased but leaf to stem ratio reduced significantly (<0.01). generally the valuable nutrient contents of the samples harvested in the afternoon were higher than the samples obtained in the morning. with the growth advancement amount of produced gas increased non-significantly (figure 1), but the interaction between the growth stage and cutting time in case of gas production was significant (<0.01). gas production rate for the early bud samples harvested in the afternoon was significantly (<0.5) higher. no noticeable trend was found for ammonia nitrogen (nh3-n) in this study. according to the results of in vitro digestion measurements amount of digestible dry matter for the early bud alfalfa harvested in the afternoon was higher significantly (<0.05). it was concluded that the nutrient contents of alfalfa forage harvested at early bud stage and in the afternoon led to higher feeding vale in case of cho, cp contents. alfalfa harvested at early bud stage in the morning (ebam), alfalfa harvested at early bud stage in the afternoon (ebpm), alfalfa harvested at late bud stage in the morning (lbam), alfalfa harvested at late bud stage in the afternoon (lbpm), alfalfa harvested at early bloom stage in the morning (efam), and alfalfa harvested at early bloom stage in the afternoon (efpm)). conclusion it was concluded that cutting time of alfalfa forage at early bud stage in the afternoon provides better hay for ruminants in case of nutritive value.