Optimum alfalfa harvesting time is critical to obtain high-quality forages for lactating dairy cows. This experiment was conducted to find the optimum maturity stage and time of the day to harvest alfalfa for high producing dairy cattle in the east of Iran. Experimental hays for dairy cattle diets were harvested from a third cut of a second year alfalfa field (4.2 ha) divided into three plots (1.4 ha) with each plot assigned to one of the following three stages of maturity: early bud (EB), late bud (LB) and early flowering (EF, local commercial harvesting stage). Half of each plot was harvested at sunset (PM) after a sunny day and the other half was harvested the next morning (AM), in 2010 on June 13/14, 19/20 and 26/27 for EB, LB and EF, respectively. After 4 field days, these hays (n6) were baled in a factorial arrangement. Advancing from EB to EF, the DM content of fresh alfalfa increased (PB0.01) and delaying the time of cutting in the day increased its DM content (PB0.01). With advancing maturity, leaf content and leaf/stem ratio decreased (PB0.01). These traits increased at PM cutting compared with AM cutting (PB0.01). Across maturity stages ash and CP content decreased and total carbohydrate (TCHO), NDF, ADF and cellulose content increased (PB0.05). These components were similar between alfalfa harvested at sunset and sunrise. Fat content was similar among the maturity stages and between PM and AM harvests (PB0.05). Alfalfa harvested at sunset had a higher true potentially degradable protein (PB) and lower buffer soluble protein (BSP) compared with sunrise harvests (PB0.05). These protein fractions were similar among three stages of maturity. There was an interaction between stage of maturity and cutting time for dry matter digestibility (DMD) after 24 h and 48 h of in situ incubation (PB0.05), which were both highest for alfalfa harvest at EBPM. Our findings confirm, as previously reported, that with advancing maturity ash, CP and leaf contents decrease and NDF, ADF and TCHO contents increase. Alfalfa harvest at sunset had higher DM, leaf and PB contents and lower BSP content. Results from the current study indicate that early bud stage at sunset is the optimum time to harvest alfalfa.

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