The study evaluated structural effectiveness of NDF from of spring harvest grass/clover forages of primary growth by assessing chewing activity and feces particles >1.0 mm in heifers. Two batches of mixed ryegrass, red and white clover harvested in 2009 on May 9 and 25 were conserved as either silage or hay. The forages early silage (ES) and hay (EH), and late silage (LS) and hay (LH) had DM contents of 45, 84, 25 and 83%, and NDF contents of 32, 44, 42 and 50% of DM, respectively. Forages were fed as sole feed to four Jersey heifers of 435±30 kg BW in a 4×4 Latin square experiment. Feeding level was 90% of individual ad libitum intake, divided in two daily meals at 0800 and 1530 h. Jaw movements oscillations (JMO) were recorded for 96 h continuously using Hall sensor fitted chewing halters. Jaw movements (JM) were identified from JMO, clustered into cycles and periods of rumination and eating and summarized into min per day. Feces were sampled 3 times daily and machine washed in nylon bags of 0.01 mm pore size. Feces particulate matter was freeze dried and divided into small (0-1.0 mm) and large (>1.0 mm) particle fractions by dry sieving. Data was analyzed by the MIXED procedure in SAS 9.2 with period, conservation, harvest time, and conservation×harvest time as fixed effects and heifer as random. Daily intake of ES, EH, LS, and LH was 8.7, 9.4, 7.2, and 7.2 kg DM and 2.7, 4.1, 3.0, and 3.6 kg NDF respectively. Early compared to late harvest caused similar NDF intake and time spent eating, but lower mean rumination per kg NDF intake (P=0.002). Hay compared to silage caused higher NDF intake (P<0.001), and less mean time spent eating (P=0.02) and ruminating (P=0.004) in min/kg NDF intake. Feeding silage compared to hay resulted in a higher proportion of washed fecal particle DM <1.0 mm, while harvest time hardly affected fecal particle size. Results indicate that NDF from late cut forages and silage stimulated rumination more effectively, and that heifers retain large forage particles in the rumen better with silage compared to hay.