Dairy heifers consuming high-quality forage diets are susceptible to excessive weight gains and over-conditioning, which often has been controlled by dilution with straw that is sortable by dairy heifers. Our objectives were: i) to compare the growth performance of dairy heifers offered a high-quality forage diet (CONTROL) with diets containing 1 of 3 diluting agents that included eastern gamagrass haylage (EGG), chopped wheat straw (STRAW), or chopped corn fodder (FODDER); and ii) evaluate sorting behaviors of heifers offered these forage diets. Holstein heifers (n = 128) were stratified (32 heifers/block) on the basis of initial BW (heavy, 560 ± 27.7 kg; medium-heavy, 481 ± 17.7 kg; medium-light, 441 ± 22.0 kg; and light, 399 ± 14.4 kg), and then assigned to 1 of 16 identical research pens (4 pens/block; 8 heifers/pen) in a randomized complete block design with the 4 research diets as treatments. Diets were offered in a 118-d feeding trial with heifers crowded to 133% of capacity at the feed bunk. Compared with CONTROL, inclusion of low-energy forages was effective in reducing DMI (11.06 vs. 10.04 kg/d; P < 0.01) and energy intake (7.39 vs. 5.95 kg TDN/d; P < 0.01). Concentrations of physically effective fiber (pef) particles did not change during the 24-h period following feeding for either the CONTROL (P ≥ 0.56) or EGG (P ≥ 0.75) diets; however, this response for pef particles masked the competing (and cancelling) responses for individual large and medium particles comprising pef, which heifers sorted with discrimination and preference, respectively. Sorting against pef particles was detected for STRAW as a linear (P < 0.01) function of time from feeding, and much more severely for the FODDER diet, which exhibited linear, quadratic and cubic effects (P < 0.01). Sorting of forage particles by heifers could not be related directly to heifer performance. Compared with CONTROL, ADG was reduced by dilution in all cases (1.16 vs. 0.91 kg/d; P < 0.01), but ADG for STRAW was approximately 0.2 kg/d less than EGG (0.98 kg/d) or FODDER (0.97 kg/d), despite exhibiting sorting characteristics intermediate between those diets.