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Effects of straw processing and pen overstocking on the growth performance and sorting characteristics of diets offered to replacement Holstein dairy heifers.

Our objectives were to evaluate the effects of pen-stocking density and straw processing on the growth performance, feed-bunk sorting behaviors, and hygiene of Holstein dairy heifers. Two corn silage-alfalfa haylage diets diluted with wheat straw were offered; diet composition was identical, except that one diet contained short (well-processed) straw (SS; 46.0% neutral detergent fiber, 12.9% crude protein, 60.7% total digestible nutrients, TDN), and the other long (poorly processed) straw (LS; 46.5% neutral detergent fiber, 12.6% crude protein, 60.0% TDN; % of dry matter basis). A 2 × 3 factorial arrangement of straw-processing (SS or LS) and pen-stocking-density [100, 125, or 150% of capacity] treatments was evaluated with 240 Holstein dairy heifers (410 ± 56.3 kg) that were blocked by weight, and then assigned to 24 pens with 4 pens/interactive treatment. For 91 d, diets were dispersed at 1100 h daily, and bunks were sampled subsequently at 1300, 1600, 1900, 2200, 0100, and 0600 h during 3 evaluation periods throughout the trial. Diets were offered for ad libitum intake, but with minimal orts (<3%); as such, particle-size concentration factors were calculated as bunk concentration/initial concentration. For the LS diet, particle-size concentration factors for large (>19 mm) particles increased linearly from 1.26 to 2.82 across sampling times, differing from the SS diet at 2200, 0100, 0600, and 0900 h (orts). Similar factors calculated for the SS diet also increased linearly across sampling times, but these responses were less severe (1.27 to 1.97). Overall, particle-size concentration factors for physically effective fiber exhibited responses similar to those observed for large particles, except they were limited to narrower ranges for both the SS (1.04 to 1.14) and LS (1.03 to 1.26) diets. Despite these differences, daily dry matter intake was not affected by treatment (mean = 9.65 kg of dry matter/d), nor was daily intake of TDN (mean = 5.92 kg of TDN/d). For SS, heifers housed within overstocked pens exhibited reduced average daily gain (ADG) compared with the 100% stocking rate (0.93 vs. 0.99 kg/d). With LS processing, ADG differed between the 125 and 150% stocking rates (0.96 vs. 0.88 kg/d), as did the within-pen coefficient of variation for ADG (10.7 vs. 18.6%). Hygiene scores (1 = clean, 4 = caked-on manure) for legs (range = 2.1 to 2.3) and flanks (range = 1.6 to 1.9) indicated heifers stayed acceptably clean, but the within-pen coefficient of variation for legs (14.4 vs. 9.0%) and flanks (34.2 vs. 23.8%) was greater for overstocked pens compared with the 100% stocking density, thereby suggesting hygiene scores were more variable without a free stall for each heifer.