Experiment 1: In Situ Disappearance

Two ruminally cannulated Simmental × Angus cows (562 ± 3 kg BW) were used to estimate rates of ruminal digestion of each botanical part of corn residue and corn steep liquor. Corn husks, leaves, and stalks were harvested from November 10, 2017 to January 16, 2018 from a 2.7 ha field located 2.5 km north of Brookings, SD (44°20′51.10′′N, 96°47′21.67′′W). The field contained a uniform mixture of 4 cultivars of corn (DKC 46-20, DKC 45–65, DKC 58-06, and DKC 54-38, Bayer Corporation, Creve Coeur, MO) planted at a rate of 12,950 seeds per ha. Standing corn plants were harvested 3 times weekly with a sickle mower (Ford 501, Ford Motor Company, Troy, MI) after DM concentration in grain had reached 84%. Whole corn plants were immediately collected after harvest and separated by hand into husks, leaves, and stalks. Measures of in situ disappearance were conducted on an aliquot of composited samples of husks, leaves, stalks, or steep liquor after samples were partially dried (55 °C) and ground to pass a 2-mm screen in a Wiley Mill (Thomas Wiley Mill Model 4; Thomas Scientific USA, Swedesboro, NJ). Four grams of ground sample (DM basis) were placed in a polyester bag (Dacron, 10 × 20 cm, 50 ± 10 μm pore size; R1020, Ankom Technology, Macedon, NY). Within each cow, triplicate sets of polyester bags containing husks, leaves, stalks, or corn steep liquor were placed in large mesh bags (38.1 × 45.7 × 2.5 cm, Mainstays Lingerie Mesh Laundry Bag, Wal-Mart Stores, INC, Bentonville, AR) that were weighted and suspended in the rumen for either 4, 8, 12, 24, 36, 48, 72, or 96 h. Cows were fed ad libitum amounts of long-stem corn residue (84.2% DM, 3.6% CP, 72.3% NDF, 47.2% ADF) for 14 d prior to measures of in situ disappearance. During in situ incubations, cows were allowed ad libitum access to long-stem corn residue and provided corn steep liquor (1 kg) twice daily at 0700 h and 1900 h. After ruminal incubation, bags were mechanically rinsed 5 times in a commercial washer (Fabric-Matic, Model A511S, Maytag, Newton, IA); each rinse consisted of a 1-min rinse followed by a 2-min spin cycle (Vanzant et al., 1998). Estimates of 0-h disappearance were achieved by rinsing polyester bags identically to bags previously ruminally incubated. After mechanical rinsing, all samples were dried at 55 °C for 24 h, and analyzed for DM, OM, and NDF.

Nearly all corn steep liquor was removed from bags used to estimate 0 h disappearance (89.8 ± 3.2% DM) and no corn steep liquor remained in polyester bags after 4 h of incubation. Thus, corn steep liquor was considered to rapidly disappear in the rumen and estimates of rates of disappearance from a slowly degradable fraction of corn steep liquor were not calculated because it was unlikely that time points used in this trial would accurately characterize rates of disappearance from the relatively small amount of corn steep liquor that did not immediately disappear. Rates of ruminal DM, OM, and NDF disappearance from husks, leaves, and stalks were calculated following the model described by Ørskov and McDonald (1979):

where ID is the proportion of a chemical component that immediately disappeared, SD is the proportion of a chemical component that slowly disappeared, Kd is the rate of disappearance (h⁻¹), and t is time (h). The equation was fitted using the Marquardt method (Marquardt, 1963) for iterative, nonlinear, least squares estimation in SAS (SAS 9.4, SAS Inst. Inc., Cary, NC). Pool sizes (ID, SD, and the proportion of a chemical component that did not disappear) and Kd were analyzed as a completely randomized design using the MIXED procedures of SAS. The model contained effects of botanical part (i.e., husks, leaves, and stalks) and effect of cow was random. Treatment means were calculated using the LSMEANS option. Differences between botanical parts were detected with the F-statistic (Fisher, 1935). When the F-statistic was significant (P ≤ 0.05) means were separated with the PDIFF option of SAS.