260. Intake, Digestion, and Rumen Fermentation Responses to Protein Supplementation Frequency and Forage Type in Beef Steers.

Abstract

Cattle consuming cool-season (C3) and warm-season (C4) low-quality forage (LQF; < 7% crude protein [CP]) require protein supplementation to improve forage utilization. However, daily protein supplementation is labor intensive, and it may be more practical for producers to adopt an infrequent supplementation strategy. The objective of this study was to assess utilization of wheat straw (C3; 5.3% CP, 81.2% neutral detergent fiber [NDF]) versus King Ranch Bluestem (C4; 4.3% CP, 76.3% NDF) by Angus steers receiving supplemental protein daily or every other day. Six ruminally cannulated steers (252 ± 23 BW) were used in a 6 × 6 Latin Square experiment with six 18-d periods. Each period included 8 d for treatment adaptation, 8 d to measure intake and digestion, and 2 d for assessment of rumen fermentation. Treatments were arranged as a 2 × 3 factorial where one factor was forage type (C3 versus C4) and the other was frequency of supplementation (none, daily [DLY, 125 mg N/kg BW], or every other day [EOD, 250 mg N/kg BW]). There was no forage × frequency interaction for any measures of intake or digestion (P ≥ 0.14) or effect of frequency (DLY versus EOD; P ≥ 0.68). Forage type (C3 versus C4) affected forage OM intake (FOMI; P ≤ 0.01), while there was no effect of protein (CON versus mean of DLY + EOD; P = 0.25). FOMI averaged 1.39 kg/d for C3 and 3.68 kg/d for C4. Total digestible OM intake (TDOMI) was affected by forage (P ≤ 0.01) and protein (P = 0.02); for C3, TDOMI averaged 0.71 kg/d (CON) and 1.00 kg/d (DLY + EOD) and, for C4, TDOMI averaged 1.59 kg/d (CON) and 2.55 kg/d (DLY + EOD). Organic matter digestibility (OMD) was not affected by forage (P = 0.86) or protein (P = 0.11), averaging 56.2%. For ruminal fermentation, there was no effect of frequency (P ≥ 0.72). Ruminal ammonia was affected by forage × frequency (P ≤ 0.01), forage (P ≤ 0.01), and protein (P ≤ 0.01). Ammonia concentrations for C3 were 3.47 mM (CON) and 6.82 mM (DLY + EOD) and, for C4, were 0.31 mM (CON) and 1.54 mM (DLY + EOD). Total VFA were not affected by forage × frequency (P = 0.75) or protein (P = 0.20) but were affected by hour (P ≤ 0.01) and forage (P ≤ 0.01). Total VFA averaged 90.5 mM for C3 and 113.1 mM for C4. These data indicate FOMI was more than double for C4 versus C3 and was accompanied by lower ammonia concentrations and greater total VFA concentrations supportive of increased ruminal microbial activity when C4 hay was fed. We observed lower than expected intake for C3 and these findings may not be generalizable to other C3 forages. Future research should replicate this study with other C3 forages and investigate the microbial underpinnings of findings.