124. Effect of Protein Supplementation Frequency on Forage Utilization by Cattle Consuming Cool-season or Warm-season Low-quality Forage

Abstract Cattle consuming low-quality forage (LQF; < 7% crude protein [CP]) require protein supplementation to improve forage utilization. However, daily supplementation is labor intensive, and it may be more practical for producers to adopt an infrequent supplementation strategy. When protein is supplemented, the type of basal forage (e.g., cool-season [C3] versus warm season [C4]) should be considered as the response to protein supplementation may depend on forage type. Previous research has quantified forage utilization response to less frequent protein supplementation, but the interaction between protein supplementation frequency and forage type has yet to be determined. 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 [125 mg N/kg BW], or every other day [250 mg N/kg BW]). There were no interactions between forage type and supplementation frequency on measures of intake or digestion (P ≥ 0.14). Forage type affected forage organic matter intake (FOMI; P ≤ 0.01); FOMI was 1.39 kg/d for C3 and 3.69 kg/d for C4. Total digestible OM intake (TDOMI) was also affected by forage type (P ≤ 0.01) and averaged 0.90 kg/d for C3 and 2.23 kg/d for C4. Organic matter digestibility (OMD) was not affected by forage type (P = 0.86), averaging 56.2%. Supplementation frequency did not affect FOMI (P = 0.50) or OMD (P = 0.24), but there was a tendency for an effect on TDOMI (P = 0.06) where CON steers consumed 1.15 kg TDOMI/d, 1.74 kg/d for daily supplementation, and 1.81 kg/d for every other day supplementation. These data indicate that forage type affected forage utilization with C3 being less effective at meeting nutritional requirements than C4. Protein supplementation improved TDOMI but frequency of supplementation did not affect this response. Future research should further evaluate how forage type affects response to supplementation and quantify the microbial underpinnings of these responses.