Differences in forage utilization between Bos taurus and Bos inducus steers fed low-quality forage and supplemented soybean meal

Abstract Text: Five Bos taurus (Angus) and 5 Bos indicus steers (Brahman) fitted with ruminal and duodenal cannulae were used in concurrent 5 × 5 Latin squares to determine effects of protein supplementation with varying levels of low quality forage access. Treatments consisted of a control (CON; no supplement and ad libitum access to hay; 2.8% CP, 83.0% NDF) and four treatments arranged as a 2 × 2 factorial: two levels of hay intake (ad libitum and restricted, 1% of initial BW) and two levels of protein (50 and 100 mg N/kg BW, provided as soybean meal 48.5% CP). Periods were 14 d long, with 7 d adaptation and 7 d of sample collection. Data were analyzed using the MIXED procedure of SAS. Terms in the model included treatment, breed, period and treatment × breed, with steer as a random effect. The repeated statement was used for fermentation responses. There were no significant breed differences or treatment × breed interactions for hay intake, digestion, or ruminal fermentation (P > 0.05). Supplementation linearly increased (P < 0.01) hay intake, total OM intake, and total digestible OM intake in steers given ad libitum access to hay. Feeding 50 or 100 mg N/kg increased total digestible OM intake 34 and 54%, respectively versus CON. Ruminal N balance decreased linearly (P < 0.01) in ad libitum fed steers from 36.6 g/d for CON to -30.1 g/d for 100 mg N/kg, suggesting a net influx of urea into the rumen for CON and net absorption of ammonia from the rumen for 100 mg N/kg. When supplement was provided at 50 mg N/kg steers with ad libitum access to hay had greater (P < 0.01) ruminal N balance (11.2 g/d) than restricted steers (-3.6 g/d); however, there was only a tendency (P = 0.09) for a difference between ad libitum and restricted steers supplemented 100 mg N/kg. Ruminal ammonia N increased linearly (P < 0.01) with increasing protein supplementation and was greater (P < 0.01) when hay intake was restricted for both levels of N supplement. Similarly, total VFA concentrations were linearly increased (P < 0.01) with increasing supplementation; however, VFA concentrations were lower (P = 0.03) for both levels of supplementation when hay intake was restricted rather than ad libitum. These data suggest that the forage utilization response to supplemental protein was similar among the subspecies of cattle.

Keywords: low quality forage, protein, supplementation