Cattle are more motivated for a high-concentrate diet than Sudan grass hay, despite low reticulorumen pH.

Sub-acute ruminal acidosis (SARA) is characterized by chronic low ruminal pH, occurs for feedlot cattle fed high-concentrate diets. Forages slow digestion and reduce acid production. We aimed to assess how motivated finishing cattle are to access forage (Sudan grass hay, SG) via their willingness to interact with an electrified barrier. Reticulorumen pH was measured to relate the results with digestive health. Twenty-eight animals fed a high-concentrate ration ad-libitum had access to 4 L of one of two treatments (n=14/treatment) fed 1x/d behind a barrier: 1) SG, or 2) an additional offering of the normal ration (TMR). To access a treatment, the steer voluntarily pushed his muzzle against an electrified barrier. The electrical current was increased exponentially every 24 h (0, 156, 312, 625, 1250, 2500, 5000 µA) until the animal ceased accessing it. Visits to the treatment were recorded continuously 24 h/d and reticulorumen pH was measured every 10 min. Rumen pH averaged 3.0±1.2 h/d for SG animals and 2.3±0.9 h/d for TMR animals, durations which meet the criterion for SARA. However, animals with access to SG were less likely to advance to the next current than TMR animals (P<0.01) and were approximately 3x less willing to interact with higher currents than TMR (mean maximum current touched: 469±169 and 1380±254 µA, respectively, mean±SE, P=0.01). Lower motivation to access SG was further demonstrated through fewer visits to the SG (2.4±0.4 vs 5.3±0.6 #/d, P<0.01), and less SG consumed than TMR (32.0±0.1 vs 74.0±0.0 %/d, P<0.01, measured as % due to weight differences of SG and TMR). Overall, finishing cattle valued the TMR more than SG, likely because of differences in the quantity offered, palatability, and familiarity. When rumen health was considered, SG animals visited more often (r=0.5, P=0.09) and showed fewer failed attempts (r=-0.5, P=0.06) to access forage as the severity and duration of pH depression below 5.6, for example, increased. No measures of treatment use were related to pH depression for TMR animals (P≥0.31). These findings provide evidence that cattle are motivated for Sudan grass hay when experiencing chronic low reticulorumen pH. However, they also contribute to the mixed evidence about motivation for forage in this life stage, because, overall TMR was valued more highly than SG. Despite widespread pH depression, TMR cattle contrafreeloaded for additional concentrate, demonstrating unexpectedly high motivation for this resource.