Restricting the time of access to fresh forage reduces intake and energy balance but does not affect the digestive utilization of nutrients in beef heifers

The effect of restricting the time of access to a high-quality fresh forage on the intake and digestion of nutrients was studied in heifers. Twenty-four Hereford heifers (body weight (BW) 153 ± 18 kg) housed in metabolic cages were randomized according to a BW-based block design and were randomly assigned to receive one of the four following treatments: 24, 8, 6 or 4 h of daily access to freshly cut pasture (Lolium multiflorum and Trifolium repens) (T24, T8, T6 and T4, respectively). Their feeding behavior (classified according to the categories of 'eating', 'ruminating' or 'other'), intake rate, intake and digestibility of nutrients, ruminal environment, microbial protein synthesis and blood markers of energy and protein metabolism were assessed. The intake for all of the feed fractions decreased at an increasing rate (from 6 h of access) as the time of access to forage decreased (linear: P < 0.001; quadratic: P < 0.035). During the first 4 h of feeding, animals linearly increased the eating time and the intake rate (P < 0.001 and P = 0.006, respectively), and decreased the ruminating time (P = 0.014) as the time of access to forage decreased. Neither forage digestibility, nor ruminal microbial protein synthesis efficiency, were affected by the restrictions. The metabolizable energy balance linearly decreased (P < 0.001) and N retention decreased at an increasing rate (from 8 h of access; linear: P < 0.003, quadratic P < 0.030), with decreasing time of access to forage. The ruminal pH and NH3-N concentration were affected by treatment and time (P < 0.001). Ruminal pH increased at an increasing rate (from 8 h of access) as the time of access to forage decreased. Meanwhile, NH3-N concentration linearly decreased as the time of access to forage decreased. There was a significant interaction between treatment and time for pH and NH3-N (P < 0.001), being both variables less stable during the day in restricted animals than in unrestricted ones. The concentrations of blood glucose, urea and insulin were not affected by the restrictions. In general, the effects of the restriction seem to have been less evident on digestion than on ingestion. Fasting beef heifers led to behavioral changes in order to maintain the dry matter intake, but these changes resulted insufficient when the time of access to forage drops below 6 h per day.