Matching plant and animal processes to alter nutrient supply in strip-grazed cattle: timing of herbage and fasting allocation.

This work aimed to assess the impact of timing of herbage allocation and fasting on patterns of ingestive behavior, herbage intake, ruminal fermentation, nutrient flow to the duodenum, and site and extent of digestion. Treatments were daily herbage allocation in the afternoon (1500 h, AHA), morning (0800 h, MHA), AHA after 20 h of fasting (AHAF), and MHA after 20 h of fasting (MHAF). Four ruminally and duodenally fistulated heifers (279 +/- 99 kg of BW) individually strip-grazed wheat pastures in a Latin-square design. Eating, rumination, and idling behavior were recorded every 2 min, and bite and eating step rates were measured hourly while the heifers were grazing (11 h MHA and AHA; 4 h MHAF and AHAF). Ruminal DM pools were measured 4 times daily (0800, 1200, 1500, and 1900 h) to estimate daily herbage DMI and its pattern. Ruminal fluid was sampled at these same times and also at 2300 h. Duodenal digesta was sampled over 2 d to determine the site of herbage digestibility. Treatments did not affect daily herbage DMI (16.5 g/ kg of BW, SE = 0.0025; P > 0.05). However, they altered the eating pattern; the evening grazing bout of AHA and AHAF was greater (P < 0.05) and more intense (P < 0.05 for bite mass and rate, eating step, and intake rates). Ruminal nonglucogenic:glucogenic VFA ratio and pH were lower (P < 0.05) for AHA and AHAF during the evening. The flow of OM, N, microbial protein, and nonmicrobial OM to the duodenum did not vary (P > 0.05) among MHA, MHAF, and AHAF; however, it averaged 970, 40, 300, and 540 g/d, respectively, greater (P < 0.05) for AHA. Total tract digestibility did not differ (P > 0.05) for MHA, AHA, and AHAF, but was lower for MHAF (P < 0.05). Apparent ruminal digestion did not differ (P > 0.05) within fasted and nonfasted treatments; however, it was greater (P < 0.05) for fasted than nonfasted treatments. True OM ruminally digested did not differ (P > 0.05) among MHA, MHAF, and AHAF, but was greater (P < 0.05) for AHA. The results demonstrate the strong link between ingestion and digestion patterns, and its impact on nutrient supply. At the same amount of resource allocation, nutrient supply to grazing cattle can be modified through strategic grazing management.