We determined if sequence of ingestion affected use of endophyte-infected tall fescue (TF) when cattle also grazed birdsfoot trefoil (BFT) or alfalfa (ALF). Based on chemical characteristics of TF (alkaloids), BFT (tannins), and ALF (saponins), we hypothesized that cattle first allowed to graze ALF or BFT would subsequently spend more time grazing TF than cattle that first grazed TF followed by ALF or BFT. Sixteen bred heifers (478 ± 39 kg initial BW) were randomly assigned to 4 replicated pasture units. Each replicated unit consisted of 4 treatment sequences (TF → BFT, TF→ALF, BFT → TF, or ALF→TF), with 2 cows per sequence. Pastures were in the vegetative stage of growth at a height of 20 to 30 cm and provided ad libitum forage to cattle. We recorded foraging on TF, BFT, and ALF using scan sampling of individuals at 2-min intervals. The study was conducted in 4 phases run sequentially, for a total of 30 d. In phases 1 and 3, cattle in group 1 grazed TF pastures for 45 min and were then moved to BFT pastures for the next 45 min (TF→BFT); cattle in group 2 grazed in the reverse sequence (BFT → TF). In phases 2 and 4, cattle in group 1 grazed TF pastures for 45 min and then subsequently grazed ALF pastures for the remaining 45 min (TF→ALF); cattle in group 2 grazed in the reverse sequence (ALF→TF). Sequence of plant ingestion affected food selection. In phase 1, scans revealed grazing of TF by heifers was cyclic, and heifers tended to have more scans (P = 0.52) grazing TF when they grazed BFT → TF; scans for heifers grazing TF were consistently greater (P < 0.05) throughout phase 3 of the trial. In phase 2, heifers that grazed in the sequence ALF→TF spent considerably more scans (P = 0.03) foraging on TF from d 4 to 10 than heifers that grazed in the sequence TF→ALF, and they remained greater throughout phase 4 of the trial. Although the sequence ALF→TF appeared to be more effective than BFT → TF, consistent with the hypothesis of a complementary relationship between the steroidal alkaloids in TF and saponins in ALF, tannin concentrations in BFT were minimal (1.8%), which likely reduced the presumed inactivation of alkaloids by tannins. We also speculate that heifers needed to learn about the positive postingestive influence of sequence, a notion consistent with more similar scans spent foraging BFT and TF early in phases 1 (BFT → TF) and 2 (ALF→TF), and with the consistent and marked increase in scans spent foraging on TF for animals foraging in phases 3 (BFT → TF) and 4 (ALF→TF).

BACKGROUND: A mixture of plant species adds biochemical diversity to pastures that may enhance productivity while decreasing reliance on herbicides and insecticides. All plants contain secondary metabolites (PSMs) that interact in plant communities in a variety of ways. Our objective was to determine if PSM diversity enhanced food intake when sheep were allowed to select from foods that varied in concentrations of tannins, saponins, and alkaloids. We hypothesized that intake of foods containing alkaloids would increase when sheep were offered supplemental foods with tannins and saponins. The alkaloid gramine is proteinaceous in nature, tannins bind to proteins in the gut, which enhances excretion of proteins, so we hypothesized tannins would bind to gramine and enhance its excretion from the body. The alkaloid ergotamine is steroidal, saponins have a high affinity for binding to lipid and steroidal compounds in the gut and then being excreted, so we hypothesized saponins would bind to ergotamine and enhance its excretion from the body.

We determined whether plant diversity and sequence of plant ingestion affected foraging when cattle chose from plants that varied in concentrations of alkaloids, tannins and saponins. We hypothesized cattle that ate high-alkaloid grasses (endophyteinfected tall fescue (TF) or reed canarygrass (RCG)) would prefer forages high in tannins (birdsfoot trefoil, BFT1) or saponins (alfalfa, ALF1), because tannins and saponins can bind to alkaloids, presumably reducing their absorption. We further hypothesized that forages with tannins or saponins consumed before, rather than after, foraging on high-alkaloid grasses would promote greater use of those grasses presumably by binding to alkaloids, thereby reducing their absorption. In Phase 1, cattle (n 5 32) grazed on either high (1) or low (2) alkaloid grass (TF or RCG) pastures for 30 min each morning at 0600 h and were then offered a choice of BFT1, BFT2, ALF1 and ALF2 for 60 min each day for 12 days. In Phase 2, cattle (n 5 32) were first offered a choice of BFT1 or ALF1 for 30 min at 0600 h and then placed on grass (TF1 or 2, or RCG1 or 2) pastures for 60 min for 12 days. In both phases, we had four spatial replications of four treatments with 2 per calves assigned to each of the 16 replications per treatment combinations. Scan samples of individuals at 2-min intervals were used to determine incidence of foraging on each plant species (%). Cattle grazed more on RCG than on TF in Phases 1 (62% v. 27%; P 5 0.0015) and 2 (71% v. 32%; P 5 0.0005). In Phase 1, cattle that first foraged on RCG1 or TF2 subsequently preferred ALF over BFT, whereas cattle offered RCG2 or TF1 foraged on ALF and BFT equally. Foraging by cattle on RCG was cyclic during Phase 1, whereas cattle foraging on TF markedly decreased incidence of use of TF from 41% to only 16% by the end of the 12-day trial (P 5 0.0029). Contrary to the cyclic (RCG) or steadily declining (TF) use of grasses in Phase 1, cattle steadily and dramatically increased foraging on both RCG and TF throughout Phase 2, when they first grazed BFT1 or ALF1 followed by high-alkaloid grasses (P 5 0.0159). Our findings suggest that in plant species the sequence of ingestion influenced foraging behavior of cattle and that secondary compounds influenced those responses.

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