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Refining Recommendations for Grazing Whole Plant Corn



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Study objectives were to assess the impacts of grazing allocation and fiber supplementation on animal performance, grazing preference and behavior, ruminal fermentation and whole-system economic costs. Field studies were carried out during the winters of 2015-16 (yr 1) and 2016-17 (yr 2) evaluating replicated (n = 2) whole-plant corn grazing systems: (i) 3d allocation with supplemental fiber (3DF); (ii) 3d allocation without supplemental fiber (3DNF); (iii) 9d allocation with supplemental fiber (9DF); and (iv) 9d allocation without supplemental fiber (9DNF). Ninety-six cows (yr 1 BW = 674 kg ± 53.1, yr 2 BW= 638 kg ± 49.9), including 16 ruminally cannulated cows, were allocated to 1 of 4 replicated systems for 84 and 88 d in yr 1 and 2, respectively. The same 2100 CHU hybrid (P7332R) was seeded in yr 1 and 2 (yr 1, CP =8.6 %, TDN = 69.5%; yr 2, CP = 8.3%, TDN = 70.72%) and a supplemental fiber source was offered each year, at 15% of total DMI in 3DF and 9DF treatment which was an alfalfa-grass mix hay (yr 1 CP = 12.9%, TDN = 60.2%; yr 2, CP =15.5%, TDN = 58.42%). Cow BW, rib fat, BCS, and ADG were not affected by treatment (P = 0.85). Total DMI was greater (P = 0.01) in 9D forage allocation compared to 3D allocation (12.8 kg hd-1 d-1, respectively). The proportion of cob was greatest (34.2%; P = 0.01) before a grazing allocation and decreased (1.1%) at the end of allocation, whereas the stem proportion was lowest (48.6%; P = 0.01) before and greatest at the end (87.4%) of allocation. When supplemental fiber was offered cattle spent less (48.6 vs. 63.5%, respectively; P = 0.03) time grazing corn compared to cows not provided supplemental fiber. Grazing selection results suggest that cobs were heavily selected for at the start of a grazing allocation, which would provide a highly fermentable starch source at the start of the grazing period. Rumen pH parameters were not affected by fiber supplementation or allocation duration (P > 0.05). The duration that ruminal pH < 5.8 was greatest (143.18 min; P = 0.01) at the start of the grazing allocation compared to middle and end (26.32 and 0.0 min, respectively) of allocation. Total SCFA concentration was greatest (102.12, 99.13 and 86.77 mM, respectively; P = 0.01) at the start and middle of allocation, compared to the end of allocation. The molar proportions of propionate (P = 0.03) and isovalerate (P = 0.01) were lower when fiber was supplemented to cows. Fecal starch was greatest (P = 0.01) at the start of 9D and middle of 3D grazing allocations but was not affected (P = 0.58) by fiber supplementation. Grazing systems costs were not affected (P = 0.73) by grazing treatment. Results from the current study suggest that trt had no impact on rumen fermentation however grazing selection as timing within a forage allocation impacts diet fluctuation in all trt. Offering fiber supplementation did not reduce selection of cob or improve forage utilization. Maintaining the current recommendation of 3 d allocation lengths will reduce the diet fluctuation by the animal and potentially improve forage utilization and economic costs.



beef cow, extensive grazing, corn grazing, forage, allocation



Master of Science (M.Sc.)


Agricultural and Resource Economics


Animal Science


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