Effect of 3- Vs. 9-d Whole-Plant Corn Allocation with or without Fiber Supplementation on Cow Performance, Grazing Preference, and Ruminal Fermentation.

Grazing whole-plant corn is becoming increasingly common on western Canadian beef farms as a winter feeding method. This 2-yr study evaluated the impact of provision of 3- or 9-d (3D or 9D, respectively) whole-plant corn allocation (Zea mays, P7332R; yr 1, TDN = 69.5%, CP = 8.6% [DM basis]) with or without fiber (FIB or NFIB, respectively) supplementation on cow performance, grazing preference, and ruminal fermentation. Supplemental fiber (hay, TDN=60.2%, CP=12.9% [DM basis]) was provided at 15% of DMI in 3DFIB and 9DFIB systems. Ninety-six Angus cows (BW=664.4 kg ± 20.05), including 16 ruminally cannulated cows, were allocated to 1 of 4 replicated (n=2/yr) systems for 84 and 88 d in yr 1 and 2, respectively. Cow BCS and BW were recorded at the start and end of the study and grazing preference and ruminal fermentation data were collected at the start, middle, and end of the grazing allocation. Grazing preference was determined based on plant part (stem, husk, leaf, and cob) disappearance. Data were analyzed as a completely randomized design with a 2 × 2 factorial arrangement using the mixed model of SAS. Year was included as a random effect and time within an allocation as a repeated measure. Cow BCS and ADG were not affected (P>0.05). As grazing progressed within an allocation, the proportion of cob, husk, and leaf decreased (P<0.01); while, the stem increased (P<0.01). For 3D cows, ruminal NH₃-N concentration did not differ within an allocation; whereas, NH₃-N increased for 9D cows (allocation × timing, P<0.05). Total short chain fatty acid concentration was greater (95.2 vs.85.6 mM, P<0.05) and acetate was greater (62.2 vs. 55.1 mM, P<0.05) for FIB than NFIB. Butyrate concentration decreased from 13.8 to 6.5 mM from the start until the end of the 9D grazing allocation, but concentrations did not differ for 3D (allocation × timing, P<0.05). Minimum pH increased during the grazing allocation, but the magnitude of the increase was greater for 9D than 3D (allocation × timing, P<0.05). The duration that ruminal pH was < 5.8 was not affected by allocation or provision of FIB, but decreased (P<0.01) from 143 min/d at the start of grazing allocation to 0 min/d at the end. The provision of fiber alters ruminal fermentation and extending the duration of the grazing allocation increases the selection of plant structures and differences in ruminal pH within a grazing allocation.