78 The impact of intercropping corn with annual forages on forage chemical composition and bred heifer performance during late fall/early winter grazing in Western Canada.

Abstract Extended grazing is frequently used by Canadian beef producers to reduce winter feed costs. Corn is a high-energy forage with increased biomass yield per hectare compared to other annual crops. However, corn has low CP concentration which may not meet the nutritional requirements of growing cattle or those in mid-late gestation when grazed alone. The objectives of this study were to compare monoculture corn (COR) on 0.76 m row spacing with corn intercropped with Italian rye and hairy vetch (INT) on 1.52m row spacing by examining forage chemical composition, individual DMI, ADG, blood SUN and enteric methane (CH4) of bred heifers grazing in the late fall/early winter. For both treatments, corn was seeded at the Glenlea Research Station in Manitoba on June 1st while the intercrop was seeded June 28th at the V4 stage of corn growth. In a complete randomized design, each treatment had 2 replicate paddocks with 12, 18-mo old, bred heifers per paddock (n=48) with an average body weight of 419.0 kg. Heifers were turned out to pasture on October 11th for a 12-d adaptation to corn only, followed by an 8-d adaptation to the experimental treatments and then a 41-d experimental period. Corn was sampled on October 16th, and intercrop forage was sampled weekly over the experimental period to determine forage chemical composition. Animal BW (2 d) and blood SUN were determined on d 1, 15 and 41 of the experimental period. Individual DMI was determined using a titanium dioxide marker and fecal recovery method from d 7-13. Data was analyzed with R (version 4.4.1) using a linear mixed effect model (lmer4) with forage treatment as the fixed effect and paddock as a random effect. Corn CP concentration was lower for COR paddocks compared to INT paddocks (4.83% vs 6.75%; P< 0.001), but there was no difference in TDN between treatments (P = 0.09). The mean intercrop CP, nitrate and TDN concentrations were 22.13%, 0.60%, and 68.83% respectively. Daily DMI was 13.41 and 16.50 kg/animal for COR and INT treatments respectively (P = 0.2649). Blood SUN for COR heifers was lower than INT at d 15 and d 41 (P = 0.03, P < 0.001). Heifers grazing COR had 44.5% lower ADG than the INT treatment (0.66 vs 1.19 kg/day; P = 0.04). Results from this study suggest that compared with monoculture corn, corn intercropped with annual forages offered higher nutritive value and improved animal performance during the late fall/early winter grazing season, thus offering a novel alternative grazing option for bred heifers.