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Abstract Three experiments evaluated delaying corn silage harvest, silage concentration, and source of supplemental protein on performance and nutrient digestibility in growing and finishing diets. Experiment 1 used 180 crossbred yearling steers (body weight [BW] = 428; SD = 39 kg) to evaluate corn silage dry matter (DM) (37% or 43%) and replacing corn with silage (15% or 45% of diet DM) in finishing diets containing 40% modified distillers grains with solubles. Experiment 2 used 60 crossbred steers (BW = 271; SD = 32 kg) to evaluate corn silage harvest DM (37% or 43%) and response to rumen undegradable protein (RUP) supplementation (0.5%, 1.4%, 2.4%, 3.3%, or 4.2% of diet DM) in silage growing diets. Experiment 3 used 9 crossbred lambs (BW = 30.1; SD = 4.1 kg) to evaluate nutrient digestibility of 37% or 43% DM corn silage in silage growing diets fed ad libitum or restricted to 1.5% of BW. In experiment 1, as corn silage concentration increased from 15% to 45%, average daily gain (ADG) and gain-to-feed ratio (G:F) decreased (P ≤ 0.04). Carcass-adjusted final BW and hot carcass weight (HCW) were lower (P ≤ 0.04) for steers fed 45% corn silage compared to 15% when fed for equal days. As DM of corn silage was increased from 37% to 43%, no differences (P ≥ 0.30) in dry matter intake (DMI), ADG, G:F, or HCW were observed. In experiment 2, as DM of corn silage increased from 37% to 43%, ADG and G:F decreased (P ≤ 0.04). Increasing supplemental RUP in the diet increased (P ≤ 0.05) ending BW, DMI, ADG, and G:F linearly as supplemental RUP increased from 0.5% to 4.2%. In experiment 3, there were no differences (P ≥ 0.56) in DM digestibility and organic matter digestibility between silage harvest DM and intake level. Neutral detergent fiber (NDF) intake was reduced (P < 0.01) for lambs fed the delayed harvest corn silage compared to earlier corn silage harvest. As silage harvest was delayed from 37% to 43% DM, NDF digestibility decreased (P < 0.01) from 64.39% to 53.41%. Although increasing corn silage concentration in place of corn in finishing diets reduced ADG and G:F, delayed silage harvest did not affect performance of finishing cattle. Delayed silage harvest in growing cattle resulted in lower ADG and G:F, possibly due to increased starch or maturity leading to decreased NDF digestibility. The addition of RUP to silage-based, growing diets improves performance by supplying more metabolizable protein and suggests RUP of corn silage is limiting.

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