Abstract #871

# 871
Dry matter intake patterns of feedlot cattle.
R. B. Hicks1, R. P. Lake2, F. N. Owens*3, 1Oklahoma State University, Goodwell, OK, 2Hitch Consulting Services, Guymon, OK, 3DuPont Pioneer, Johnston, IA.

Most intake equations for feedlot cattle strive to predict mean intake for the total feeding period. Yet, DMI of feedlot cattle typically plateaus after 2 to 4 wk on feed and declines steadily thereafter. Consequently, dietary requirements change over time. To quantify intake patterns, weekly DMI data were compiled from 2,329 pens (minimum of 50 steers per pen) fed high concentrate diets an average of 158 d in one southern Great Plains feedlot. Intake of DM differed with initial weight (P < 0.01), so pens were grouped by initial weight (216 to 443 kg) into 10 sets. Adjusted for death loss and sick pen days, DMI averaged 9.56 kg/d. Within weight groups, DMI later (wk 8 to 17) was correlated (R2 = 0.77) with DMI earlier (wk 4 through 7). Based on DMI and steer weights, ME realized by each pen of steers was calculated (mean ME = 3.45 ± 0.18 Mcal/kg). Based on initial weight and ME intake, mean weight of steers within each pen each week was calculated. Following 3 wk for diet adaptation, Intake of DM as a percentage of mean weight (DMIpc) was lower for pens of steers with greater initial weight and decreased linearly over time (DMIpc = 2.954 – 0.04776 × week on feed – 0.000859 × initial weight, kg); calculated DMI was related to observed DMI (R2 = 0.79; RMSE = 0.61 kg). Pens of cattle with greater initial weights had greater DMI and ADG but lower DMIpc and feed-to-gain ratios. Adjusted for initial weight, and DMIpc, expressed as a fraction of current body weight, was greater (P < 0.05) during October and November than in January through May or during a summer intake slump (July and August). The dietary CP percentage required, assuming a 64% efficiency of retention of CP intake, added to inevitable N losses was greater for lighter cattle, peaked early, and declined steadily (CP = 17.1 – 0.1407 × week – 0.0142 × initial weight; R2 = 0.80). In conclusion, later DMI can be predicted from DMI early in a feeding period. Because DMI and performance vary with initial weight and time on feed, data on interim pen weights, DMI, and ADG should permit nutrient requirements to be predicted more precisely than DMI means for the total feeding period.

Key Words: intake, feedlot, protein requirement