A Mathematical Model of Forage-Based Post-Weaning Growth of Beef Cattle

Four three-year experiments were conducted using 1. 2 ha pastures to evaluate the productivity of various forage species combinations and the performance of stocker steers grazing them. Forage systems used were (1) Common bermudagrass (Cynodon dactylon L. var. dactylon) + 112 or 224 kg N/ [CB] ; (2) Common bermudagrass overseeded with high endophyte (Acremonium coenophialum Morgan-Jones & Gams) tall fescue (Festuca arundinacea Schreb.), ladino clover (Trifolium repens L.) and Kobe lespedeza (Lespedeza striata (Thunb.) H & A) [CB + HF + LEG]; (3) HF + 67 kg N/ha [HF]; (4) HF + LEG; (5) Low endophyte tall fescue + 67 kg N/ha [LF] (6) LF + LEG; (7) Midland bermudagrass + varying rates of N [MB]; (8) MB + LEG; (9) MB + HF + 224 kg N/ha [MB + HF]; (10) MB + HF + LEG; (11) MB + LF + 224 kg N/ha [MB + LF]; (12) MB + LF + LEG; and (13) Orchardgrass (Dactylis glomerata L.) overseeded with ladino clover [OG + LC].

Systems analysis was used to determine relationships among climate, forage dry matter production, dry matter intake, and animal performance. This information was incorporated into a mathematical model which would provide estimates of productivity with varying inputs in southwest Tennessee. Data from Tennessee Feeder Calf Sales (1972-1988) were incorporated to estimate animal value. Forage production budgets and stocker cattle budgets were included to provide estimates of the economic viability of stockering cattle on various forage species combinations.

Six components comprising the model were user input area, climatic characteristics, forage yield, forage intake, cattle gain, and price and budget. User supplied inputs included expected yearly precipitation, yearly average temperature, forage system, soil type, animal sex, muscling score, stocking rate, and animal purchase price, weight, and month. Monthly output variables from the model were forage dry matter yield, dry matter intake, pasture carrying capacity, cattle weights, gains, average daily gains, beef production, cattle value per animal and per hectare, and net income per ha.

Output for each forage system simulated under normal climatic conditions was compared. Pastures containing HF were the least productive in terms of animal performance and estimated net income. Bermudagrass pastures receiving nitrogen fertilization produced the greatest amounts of forage and supported the most animals. However, animal performance was similar to that of HF forage systems. Steers grazing pastures with LF and OG gained more weight than steers grazing any other forage system. These gains resulted in a large economic advantage for these forage systems.