Does Initial Body Size Influence Weight Gain of Stocker Steers During the Growing Season in Semiarid Western Great Plains Rangeland?

Ranchers in the western Great Plains grazing yearlings (i.e., stockers) during the growing season need to understand how variation in starting animal weights influences subsequent end weights for marketing opportunities and prices received. Whether variation in stocker steer body size (i.e., entry weight) at the start of the growing season influences weight gains during the summer grazing (midMay through September), period on semiarid shortgrass prairie rangeland remains unclear. We used 10 yr (2014–2023) of weight gains from 2 162 stocker steers (Bos taurus) that had entry weights ranging from 222.7 to 370.0 kg to assess if grazing season weight gains under traditional, season-long grazing management with moderate stocking rate were influenced by entry weights under varying levels of spring (April through June) precipitation and pasture forage productivity (low vs. high productivity soils). Entry weight had no effect on grazing season weight gains regardless of precipitation level and soil type (as measured by ecological site). Stocker operations in this rangeland can anticipate steer weight gains of 135 kg steer⁻¹ under average precipitation and forage productivity. Summer grazing season weight gains did exhibit a quadratic relationship between spring precipitation and pasture productivity. A 25% reduction in spring precipitation decreased weight gains by 16% and 23% in the high and low productivity pastures, respectively; 50% reduction in spring precipitation lowered weight gains by 40% and 55%, respectively. Conversely, increasing spring precipitation by 50% increased weight gains by only 15% and 20% in the high and low productivity pastures, respectively. These findings highlight that abundant precipitation can result in substantial forage production, but forage quality reductions will limit additional weight gain. The plateau in steer weight gain when April to June precipitation exceeds 170 mm suggests that forage quality limitations could potentially be ameliorated by strategic protein supplementation.