Perennial warm-season grasses can still look green in midsummer even as their nutritional value declines. Forage analysis from several Oklahoma forage systems shows that this change begins earlier than many producers expect.
Tallgrass prairie dominated by big and little bluestem, Indiangrass and switchgrass at the OSU Range Cow Research Center near Stillwater peaked in nutritive quality in May with 10.55% and 75% digestibility. After May, protein and digestibility declined steadily as forages mature with less than 6% crude protein and 65% digestibility in the late summer.
The same pattern has been observed in other warm-season pastures. In OSU data from midgrass prairie, crude protein declined from 9.8% in May to 7.1% in July, while total digestible nutrients declined from 64% to 61%. Old World bluestem declined even more sharply, from 12.9% crude protein and 60% TDN in May to 7.4% crude protein and 55% TDN in July.
One useful way to evaluate this change is the ratio of TDN to crude protein. Ratios between about 4:1 and 7:1 are generally considered balanced. When this ratio approaches 7:1 or above the crude protein is too low for optimal forage fiber breakdown in the rumen, limiting forage digestion and intake. In the early summer the TDN:CP ratio is 7:1, 6.5:1, and 4.6:1 for tallgrass prairie, midgrass prairie, and Old-World bluestem. But, July, the ratio had increased to 10.8: for tallgrass prairie, 8.6:1 for midgrass prairie and 7.4:1 for Old World bluestem, indicating that rumen-degradable protein was becoming limiting.
This determines the kind of supplement cattle need. When adequate standing forage remains but protein has declined, a pound of high-protein supplement increases microbial activity, fiber digestion, passage rate and forage intake; but only if adequate forage is available. This is the basis of the Oklahoma Gold supplementation program, where 1-pound of a high protein supplement (38 to 40% crude protein) increases late summer stocker gains by 0.4 to 0.5 pounds per day. This is a very efficient program requiring only 2 to 2.5 pounds of supplement for each pound of added gain. The Oklahoma SuperGold supplementation program is based on feeding 2.5 pounds per day of a moderate (25-30%) protein supplement to provide both protein and energy to the calf. This can improve daily gains by 0.6 to 0.7 pounds per day with supplement efficiency of around 3.5 pounds of supplement per pound of added gain.
If forage quantity is also limited, protein alone cannot replace missing forage. Higher levels of moderate protein-energy supplement will be needed to maintain performance and stocking rates. High feeding rates of supplements containing a large amount of grain can reduce forage intake and fiber digestion.
The first question should not be, “What supplement should I buy?” It should be, “What is limiting performance?” Forage testing provides the best answer, but pasture maturity, manure consistency, cattle fill and weight gain also provide clues. Start supplementation before gains fall sharply, match the supplement to forage quality and availability, and remember that the supplement that fits one forage—or one month—may be the wrong tool for another.