As we move into the heat of summer, forage plants begin to reach maturity which means that the amount of lignin deposited in the plant cell wall increases and the amount of protein decreases. This is a continual process as the plant matures, but when the plant begins to put up seed heads is usually when forage digestibility begins to decline, although this can be forage species dependent. Lignin is not readily digested by rumen bacteria, and it also inhibits digestion of the rest of the plant cell wall. Additionally, the decrease in protein concentration becomes limiting for growth of rumen bacteria which decreases their ability to digest plant material. The increase in lignin and decrease in protein combine to reduce forage digestibility which in turn decreases the amount of forage the animal can consume. This becomes a double-edged sword in that the animal does not get as much nutrition from each bite and cannot eat as much.
It is impossible to remove the lignin once it is deposited in the plant cell wall. The only management strategy is to slow down the rate of plant maturation by frequent grazing. To accomplish this effect, cattle must be moved to a new paddock every day which is not practical for many ranches. Also, there is discrepancy as to whether frequent grazing/rest periods has the same effect with all forage species in all regions of the country.
Even though maintaining low lignin concentration of forages is not always practical, increasing the protein supply to rumen bacteria is beneficial and practical. Digestibility of forage can be improved with protein supplementation when forages mature in late summer. Previous research at Kansas State University indicates a 13% improvement in digestibility of native prairie hay with protein supplementation. Available protein sources that work well are soybean meal and cottonseed meal that provide large amounts rumen degradable protein meaning that this protein is available to the rumen bacteria. Other feedstuffs such as distiller’s grains are lower in rumen degradable protein and higher in energy and are better suited when both energy and protein need to supplemented.
Protein supplementation can be easily implemented in many ranch situations due to the ability of the ruminant animal to recycle nitrogen within the body. Because of this ability to recycle nitrogen back to the rumen, beef cows and stocker calves can be supplemented with a high protein feed every 3 to 6 days rather than daily with similar benefits in forage digestion. Be sure to monitor the maturity of forage plants in pastures over the next few weeks to determine the appropriate time to begin protein supplementation. Work with your veterinarian or county extension agent to determine the appropriate time and amount of protein supplement.