How Low Can Rangeland Forage Quality Go?

As I look out the window, the snow is flying sideways in one of those spring snowstorms that are the root for the many jokes about Wyoming not really having a spring. Although we know that warmer days are around the corner, we are still dealing with below freezing temperatures, debilitating winter weather and mostly dormant forage on rangelands. I have also been fighting these conditions to collect forage samples to understand how low rangeland forage quality can go in the winter. I say “fighting” because, trust me, it is hard to keep a paper sack from blowing away as you are trying clip grass samples in March in Wyoming!

In this study, I am looking at three very common grasses on Wyoming rangelands. The first grass species is blue grama (Bouteloua gracilis), a native warm-season grass that forms small bunches and mats. Blue grama is less productive than many other grasses due to its small size, but it provides good grazing, is palatable, drought tolerant and can increase under heavy grazing.

The second grass species is crested wheatgrass (Agropyron cristatum), a non-native, cool-season bunch grass. Crested wheatgrass was introduced to the United States from Asia. This exotic species is capable of establishing very easily and producing a lot of forage. Typically, crested wheatgrass starts growing before other native cool-season grasses and is good for early spring grazing but digestibility may decline rapidly through the growing season.

The third grass species is western wheatgrass (Pascopyrum smithii). Western wheatgrass is a native grass that is rhizomatous and can produce a moderate amount of high quality forage for grazing.

In January, I collected four independent samples for each species on the University of Wyoming’s Agricultural Experiment Station pastures west of Laramie and sent them to the laboratory to determine crude protein, true digestibility at 48-hours in vitro and total digestible nutrients (TDN). I have put all of this data in a bar graph to the right so you can see the differences between the forage species.

Average crude protein was the highest for western wheatgrass at 6.5 percent. Blue grama and crested wheatgrass both had lower crude protein values than western wheatgrass at 4.8 percent and 4.2 percent, respectively. Average digestibility was the highest for blue grama at 66 percent, intermediate for western wheatgrass at 53 percent and the lowest for crested wheatgrass at 47 percent. TDN, or energy, was the highest for western wheatgrass and blue grama at 50 percent and 49 percent, respectively, and was lower for crested wheatgrass at 46 percent.

So what does this mean for ranchers? First and foremost, in January, all of the rangeland forage species had crude protein values lower than the needs of all classes of cattle. Remember, general rules of thumb are dry cows need about seven percent crude protein, growing heifers need about 10 percent crude protein, and lactating cows need about 12 percent crude protein.

It also illustrates that some species, such as western wheatgrass, retain crude protein better than other species.

Secondly, just because a forage species is low in crude protein does not mean it is low in digestibility. Consider the high level of digestibility for blue grama in this study, at 66 percent.

Third, TDN requirements for all classes of cattle typically exceed 50 percent, so all three forage species were below that level.

So, depending on how much dormant forage is available, protein supplementation to increase dry matter intake and increase total TDN in the diet is something that has to be considered. This is all especially relevant depending on your breeding and calving program and the stressful periods when cows have a particularly high nutritional demand such as late gestation and early lactation.

Thus, you have to consider your reproductive management and calving or lambing periods relative to forage quality and animal demand. Finally, it is important to realize that not all rangeland forage species offer the same level of quality, digestibility or energy. Maintaining a mixture of forage species can optimize different plant nutritional traits through the year.

So the short answer to the question, “How low can rangeland forage quality go?” is, lower than animal needs for all three forage species we tested in January.

For more information, contact me at jscasta@uwyo.edu and check out my blog “Rangelands4u” at wyoextension.org/rangelands4u.