Overlap in Spring Growth from Warm- and Cool-season Grasses – Which Should I Graze Now?
Native grass pastures begin producing forage at a time during spring when our cool-season pastures are still productive. Certainly by mid-May, producers who have both warm- and cool-season pastures will be faced with the question of which they should be grazing. Simply put, there are two options. First, stay on the cool-season grass until it tells you it is time to move off. Depending on the spring (and your management), this could be up until late June. The other option is to move off the cool-season pasture when the warm-season grass is first ready to graze. Which makes the most sense?
Although either option is fine, and one or the other may ultimately fit your operation best, I think your best bet is to graze the warm-season grass once it tells you it is ready. Why? There are several reasons.
First, the quality of native warm-season grass at this point is much greater than that of cool-season grass. By mid-May when most of our cool-season grasses are past boot stage, forage quality begins to decline more rapidly. At this same point, warm-season species such as switchgrass, big bluestem, and indiangrass are producing gains above 2.5 pounds per day. Although this rate declines somewhat in June, the gap between cool- and warm-season grasses during late spring and early summer continues to increase. Thus, substantial gain is foregone by staying on the cool-season grass.
A second reason for moving off cool-season pastures at this point is that during May and June, toxins within tall fescue are increasing leading to reduced animal growth and reproductive performance. For spring herds, where April breeding is typical, having recently bred animals removed from toxic fescue is a good practice. A recent study at Clemson University found that switching bred animals to a non-toxic summer forage substantially increased pregnancy rates (as much as 20-30%). This same toxic forage, when harvested as hay, has much lower levels of ergot alkaloids.
A third reason to move off of cool-season forages earlier is stand vigor and longevity. Stressing cool-season grasses during summer when they are becoming semi-dormant has a greater negative impact than if that same stress occurred at a time when they were vigorously growing. Erring on the side of getting off somewhat early makes more sense than going the other way and getting off too late.
Finally, because of the rapid growth of tall-growing natives during May and June, their quality will decline as they mature. Obviously, they can be cut for hay if they do get too tall for effective grazing, but if your timing is not good, you could lose considerable potential nutritional benefit. I have seen a couple of studies in which the researchers did not use the warm-season grasses until much later in the growing season. In one case, the result was a loss of more than 50% of the per acre production! Recently a colleague observed that if you do not move to the warm-season grass once its ready, you will ruin both your cool-season and warm-season forages. For me, I think he had it right.