I hear people talking about the need to provide supplemental nutrients during drought because the brown, dormant forage we see on rangeland is low in quality. On the other hand, I have often heard producers say they weaned their heaviest calves in drought years. These two statements don’t make sense if one assumes that lower nutrient intake should lead to lower gains and therefore weaning weight. I would like to provide some evidence in this column to sort this conundrum out.
I can cite evidence from 2 research trials that were each conducted over 2 years that encompassed one year with relatively normal weather and one drought year. The first study is one that I conducted in Kansas in the 1980s (Figure 1). In that case, 1987 was the near-normal year, and 1988 was the drought year. These graphs display the crude protein content and digestibility of diets from steers that were fistulated so we could collect samples of what they selected. Note that crude protein generally declined as the season progressed in 1987 (dashed line). This is what would be expected as vegetation matures through the summer. The year of 1988 (solid line) was a year with a hot, dry spring and early summer, much like this year. As a result, we turned the steers out in early May that year. While crude protein levels were somewhat lower from early May to mid-July in 1988, they are not in the range that we would consider as low-quality forage (less than 7%). In fact, one of the major reasons that we supplement protein on winter range or crop residues (true low-quality forages) is to stimulate digestion of the fiber in the forage. Digestibility of diets was actually higher in the drought year (1988) than the normal year (1987). Our problem in Kansas in 1988 was a lack of forage growth because of drought, not forage quality.
Figure 1. Cattle Diets on Kansas Shortgrass Range
The other study was conducted by Joe Wallace and others on sandhills rangeland in northeast Colorado. In their case, the drought year was 1964 and the near-normal year was 1965 (Figure 2). Again, digestibility (called digestible dry matter in their graph) did not decline in 1964 (the drought year) and was higher throughout the drought year than the normal year. Digestibility generally declined throughout the year in 1965 as would be expected as forage matured under normal conditions.
Figure 2. Digestibility of Rangeland Forage During a Drought