Cattle diet quality during drought

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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

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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

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Why does this happen? In a normal year, forage turns brown in the fall after it has completed it entire process of growing and maturing. That process of maturing involves development from vegetative to flowering and finally the setting of seed. Nutrients like protein in the forage naturally decline throughout this process, while fiber and lignin naturally increase. Lignin is a part of the cell wall that provides strength for the plant, but it is indigestible and interferes with digestion of nutritious parts of the plant. On the other hand, when plants run out of water in a drought year, they stop growing without going through the normal process of maturing. Thus, they don’t decline in nutritional value at the normal rate. In the case of drought, we can’t assume that brown color signifies low quality.

The point is: forage quality is a function of how mature the plants are. That means that the timing of the drought onset can influence whether or not forage is high or low in quality in a drought year. Notice in the Kansas data in Figure 1 that there was a large increase in crude protein content and digestibility in August, which coincides with the drought breaking and some nice late-summer rains. Steer gains dramatically increased and average daily gain was actually higher in August and September of that year rather than in early summer as would be more normal.

On the other hand, if the drought had not started until late summer or fall so that plants had a chance to mature before they were impacted by drought, we should expect that forage quality would be low. Last year in most of the Northern Great Plains would be a good example of that; it was wet in spring and summer so we had a lot of mature forage by September, and then the drought started. As a result, we had a lot of low-quality forage last fall.

How do you know which situation you are in on your place? First, think about when the drought started. Look at the forage and see if it is fully headed out or if it never matured. Watch body condition on your cows. If they start slipping, is it because of a lack of forage or poor quality in the forage? If there is adequate grass in the pasture, then the problem may be low quality. One tool that could help with this is something through NRCS called NUTBAL. For this program, a fresh fecal sample is analyzed using NIRS to predict the crude protein content and digestibility of the diet that was consumed. These predicted values will provide a reasonable estimate of the adequacy of protein and energy in the diet.

In general, the problem in drought is a shortage of the amount of forage, not the quality. Often, when we consider supplying feeds to grazing cattle during drought, we seldom need supplemental nutrients. Instead, we need to consider supplying substitute feeds to extend the limited pasture. Other alternatives are to reduce cattle numbers to match the limited forage supply or some combination of reducing cattle and providing substitute feeds. All of these options create economic hardships, but providing unneeded supplemental nutrients would be the most damaging.

Source: Ken Olson


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