Thursday, June 11, 2009

Climate, the nutritional value of grass, and the Wisconsin paradox

Map of grass protein concentrations as derived from cattle fecal chemistry. Red implies higher protein concentrations, blue lower. Craine et al. in review, Global Change Biology

The perennial native grasslands of North America are often dichotomized as being either tallgrass or shortgrass. At its most basic, the humid tallgrass produce a large quantity of low quality grass, while the xeric shortgrass produce a small quantity of high quality grass. For grazers, the tallgrass is sour and the shortgrass sweet.

The generalizations about tallgrass and shortgrass are certainly true, but raise the question about the Wisconsin paradox. If wetter grasslands are lower quality to grazers then why are the best grasslands for cattle in Wisconsin and not in Montana? 

To help understand the pattern of grass protein concentrations, we analyzed a dataset from Texas A&M's Grazinglands Animal Nutrition Lab. Over 15 years, they had accumulated a large dataset on grass protein concentrations across the US as derived from cattle fecal chemistry. When we analyzed the data, we found that in contrast to expectations, wetter grasslands had higher protein concentrations than drier grasslands. Here, tallgrass was sweet and shortgrass was sour.

The paradox can be ascribed to management of grasslands. In native grasslands, tallgrass is sour. Yet, for managed grasslands, tallgrass is sweet. What do they do in places like Wisconsin to turn sour sweet? One hypothesis is that it is the grazing itself. Managers make sure that the pastures are intensively grazed so that quality never declines. Another is that managers make sure that the grasslands don't burn. A third is by controlling species composition, managers can favor palatable species. Planting legumes and cool-season European grasses might be enough to turn the sour sweet.

The different patterns in native and managed grasslands raise some important questions about how well we understand grasslands. At its most basic level, we still aren't sure what drives the fundamental characteristics of tallgrass and shortgrass. 


  1. Did you look at C3/C4 ratios and how they interact with the climate?

  2. Absolutely. Is this a map of C3/C4 ratio? No surprise, unless I'm missing something wildly obvious.

  3. Nice thought. No specific data on C3/C4 ratio with these data, but in native grasslands C4 fraction only increases with temperature, not precipitation. It can't explain why protein increases with increases precipitation. Also, the idea that C4's have lower protein than C3's does not always bear out. NAD C4's actually tend to have high N concentrations and many C3 grasses have quite low N concentrations.