Monday, August 18, 2014

More forensics on burning

There still is debate smoldering over the timing of burning on grasslands here in the Flint Hills.

Much of the debate stems from research conducted here in Kansas over 50 years ago.

A little more forensics is illuminating.

The key evidence to suggest that burning should be done in late spring is from the weight gain of cattle in an experiment that burned pastures at different times: early-, mid-, and late-spring with an unburned contrast, too. In the experiment, each month, the cattle would be taken off pasture and weighed to examine monthly weight gain. The experiment was carried out from 1950-1966.

In the first reporting of the results from the experiment (Anderson et al. 1970), the authors showed no significant difference in monthly weight gain for cattle placed on pastures with early- and late-spring burns.

Their conclusion was a lot more certain than their data.

“Mid- and late-spring burning produced more weight gain on steers than nonburning. Late-spring burning also increased steer gains over early-spring burning. The weight gain obtained with early-spring burning was essentially equal to that obtained with nonburning.”

So, although there were never any significant differences in weight gain, the conclusions were that there were.

It is possible that when you add up all the monthly gains for a year, you get significant increases in weight gain. As far as I can tell, that was never tested.

When the data were summarized in a later publication, the monthly weight gains are compiled into a 5-month weight gain. But there are no error bars. No tests of significance.

Often, these results have been reported as stating that burning in late spring leads to 32 pounds greater weight gain over a 5 month period (May – September). I’m not sure where that number comes from because the graph only shows 26 pounds difference. Yet, most of the cattle in the Flint Hills are only left out on pasture for 3 months. May, June, and July. If you go back to the 1970 paper, over the 3-month May-July period, differences in weight gain were measured at just 9 pounds.

9 pounds is still 9 pounds, though. Yet, is it? There is no evidence to show that the 9 pounds, no less 26 pounds, is actually significant. That means 9 pounds might be 0 pounds.

There are other parts of the research that are curious. For example, if you look at the productivity data, burning just 42 d earlier results in a 25% reduction in grass productivity. In contrast, 20 years of data at Konza shows no significant reduction in grass productivity. What would cause such a marked reduction in productivity? In the past, it was suspected that soils dried out a lot faster without any cover, but there is actually little evidence to support this.

Could it have been differences in forage quality? Again, no data were ever taken on forage quality for pastures burned at different times.

Also, the average date of burn for the late-season burn was May 1. The stocking date for all the animals? May 1. How that actually happened, I have no idea.

So what might be going on here?

One limitation of the work is that there was no spatial replication for the experiment. Each pasture had a different treatment. There was only one pasture for each treatment. Treatments were not rotated among pastures. Replication came from measuring the same pasture year after year.

In scientific terms, that means site differences and treatment differences were confounded. In lay terms, we have no way to know whether any differences in weight gain were because of the pasture that happened to be picked for the early-season burn happened to have worse forage than the late-season pastures.

This is exactly why scientists replicate.

Still, replication at this scale is certainly hard. We only have so much land to work with. You do the best you can.

Yet, is it impossible to do? No. Can it still be done? Yes.

Scientists could easily work with ranchers to use their operations as experiments. Different ranchers could burn at different times and they could record their animals weight gains. That’s a whole lot better than extrapolating from a few hundred acres at one spot to a much broader area.

In all, was the past work suggestive that late-spring burning was optimal. Certainly. But there are too many questions about the certainty of the results and their broader relevance.

At this point, the most conservative, commutative interpretation of all the data is that there are no significant effects of early-season burning on weight gain.

Interpreted within the context of the experimental design and more recent data, it is hard to be convinced of the necessity to burn in late April in the Flint Hills.

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