Brassicas

 

Understanding Deer Attraction to Brassicas: The Role of Sulfur

 

Brassicas are one of the most popular food plots due to their high protein content and ease of growth. However, many food plotters have noticed that it may take time for the deer to "learn" to eat brassicas. We've all heard stories of food plotters saying their deer wouldn't touch the brassicas until the second or third year after planting. This interpretation, however, doesn’t align with the reality of deer behavior. Deer have some of the most sensitive noses of all big game species and there's no evidence it takes them time to "learn" to eat a new food. The real issue comes down to the palatability of brassicas to deer, which is largely affected by the sulfur content of the plants as we will explain next.

 

Brassicas uptake a significant amount of sulfur from the soil, typically around 18-22 lbs per acre annually. This high level of sulfur uptake is due to the large amount of sulfur present in brassicas' leaves and roots. The greater the available sulfur content of the soil, the greater the sulfur concentration of the brassica plants. One study published in the Journal of Experimental Botany (Volume 61, Issue 10, June 2010, Pages 2635–2646) found that plants given water containing a 0.3% sulfate solution accumulated as much as 26 times more of that sulfur than plants given water with a 0.015% sulfate solution. For perspective, soil concentrations around the contiguous 48 US states range from 0.01% to as high as 2.48% of topsoil weight! However, 90% of the topsoil has a concentration between 0.01% and 0.6% sulfur.

 

Replace

High concentrations of sulfur are toxic for ruminants, including deer. Just like cows and other ruminants, deer have a complex system of four stomach chambers that they use to digest the large amounts of cellulose in their diets from grasses and woody shrubs. The second stomach chamber is called the reticulum where food is broken down by microbial fermentation. In the process of fermenting, foods containing a lot of sulfur produce a significant amount of a poisonous gas called hydrogen sulfide. This is the gas that gives sewage and rotten eggs their characteristic dreadful odor. In addition to sulfur compounds that are directly absorbed into the bloodstream during digestion, ruminants will actually end up breathing the poisonous hydrogen sulfide from their own stomach. Both of these routes of exposure cause a wide range of toxic neurological and hematological problems in ruminants. So what does this have to do with deer "learning" to eat brassicas?

 

Just as the microbial fermentation breaks down the brassicas in a deer's second stomach, the same process happens to brassicas left uneaten in the field. Similarly, as the brassica crop rots it gives off large amounts of hydrogen sulfide which is carried off into the atmosphere. Each subsequent planting of brassicas that goes uneaten and rots will remove more sulfur from the soil and carry it away as sulfur-bearing gases like hydrogen sulfide. That means every year the soil will have less sulfur and consequently the brassica crop will contain less sulfur each year unless it's supplemented. So it's not that deer are "learning" to eat brassicas after a couple of subsequent plantings in your food plot, but rather that the sulfur concentration in areas that started out high will come down to a more normal value and thereby increase the palatability of the brassicas for the deer.

 

Now the converse can also certainly be true that if soil sulfur is too low, it can also reduce the desirability of the food plot to deer, but that will be saved for another article.

 

So what should you do if you plant brassicas in your food plot and the deer aren't showing interest?

 

Get a soil sample or use one of our online tools (coming soon) to understand your local geochemistry. If your sulfur is above 0.04% (or 400 ppm) sulfur, you'll want to start a sulfur reduction plan. First off, you will want to avoid any sulfur-bearing fertilizers like ammonium sulfate (aka 21-0-0) or any fertilizers with a +S mark at the end of their NPK ratings (25-5-10+S for example). Secondly, you'll want to continue planting a brassica crop in your food plot, ideally one that produces an above-ground root bulb like turnips. You know there's a good chance the deer aren't going to eat the brassica crop so you should limit it to perhaps 1/4 of your total food plot and plant the rest with more desirable offerings for the deer. Radishes are another good option because the leaves tend to be attractive to deer even if they won't eat the radish root due to the high sulfur concentration. Whether it's radish or turnip, you'll want to let these rot entirely--do not till them under until they have completely deteriorated.

 

Continue subsequent brassica plantings as 1/4 of the total food plot (you can do this in monocultured sections or dispersed throughout your entire field as long as you monitor your seed rate carefully and only apply brassica seed at 1/4 the recommended planting rate per acre). You can monitor this process through annual soil samples or by simply waiting until the deer start to eat the brassicas.

 

In conclusion, there is no such thing as deer needing to "learn" to eat a new crop. If they're not eating it after the first planting it's because you have a soil chemistry issue that needs to be addressed. Keep an eye out for our online tools coming soon to help understand your local soil chemistry with or without a soil sample. Or reach out for a custom consultation.

 

Good luck and happy hunting.

this text with information about you and your business or add information that will be useful for your customers.