ONA REPORT

published in

THE FLORIDA CATTLEMAN AND LIVESTOCK JOURNAL


May 2013

Know the Sulfur Content of your Forage Test It


For questions or comments regarding this publication contact: John Arthington

Late spring and early summer is a good time to assess the sulfur content of your pasture forage.  Sulfur is an essential nutrient for beef cattle, and when concentrations are elevated, an antagonist of copper and selenium absorption.  It is the latter that is most concerning for grazing beef cows in Florida because both copper and selenium are essential nutritional elements.  Both elements are cofactors in numerous enzyme systems, which are known to be essential for optimal growth, reproduction, and immunity. Deficiencies may occur through the prolonged consumption of forages low in copper or selenium, and/or the consumption of forages containing elevated concentrations of sulfur. 

In addition to being a mineral antagonist, sulfur is also an essential nutrient for cattle as a component of certain amino acids, vitamins, and other compounds.  Sulfur deficiency in cattle is linked to decreased ruminal function, which may lead to a decline in forage digestibility and ultimately a decrease in animal growth.  Fortunately, sulfur deficiency is seldom a problem in well-managed grazing cowherds.  In contrast, diets high in sulfur are typically more of a concern. High dietary sulfur can lead to a reduction in copper and selenium absorption.  The maximum tolerable concentration for sulfur is estimated to be 0.40% of the total diet (dry matter).  Although over time, cattle may become tolerant to higher levels of dietary sulfur, concentrations exceeding 0.30% of the total diet dry matter are sufficient for the antagonism of copper and selenium to occur. Therefore, a reasonable upper limit for sulfur intake would be approximately 32 g daily for mature grazing beef cows (Table 1). 

Dietary sulfur can be derived from a variety of sources, with the major contributors coming from forage, supplement, and water. Because forage intake constitutes the greatest amount of total daily dry matter intake, forage sulfur concentrations tend to be the leading provider of dietary sulfur. Our experiences suggest that a common range for the sulfur content of pasture forage is 0.15 to 0.25%.  When concentrations exceed 0.25%, an exogenous sulfur-containing source is typically responsible. These may be derived from sulfur-containing nitrogen fertilizers (i.e. ammonium sulfate) and to a lesser extent, animal manure.  Depending on the region of focus, atmospheric deposition of sulfur may be a concern.  This sulfur source is derived from rainfall containing weak sulfuric acid from coal-fired energy plant emissions (i.e. acid rain) and can be an important source of sulfur contributing to the overall forage content. Regarding forage sulfur accumulation as a result of fertilization, our research has found that repeated annual applications of ammonium sulfate, as a source of nitrogen, can result in plant sulfur concentrations as high as 0.50%. Cows grazing these pastures were found to have a lesser copper status (i.e. reduced liver copper concentrations) at the end of the summer grazing season compared to cows grazing unfertilized pastures or pastures fertilized with ammonium nitrate.

Supplemental feeds provided to grazing cattle can be another significant source of dietary sulfur.  This has become more pronounced with the increased prevalence of dried distillers grains, a high-sulfur feed ingredient, in cowherd supplement formulations. Several ingredients, commonly found in supplement formulations, contain high levels of sulfur (Table 2).  Although their contribution to total dietary sulfur intake may be significant, these feeds are typically only provided during the winter supplementation period.  Fortunately, both copper and selenium can be stored in body tissues and called upon during instances of deficiency.  Although sulfur excesses may reduce copper and selenium stores during periods of winter supplementation, well-managed cattle will likely have adequate tissue reserves to handle short periods of trace mineral loss.  Be cautious, however, to limit high-sulfur intake to as short a time as possible to ensure cattle have an opportunity to replenish these tissue losses.

In Florida, the use of molasses-based supplemental feeds is common. Previously, we reported on the absorption efficiency of supplemental copper provided in corn- versus molasses-based supplements (Journal of Animal Science. 2002. 80:2787).  Our results indicate that the sulfur concentration, present in cane molasses, may interfere with normal copper absorption in cattle.  In these studies, heifers receiving supplemental copper through corn experienced a 46% increase in liver copper concentration compared to a 9% decrease in heifers receiving the same amount of copper through a molasses supplement.  In a following experiment a third treatment was included that provided copper in a corn-based supplement, but also fortified with sulfur at a level equal to the amount obtained by the molasses supplement.  Increases in liver copper concentrations were different for each treatment (155, 87, and 13 ppm for corn, corn+sulfur, and molasses supplements, respectively.  Other studies have shown similar responses with selenium in growing cattle consuming high-sulfur supplements.  Attempts to overcome this sulfur-induced antagonism by replacing inorganic copper and selenium sources with organic alternatives (i.e. selenium yeast) have been ineffective (Journal of Animal Science. 2001. 81:1357 and Journal of Animal Science. 2008. 86:1472).

Beef cow/calf producers should not avoid sulfur-containing fertilizers or feed ingredient sources.  These resources are typically important contributors to economically relevant management decision.  Instead, producers should attempt to estimate total sulfur intake of their cowherd.  This can be achieved by having both harvested and grazed forage analyzed for total sulfur.  In addition, the sulfur content of supplemental feeds can be analyzed or estimated using tabular values.  From these analytical resources, a calculation of total daily sulfur intake can be estimated.  For mature grazing beef cows, total sulfur intake should not exceed 32 g daily.  If this threshold is surpassed, cows will begin to experience a net reduction in copper and selenium status.  Prolonged exposure to these high-sulfur conditions will likely result in deficiency of these two essential nutrients resulting in reduced cowherd productivity.  Understand your forage’s sulfur contribution by having it tested this year.  Many commercial forage and feed testing laboratories offer this service.  For questions regarding sulfur nutrition in grazing cattle, please contact Dr. Arthington at jarth@ufl.edu.  Portions of this article have been published previously in Feedstuffs.

Table 1.  Amount of sulfur derived from pasture forage containing a range of sulfur concentrations (dry matter basis)a


Forage sulfur, %

Intake, lb/d

Sulfur Intake, g/d

0.15

20

14

0.20

20

18

0.25

20

23

0.30

20

27

0.35

20

32

aAssumes a 1000 lb cow consuming 2.0% of body weight in dry matter forage daily.  Daily sulfur intake should not exceed 32 g/d for mature beef cow.

Table 2.  Amount of sulfur derived from feedstuffs commonly supplemented to cowherds in Floridaa

Itemb

Intake, lb/d

Sulfur, % as-fed

Sulfur provided, g/d

Molasses (Heavy Mill Run)

5

0.70

16

Molasses, 32%

3.5

0.70

11

Brewer’s grains

5

0.36

8

Distillers grains

5

0.40

9

Corn Gluten Feed

5

0.23

5

Soy hulls

5

0.10

2

Citrus pulp

5

0.07

1.5

a Daily intake values are selected estimates for commonly used supplementation rates.  If your supplement rate differs, then actual sulfur intakes will also differ.
bLiquid molasses values provided by United States Sugar Corporation, Clewiston, FL for molasses derived from sugarcane processing (06/18/07).  Dry feed values derived from NRC, 1996.