Results of recent University of Minnesota research trials suggest dietary sulfur level is a significant contributor to odor and hydrogen sulfide levels in confinement nursery facilities.

The researchers were attempting to quantify the amount of sulfur consumed, retained and excreted from pigs weaned at 18 days of age when fed either typical or low-sulfur Phase I, Phase II and Phase III diets. They also wanted to find out the amount of elemental sulfur, sulfate and sulfide consumed in feed and excreted in feces and urine when feeding typical and low-sulfur Phase I, II and III diet sequences, and the effect on energy and nitrogen retention during the three phases.

The researchers also investigated the effect low-sulfur diets would have on pig performance, says Jerry Shurson, University of Minnesota swine nutritionist. Hydrogen sulfide, odor and other gas levels were measured in confinement nursery rooms in which low-sulfur diets were fed. Odor was measured with an olfactometer and gases were measured with a Jerome meter.

High- and low-sulfur Phase I, II, and III diets were formulated at the beginning of the study. The researchers calculated the conventional diet should contain 0.45% sulfur. They figured the modified, low-sulfur diet should contain 0.24% sulfur.

"By adjusting inclusion rates of some ingredients that contribute major amounts of sulfur, we theoretically should achieve a 47% reduction in sulfur content of the diet while satisfying all other required nutrient levels," Shurson says. "However, due to the wide disparity between National Research Council (NRC) values, and our analyzed sulfur values for dried whey and the source of fish meal, the actual reduction of total sulfur in our low-sulfur Phase I diet was only 13%. This points out the importance of analyzing each ingredient for sulfur content before formulating low-sulfur diets."(See sidebar story on page 18 for more information.)

The primary sulfur contributors in the experimental high-sulfur Phase I diet were: spray dried plasma (20.5%), fish meal (15.3%), spray dried whey (13.7%), soybean meal (13%), corn (9.8%), oat groats (6.2%), skim milk (6.2%). Shurson says these ingredients collectively accounted for nearly 85% of total sulfur in the experimental diet formulation. Dicalcium phosphate, DL methionine, trace mineral premix and copper sulfate contributed the remaining 15% of sulfur to the diet.

The researchers made small adjustments to help reduce total diet sulfur content. The reduction was achieved by replacing dried whey with lactose, replacing dicalcium phosphate with tricalcium phosphate, replacing copper sulfate with zinc oxide, and by using a no-sulfate trace mineral premix in the formulation while keeping all other nutrient levels except sulfur constant.

The researchers reduced total sulfur content of the low-sulfur Phase II diet by 39%. This reduction was achieved by minimizing the use of high-sulfur ingredients, and replacing them with lower-sulfur alternatives.

The Phase III low-sulfur diet was only reduced by 19% compared to the high-sulfur, Phase III diet due to use of fewer ingredients and less flexibility for diet manipulation.

First Experiment The first experiment was conducted using a three-phase nursery diet sequence. The feeding sequence included either typical high-sulfur diets or modified, low-sulfur diets. The diets provided equal concentrations of nutrients except sulfur. Shurson expected the modified, low-sulfur diet should have reduced total sulfur consumption by 30%, based on the initial calculations using the NRC sulfur values for feed ingredients established in 1988.

The experiment was conducted using 20 PIC barrows (10 pigs/treatment). The pigs were weaned at 18 days of age. Pigs were placed in individual stainless steel collection cages and were fed either the high-sulfur or low- sulfur Phase I diet for seven days. Pigs were fed an amount of feed equivalent to 2% of their initial body weight, and they were fed twice daily. Total fecal and urine excretion was collected and analyzed for three days between days 5-7.

Pigs receiving the high-sulfur Phase I diet were weighed on the eighth day of the experiment and were switched to the high-sulfur Phase II diet for a 14-day feeding period. The same procedure was followed with pigs receiving the low-sulfur Phase I diet. Pigs were again fed an amount of experimental diet equivalent to 2% of their body weight for one week, beginning on day 8. Urine and feces were collected from days 12 to 14.

The same procedure was used for each of the five weekly collection periods for pigs fed both experimental diets in all phases.

Feed samples of each diet, as well as fecal and urine samples from each collection period, were analyzed for nitrogen, sulfur, sulfate and sulfide content.

Shurson says it is difficult to accurately quantify various sulfur compounds in feces and urine. However, the researchers did find the amount of sulfate excreted in urine was dramatically higher than in feces, and was higher for pigs fed the high-sulfur diet compared to pigs receiving low sulfur in Phase II, Phase III and for the average of the five-week experiment.

Shurson says the measuring techniques for measuring chemical forms of sulfur need improvement before much more in-depth research can be done in this area. Current available measurement techniques are not sophisticated enough to measure the forms of sulfur excreted by animals.

The prominent form of sulfur being excreted by the pigs was sulfate. Pigs eating the low-sulfur diet had low sulfate excretions.

Shurson says the researchers' goal was to formulate low-sulfur diets that would help reduce sulfur excretion without reducing energy and nitrogen digestibility, retention and pig performance. "There were no differences in digestible energy and nitrogen retention between pigs fed either high- or low-sulfur diets while still reducing sulfur and sulfate excretion," he explains.

Second Experiment The University of Minnesota researchers conducted a second experiment to determine if feeding low-sulfur diets would reduce hydrogen sulfide gas and odor from manure. The same experimental diet sequence and formulations used in experiment 1 were used. A total of 128 pigs with an average initial body weight of 15.33 lb. were used. The pigs averaged 20 days of age and were housed in raised deck nursery pens and were randomly assigned to one of two experimental diets according to the room in which they were housed.

Pigs were weighed, and feed consumption was determined weekly. Researchers calculated average daily gain, average daily feed intake and gain/feed for a five-week feeding period. Odor, hydrogen sulfide and ammonia measurements were recorded weekly to determine the effect of diet on odors and gases.

Growth performance was not different between pigs fed either the high- or low-sulfur diet sequences.

Odor levels were low, but tended to increase linearly during the five-week feeding period (Figure1). Odor levels tended to be higher in rooms where pigs were fed high-sulfur diets during weeks 3-5. Shurson says this is likely due to increased microbial fermentation of manure due to increased manure volume.

Hydrogen sulfide levels tended to be similar between dietary treatments during weeks 1 and 2, but tended to be higher for rooms where pigs were fed the high-sulfur diet sequence during weeks 3-5 (Figure 2). Ammonia level tended to increase linearly each week, but was not affected by feeding a low sulfur diet (Figures 3 and 4).

Shurson emphasizes this was the first research into the area of manipulating manure hydrogen sulfide levels through low-sulfur diets. The researchers plan to conduct additional experiments to reduce sulfur levels in grow-finish diets.

Researchers: Jerry Shurson, Mark Whitney, Department of Animal Science, and Richard Nicolai, Department of Biosystems and Agricultural Engineering, University of Minnesota. Phone Shurson at: (612) 624-2764.