Manure Manager

Features Applications Swine
Changing animal diets can help control odor

March 6, 2008  by Wendy Powers

Odor is not new to dairy and
livestock farming. Cows, pigs, and even sheep and chickens have always
had special scents attached to them. But as farms get larger and
non-farm neighbors move into the countryside, the smell of productive
animals is no longer welcomed.

Odor is not new to dairy and livestock farming. Cows, pigs, and even sheep and chickens have always had special scents attached to them. But as farms get larger and non-farm neighbors move into the countryside, the smell of productive animals is no longer welcomed.

Odor complaints often occur when manure is spread on the land. But odor and gaseous emissions are a daily occurrence wherever animals are housed or manure and feed are stored. Ammonia, hydrogen sulfide and methane (odorless) are emitted from storage lagoons, barns and feed storage areas as well as land where manure is applied. Though all of those areas can be a problem, you get the most for your money by evaluating and controlling emissions in manure storage areas.

Efforts to control air emissions from manure can be divided into two categories: post-excretion strategies, which focus on reducing or changing emissions once manure has been excreted; and pre-excretion strategies, which focus on changing emissions prior to manure excretion.

Post-excretion strategies are largely an engineering issue: treat manure or design a cover to trap emissions. These strategies may also be chemical, such as using manure additives to change the chemical properties of manure and, hence, the resulting emissions.


Pre-excretion strategies are primarily dietary: change the composition of the diet (without changing animal performance) to change manure composition and, in turn, change air emissions. A pre-excretion approach can either reduce the nutrient precursors to emissions (source reduction) or change the form of the excretions.

Reducing the nutrient precursors means feeding nutrients as near to animal requirements as possible, thereby eliminating any extras. If an animal consumes only as much nitrogen as it needs, for example, it does not have any surplus to excrete in manure that can ultimately be released as ammonia. Similar diet formulations could focus on sulfur, which can be released into the air as hydrogen sulfide. This approach is applicable to all species, but how the diet is formulated is species-specific and, should be discussed with a nutritionist.

Using the source reduction approach, we have demonstrated ammonia emission reductions in excess of 40 percent with swine, broiler chickens and beef cattle. Feed additives that improve nutrient utilization, such as pre-biotics or pro-biotics, ionophores (Rumensin) or beta agonists (Ractopamine or Posilac) can be useful in reducing dietary nutrient content without compromising animal performance. Other management strategies – such as ventilation schemes to improve indoor air quality or lighting schedules to maximize feed intake – may allow producers to maintain growth and performance while reducing the nutrient content.

Nutrient form modification strategies change the form in which nutrients are excreted. For example, in a laying hen study, calcium sulfate (gypsum) and zeolite were included in the diet formulation to acidify the diet and the resulting excretion. Acidifying the excretion reduced ammonia emissions by 40 percent because nitrogen in the excreta was in a more acidified form and therefore less able to be released to the air. The trade off of feeding the gypsum was that hydrogen sulfide emissions increased significantly because the diets then contained more sulfur. Farmers should always be cautious when modifying diets to ensure there are no unintended consequences.

We do not know how diet strategies affect emissions once manure is moved into long-term storage. Theoretically, source reduction strategies should produce permanent results because they reduce the precursors to emissions, therefore, the maximum potential emission of a nutrient. On the other hand, form modification strategies may not produce a permanent result; ammonia that is not volatilized initially may be lost to the atmosphere during long-term storage because of adaptation of microbes or environmental changes.

The effect of specific feedstuffs needs also to be considered when implementing an air emissions mitigation plan. Some feeds contribute more to excesses of specific nutrients than others. For example, co-product feeds, such as distiller’s grains, concentrate nutrients relative to the primary source of feed. This could be problematic if formulations do not include accurate nutrient composition data. In some cases, the nutrients may be less available in co-product feeds. However, in other cases, the opposite occurs and nutrients are more available and/or in a form that better meets animal needs. The effect may also be species-specific. While accurate nutrient information is important for all feedstuffs, special care should be paid when co-product feeds are included in the formulation.

Most research has focused primarily on reducing ammonia emissions. Unfortunately, reducing ammonia may not reduce odor. Furthermore, the most commonly studied strategy has been to reduce nitrogen intake as a means of reducing nitrogen excretion and, ultimately, nitrogen emissions. To significantly reduce odor and gases in addition to ammonia, a combination of pre-excretion (source reduction plus form modification) strategies may produce more desirable results than one of the options alone. Limited, if any, work is currently in the scientific literature to document the effectiveness of combining both types of dietary strategies.

Diet modification strategies have demonstrated 40 percent or better reductions in ammonia emissions, and further benefits may be realized by adding on post-excretion strategies. The decision is complicated. Consider all options and weigh the results against the desired outcomes.

Wendy Powers, PhD, is director of environmental stewardship
for animal agriculture at Michigan State University.


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