Does pasture diversity matter?

Recent trials show it boosts production and aids health

by Allen Williams, Ph.D.

The vast majority of established pastures in the U.S. are dominated by what I would term a “near monoculture”, meaning that most of the forage yield, or biomass production, is obtained through two to three primary forages in the mix.

Natural prairies are a different story, as we see literally scores of plant species in mixes consisting of grasses, legumes and forbs. I have been on “species counts” in native prairie where experts identified more than 150 different plants, sometimes more than 200.

Does it matter whether our pastures are comprised of a near monoculture, or is managing for greater complexity and diversity important? Let’s look at what some recent research tells us.

Five species vs. two
A recent Pennsylvania State University study conducted at the Hawbecker Research Farm from 2005 to 2013 examined two- and five-species perennial mixes in terms of their impact on forage biomass production and soil organic carbon (SOC) analysis (Skinner and Dell, 2016).

In August 2004 the two-species pastures were sown into orchardgrass and white clover, while the five-species pastures were sown into orchardgrass, tall fescue, white clover, alfalfa and chicory. There were a total of four replications per treatment.

Grazing started in the spring of 2005 and continued through 2013. Paddocks in each treatment were grazed the same. The number and class of animals varied between years and seasons, but stocking rates were consistent between treatments each grazing event.

Grazing started when the forage in each treatment reached 10 inches (25 cm) and stopped when the cattle had grazed down to 4 inches (10 cm). This resulted in five grazing events per treatment every year except 2006, when there were six. Grazing typically started in early to mid-May and ended in late October.

Forage biomass production was measured prior to each grazing by taking clippings, drying them down and comparing that amount of dry matter with what was measured again post-grazing.

Results showed that the five-species mix produced 31% more total forage biomass annually than the two-species mix, with the differences more profound in wet years. In other words, 31% more forage dry matter was available for livestock simply by having a little more diversity in the mix.

The five-species mix produced 34% greater forage biomass in the spring, 30% more in the summer, and 26% more biomass in the fall compared to the two-species mix.

The five-species mix generally grew faster and recovered quicker between grazings, and was thus an average of 11.2 inches in height when a grazing event started, compared to 10 inches in the two-species paddocks. Since both treatments were grazed to the same residual height, the five-species paddocks had an additional 1.4 inches of forage growth removed with each grazing.

Soil organic carbon (SOC) was measured in alternate years from 2004 through 2012, with samples taken at six different soil depths (strata) starting at 2 inches (5 cm) down to 39 inches (100 cm). SOC accumulation down to 39 inches averaged 1.8 tons of carbon per hectare annually (0.72 ton/acre) with the five-species mix, compared to 0.5 ton of carbon per hectare annually (0.20 ton/acre) with the two-species mix.

The most significant increases in SOC accumulation with the five-species mix occurred within the 4-8 and 8-12-inch strata.

The differences in production and SOC remained strong through the nine-year period even though the five-species mix lost virtually all its chicory by 2008 and 95% of the alfalfa by 2012. So the residual effects of a five-species mix that became essentially a three-species mix were quite important.

An 18-species mix
In a trial conducted by Green Acres Research Farm near Cincinnati, Ohio, annual cover crop cocktails called “biological primers” were planted in the spring of 2015. An 18-species, warm season cocktail mix developed by Dave Brandt at Walnut Creek Seeds was planted with a no-till drill on June 4, 2015.

Just 55 days after planting, scientists at Green Acres measured 58,000 lbs. of wet forage biomass per acre. That translates into more than 8,000 lbs./acre of forage dry matter.

Stocker calves were grazed using adaptive high stock density (AHSD) practices with daily moves on one-quarter to one-third acre paddocks. After the initial grazing, each paddock was rested approximately 30 days prior to re-grazing.

Calculated forage dry matter (DM) for the second grazing was 4,500 lbs./acre. At the end of the warm season grazing period, the crop was terminated by intentionally overgrazing, and a 12-species cool season cocktail was no-tilled in for winter grazing.

Cattle performance and soil response were impressive. Cattle weighed before and after grazing the warm season cocktail gained an incredible 3.8 lbs./head/day. This was without the benefit of compensatory gain, as the cattle had just come off cool season perennial pasture.

Soil organic matter (SOM) increased 0.8%, from 3.6% to 4.4%. The USDA-NRCS states that for every 1.0% increase in SOM, water holding capacity increases 25,000 gallons per acre.

At the 0.8% gain, the SOM increase in this trial added 20,000 gallons/acre of water holding capacity. Over a 100-acre farm, that would be an additional 2 million gallons of water being absorbed into the soil with a moderate to heavy rainfall instead of running off or ponding and pooling.

Increases were also measured in soil nitrogen, soil mineral value, earthworm populations and soil microbial respiration (as measured by CO2 release). Soil N increased 58 lbs./acre  because N fixation from the atmosphere increased due to enhanced rhizobia activity from the legumes included in the 18-seed warm season mix.

Soil mineral value increased $105/acre due to the complex mix, adaptive grazing and enhanced microbial population. This is $105 less in spending on the external inputs required for similar performance. The earthworm population exploded to an estimated 130,000 per acre.

Soil microbial activity was measured indirectly using the Solvita method of measuring CO2 respiration. Measurements showed a 44% increase in soil microbial activity, with CO2 respiration rising from 128 to 186.

Self-medication with plants
Diverse and complex forage mixes also produce significantly more — and more diverse — secondary and tertiary nutrient compounds. We know much about primary compounds such as crude protein, calcium, potassium and magnesium, but there are also hundreds of nutrient compounds that are very important to the health of our animals, plants and soil. Different plant species produce different arrays of these compounds, and each has a specific role.

Dr. Fred Provenza has spent his academic career researching these compounds and their effects on the animal-plant-soil complex. His research shows that animals will purposely select for these compounds in their diet if given the opportunity.

Our problem is that we have often created situations where livestock cannot select from a broad array of secondary and tertiary nutrient compounds because our pastures do not have any complexity and diversity.

Dr. Provenza has shown that the medicinal benefits of these compounds are important in animal health and performance. For instance, some plants produce tannins — natural anti-parasites that can significantly reduce internal parasite loads in ruminant livestock (Lisonbee, et. al. 2009).

Livestock that consume plants containing tannins have lower fecal egg counts and nematode loads, and better animal gains (Niezen, et. al. 2002; Coop and Kyriazakis, 2001; Min and Hart, 2003; Min,, 2004). Athanasiadon and co-workers (2000) found that tannins have a direct anthelmintic effect. Tannins also increase the supply of bypass protein in the gut (Reed, 1995; Foley,, 1999) and enhance immune response to internal parasites (Niezen,, 2002; Min and Hart, 2003).

Other studies show that foraging behavior is influenced by the availability of secondary and tertiary nutrient compounds in the diets of animals (Provenza and Villalba, 2006; Villalba and Provenza, 2007). Animals can self-medicate if given the opportunity (Huffman, 2003; Villalba,, 2006).

The very short answer
So does plant species diversity and complexity in our pastures matter? The very short answer is “Yes”.

We have to remember that in nature there are no singular effects. Everything we do in our management produces compounding and cascading effects whether we realize it or not.

Selecting or planting for monocultures and near-monocultures produces compounding and cascading effects that reduce soil health, plant growth and animal performance. Selecting for complexity and diversity do just the opposite.

Dr. Allen Williams is president of Livestock Management Consultants, LLC, based in Starkville, Mississippi.

Literature cited
Skinner, H.R., Dell, C.J. 2016.  Yield and Soil Carbon Sequestration in Grazed Pastures Sown with Two or Five Forage Species.  Crop Science, Vol. 56. July-Aug. 2016.

Lisonbee, L.D., Villalba, JJ, Provenza, F.D., Hall, J.O. 2009.  Tannins and self-medication: Implications for sustainable parasite control in herbivores.  Behavioural Processes (2009) 184-189.

Athanasiadou, S., Kyriazakis, I., Jackson, F., Coop, R.I. 2000.  Effects of short-term exposure to condensed tannins on adult Trichostrongylus colubriformis.  Veterrinary Record, 146, 728-732.

Coop, R.L., Kyriazakis, I. 2001.  Influence of host nutrition on the development and consequences of nematode parasitism in ruminants.  Trends in Parasitology, 17, 325-330.

Min, B.R., Hart, S.P. 2003.  Tannins for suppression of internal parasites.  J. Anim. Sci. 81: E102-E109.

Min, B.R., Pomroy, W.E., Hart, S.P., Sahlu, T. 2004.  The effect of short-term comsumption of a forage containing condensed tannins on gastro-intestinal nematode parasite infections in grazing wether goats.  Small Ruminants Res. 51, 279-283.

Reed, J.D. 1995.  Nutritional toxicology of tannins and related polyphenols in forage legumes.  J. Anim. Sci. 73, 1516-1528.

Foley, W.J., Iason, G.R., McArthur, C. 1999.  Role of plant secondary metabolites in the nutritional ecology of mammalian herbivores.  Proceedings: Fifth International Symposium on Nutritional Herbology.  American Society of Animal Science, IL, pp.130-209.

Niezen, J.H., Charleston, W.A.G., Robertson, H.A., Shelton, D., Waghorn, G.C., Green, R. 2002.  The effects of feeding sulla or Lucerne on lamb parasite burdens and development of immunity to gastro-intestinal nematodes.  Vet. Parasitology 105, 229-245.

Provenza, F.D., Villalba, J.J.  2006.  Foraging in domestic vertebrates: Linking the internal and external milieu.  In: Bels, V.L. (Ed.), Feeding in Domestic Vertebrates: From Structure to Function.  CABI Publications, Oxfordshire, UK, pp.210-240.

Huffman, M.A. 2003.  Animal self-medication and ethno-medicine: Exploration and exploitation of the medicinal properties of plants.  Proceedings of the Nutritional Society 62, 371-381.

Villalba, J.J. Provenza, F.D., Shaw, R. 2006.  Sheep self-medication when challenged with illness-inducing foods.  Animal Behavior. 71, 1131-1139.

Villalba, J.J., Provenza, F.D. 2007.  Self-medication and homeostatic behavior in herbivores:  Learning about the benefits of nature’s pharmacy.  Animal 1, 1360-1370.