by Brian Campbell, NRCS-Maryland Grazing Specialist
“Pigweed” can refer to any weedy member of the genus Amaranthus, which includes the popular love-lies-bleeding flower. In Maryland agriculture, the most problematic of these summer annuals include redroot pigweed (A. retroflexus), smooth pigweed (A. hybridus), tall waterhemp (A. tuberculatus), and the notoriously herbicide-resistant Palmer amaranth (A. palmeri). By far, though, the most prevalent pigweed in Maryland pastures is spiny amaranth (A. spinosus). It is important to be aware that there are other pigweeds in Maryland and that species can cross to create hybrids. In 2011, an herbicide-resistant cross between spiny amaranth and Palmer amaranth was discovered in Mississippi. Identification of pigweeds can be challenging, even at maturity. Spiny amaranth’s most distinguishing characteristic is the painful spines located where its branches meet the stem.
Livestock usually eat pigweeds without any apparent harm and the foliage can be a high-quality forage – low in cellulose and high in crude protein. However, pigweeds can store relatively large amounts of nitrates, making them potentially dangerous to livestock via nitrate poisoning. Ruminants like cattle, goats, and sheep are most at risk, with hogs and horses less so. Many other forage plants and pasture weeds that can be excellent forages also pose nitrate-poisoning potential when grown in nitrogen-rich environments. These include cereal grains – especially corn, millet, oats, rye, and sorghum; close relatives johnsongrass and sorghum-sudangrass; and forbs like dock, lambsquarter, ragweed, smartweed, and sunflower.
Pigweeds tend to be tolerant of drought and a wide range of soil conditions. In my experience, spiny amaranth is most prevalent in heavy-use areas like dirt sorting pens, holding areas, sacrifice lots, and around watering troughs. Because it is one of the few plants that can thrive in these highly compacted soils, spiny amaranth gets to take advantage of the large levels of nutrients that livestock deposit at these sites. For these reasons, the places where spiny amaranth is likely to be prolific are also the nitrogen-rich places where it is most likely to cause nitrate poisoning.
Herbicide resistance in pigweeds is relatively high and will continue to grow, even as farmers increasingly rotate through different classes of herbicides to fight that resistance. Some states are experiencing pigweed that is resistant to 3 or 4 classes of herbicide: a scary situation for row crops that rely on herbicide applications! Managers of pastureland have much more flexibility than those growing row crops in that they can spot-mow clusters of pigweed as needed to prevent these annuals from producing seed. A targeted mowing campaign that cuts the plants near the ground before or during flowering should greatly reduce the incidence of pigweed over time. Pre-emergent and post-emergent herbicides with multiple modes-of-action (that are labelled for pigweed) are also important options.
The second half of any battle with weeds is to fill the void with plants that you actually want. One suggestion for the nutrient-rich, compacted soils dominated by spiny amaranth is sorghum-sudangrass. Without vegetation, heavy-use livestock areas will erode or become extremely compacted. Even if nothing else about the plant is appealing, spiny amaranth does a good job of repairing soils degraded by compaction and excess nutrients.
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