Feeding weedy forages

 Craig Shaeffer, Professor, Department of Agronomy and Plant Genetics, Krishona Martinson, Extension equine specialist, and Jared Goplen, Extension educator-crops

Weeds generally compete with crops for water, light, and nutrient resources, and may affect human and animal health. While the goal of harvesting forages usually includes minimal weed infestations, weeds can provide valuable forage during a drought.

Key Points

• Weeds can provide nutrients and contribute to the needs of some livestock.
• Proper weed ID is essential to ensure weeds are not toxic to livestock. Weeds may also have antiquality traits, including plant chemicals, thorns, or plant hairs that affect palatability.
• Nitrates can accumulate in drought stressed weeds. Forages suspected of being high in nitrates should be tested before feeding.
• Before feeding forages containing weeds, test to determine nutritive value to allow adjustment of the ration.
• Weedy forage should be introduced slowly to livestock rations to allow for livestock to adjust to the change in diet.

Unique traits of weeds

Weeds present in our forage cropping systems are survivalists. Some unique traits of these weeds include:

• Adaption to weather extremes. For example, pigweeds have a photosynthetic pathway similar to corn and can thrive under high temperature and moisture stress.
• The ability to survive defoliation and regrow from buds near the soil surface.
• Both annual and perennial weeds have the potential for some nutritive value to livestock (Table 1). For example, pigweeds, lambsquarters and Canada thistle are high in crude protein and digestibility. However, like forages, nutrient content is affected by maturity. Less mature plants have a higher nutritive value than more mature plants.



Redroot pigweed and lambsquarters plants in an
empty field. Both species are nitrate accumulators
and should only be used as feed with caution.

• Chemicals in plants like mustards, ragweeds, cocklebur, and milkweeds can affect taste and greatly reduce palatability (the willingness of livestock to ingest the plant). Several weeds like lambsquarters and pigweeds are nitrate accumulators. Under drought conditions these plants can accumulate excess nitrate in the leaves and stems. When ingested, forages high in nitrates can prevent normal oxygen transfer and cause health effects or death. To read more on nitrate poisoning and its prevention, see Nitrate poisoning of livestock from NDSU.
• Morphological traits that have an antiquality influence are hairiness and thorns possessed by weeds like ragweeds, velvetleaf, and Canada thistle. Some weeds with thorns and awns on seeds can injure the mouths of animals and cause additional health concerns.
• Intake potential in ruminants is best associated with the concentration of cell walls (NDF) in forages, which limits the volume of forage within the rumen. Lignin is also a factor that affects the rate of digestion and movement of forage from the rumen. Grass weeds like quackgrass and foxtails have higher NDF and lower intake potential than forbs like dandelion (Tables 1). Dandelion has low NDF and high intake potential when vegetative, but dandelion flowers reduce its palatability. Quackgrass has very similar nutritive values and intake potential as smooth bromegrass.

Livestock tolerance to antiquality traits

Ruminant livestock vary in their nutritional needs and their tolerance of weed antiquality components. Sheep and cattle are susceptible to nitrate poisoning. Goats are among livestock known for consumption of a diversity of plants that other livestock will not consume. In contrast, horses can be very selective and adversely affected by the consumption of weeds. Younger animals with a smaller bodyweight tend to be more susceptible to some antiquality chemicals.

Grazing requires a high level of animal management and knowledge of weed characteristics. When grazing, there is always some chance for consumption of toxic weeds. Overstocking drastically increases the likelihood of animals consuming toxic weeds or weeds high in nitrates. Animals can adapt to increased amounts of some antiquality components if forage is introduced slowly into the ration.

Feeding weeds as part of hay or silage is a better approach than grazing since intake can be controlled. Weeds can be harvested using typical hay or silage-making strategies. Factors to consider during harvest are the high moisture content of weeds like dandelion and pigweeds that may require extra drying time. The thick stalks of weeds like velvetleaf and pigweeds will require conditioning, especially when making dry hay. Forage harvesting will not usually reduce the concentration of antiquality components. 

Ensiling will reduce the concentration of nitrates although the reduction is dependent on the extent of fermentation. Strategically feeding weedy bales or silage by including them as part of a total mixed ration will allow for smaller amounts of antiquality components to be fed at any time and allows an acclimation period for livestock. The bottom line is that weedy forages can provide valuable forage but should be fed with caution and only after proper identification to rule out poisonous plants.

Be sure to consult with your veterinarian and nutritionist if you are unsure about feeding weedy forages to your livestock.

Table 1. Palatability and forage quality of common annual weeds in different stages of maturity.

Weed/Crop	Palatability*	Digestibility	Crude protein	NDF**	Maturity stage
	%	%IVDDM	%	%
Alfalfa	60	68	14	54	flowering
Barnyardgrass	83	79	22	49	vegetative
Canada thistle	0	64	18	50	flowering
Cocklebur	0	77	24	27	vegetative
Common ragweed	58	72	26	25	vegetative
Curly dock	15	50	20	29	seed
Dandelion	45	74	20	29	vegetative
Giant foxtail	35	72	21	51	vegetative
Giant ragweed	0	72	22	37	vegetative
Hoary alyssum***	0	58	7	60	flowering
Lambsquarters	73	72	26	25	flowering
Oat	73	71	19	51	early head
Perennial sowthistle	20	66	13	45	vegetative
Penn. smartweed	75	62	23	26	flowering
Quackgrass	55	69	7	69	anthesis
Redroot pigweed	80	79	24	25	flowering
Smooth bromegrass	50	66	7	65	seed
Swamp smartweed	15	49	14	44	bud stage
Velvetleaf	43	80	28	25	vegetative
White campion	35	67	10	58	flowering
Wild mustard	3	69	20	38	seed
Yellow foxtail	90	74	22	44	seed

*Palatability or acceptance by sheep when a choice was given.
**NDF-neutral detergent fiber is a measure that is negatively correlated to rumen uptake.
***Hoary alyssum is toxic to horses.

Source: Marten, et al., 1987. Agron. J. and Marten and Anderson, 1975. Agron. J.