As it turns out, weeds that are fascinating to agronomists and weed scientists are rarely good news for a farmer. Yellow rattle is one such plant. This unimpressive looking annual forb tends to favor moist areas of pastures and hay fields. It is spreading rapidly in Vermont and New Hampshire, and exists in many other states. 
While yellow rattle has leaves of its own and carries on some photosynthesis, what makes this annual weed so interesting is that it is one of the few parasitic plants in our pastures and hay fields. It literally taps into the roots of other plants, stealing nitrogen, sugars, and other nutrients from them. It is very good at tapping into the resources of some species of host plants, where it can form a tight barrier-free connection with the “plumbing” of the roots. Interestingly, this plant is not able to form such a good parasitic connection with certain other plants, particularly forbs (buttercups, plantain, etc), where the connection can look more like a bad welding job. Yellow rattle generally parasitizes grasses more effectively than legumes, but it will attempt to parasitize anything that resembles a plant root, even twigs from a dead plant. It is reported to be particularly good at parasitizing Kentucky bluegrass, but the literature is sparse about how well it parasitizes orchardgrass, and other desirable grasses and legumes.
Because there is not much information available about practical management strategies for this weed, we must rely on general principles at this time. Several points to keep in mind include:
- Moist soils with low fertility give yellow rattle the best competitive advantage. When possible, improving drainage and fertility should be helpful to improve the competitiveness of desirable forages species.
- Forage grasses that are supplied with extra nitrogen seem to have a better chance to out-compete this weed. Judicious use of manure or nitrogen fertilizer could promote grass growth enough to help crowd out yellow rattle over time.
- Harvesting forages before the yellow rattle seed is viable will reduce the amount of reseeding that takes place. Some grazing animals will eat this weed before it gets excessively mature; grazing yellow rattle-infested areas just as flowers are beginning to appear should reduce the amount of seed produced.
- It is reported, but not clear from the available literature, that yellow rattle is less able to parasitize legumes than grasses. If this report is true, the presence of yellow rattle might make it easier to establish some forage legumes in the affected area.
Agronomator 