Biologists have found that plants lose their ability to defend themselves against pests if they grow too fast, creating a paradox for farmers looking to make the perfect crop.

A study published in the March 25 online edition of the journal Science revealed that plants that grow too quickly, while having the positive benefit of fast turnover for farmers, are less able to prevent damage from herbivores, as they cannot protect themselves with defensive measures such as toxins or thorns. This weakness causes them to be dependent on predators — who are not necessarily reliable — for defense.

The plants that are able to fend off herbivores without predator bodyguards have the downside of being more difficult to eat and of growing slower, leaving researchers with a natural dichotomy that seems to leave a fast-growing, herbicide-proof crop out of reach.

“Milkweed has been evolving for as many as 20 million years. Natural selection favors faster-growing plants and those that easily fight off insects,” Kailen Mooney, assistant professor of ecology and evolutionary biology at UCI and first author of the study, said in a press release. “If nature hasn’t found a way to combine both, perhaps it’s something that cannot be done.”