Date: Sun, 4 Dec 1994 16:56:41 -0800 From: Brian Baker WHAT IS A WEED?
Thought I'd try to break a few threads and seek clarification on something that has led to several meandering conversations where it appears to me you have different perceptions of the same problem. Looking at the various posts by (in alpha order) Aquilla, Benbrook, Hoogheem, Hovermale, Meade and Schettino, among others, on herbicides in water, nutrient mining crops, and tillage systems appear to differ on perceptions on what role weeds play in farming systems. I'd like to ask a fundamental question--What is a weed?--and challenge you SANetters to work toward a more sophisticated view of weeds in farming systems. My apologies for making this so long. _Weeds Conserve Soil_ in some farming systems, having something fill that niche may be far better than nothing. The exchange between Tracy and Sal illustrates this. While no-till might conserve soil in the corn belt, heavy use of herbicides in orchards on the steep slopes in Sal's area has led to serious erosion, even in no-till systems.
Another place where this is a serious problem is the humid tropics. During the Hawaiian rainy season, I watched soil washed down the slope of a coffee and macadamia nut grove that was no-till but kept free of weeds by herbicides. In a semi-arid subtropical perennial system, like Sal's farm, an understory is needed to keep the soil from washing away when the rains finally come after a long dry season. Someone who is criticizing Sal's observation from the perspective that the whole world grows corn and soybeans misses his point. But even in the semi-humid to humid temperate zones that characterize most of the US east of the 100th parallel, one should consider and critically evaluate the role weeds and tillage systems play at conserving resources.
Most organic farmers that I know don't even own a moldboard plow. While tools differ from soil to soil, and even season to season, most will use cultivation implements like chisels, rippers, disks, rotary hoes, spaders, as well as tool-bar devices like sweeps and shovels. A few of the more adventurous row-croppers are working with permanent beds with buried drip irrigation, following the Sundance (Arizona) system. Sundance still uses herbicides, but have found that their rates have been greatly diminished with a permanent bed with minimal disturbance, and improved water control. Others have eliminated herbicides entirely. One can argue whether the use of any or all of these tools are minimal till. Not only do they suppressing weeds at a critical time, but also aerate the soil and allow soil preparation for annual crops.
_Weeds Conserve Water_ In arid, semi-arid and Mediterranean climates, where water conservation is arguably a more serious and persistant challenge than soil conservation, weeds play a role in helping water penetration, storage and shade, conserving soil moisture that matches or outweighs water competition. Savory covers this in rangeland management systems. Tap-rooted plants create macropores as their mass slowly decomposes. These pores are destroyed by tillage, but herbicides in a no-till system can prevent the tap-rooted of these plants from ever forming. The result, especially in sun-baked alkali conditions is a hard cap that prevents water from penetrating. Water puddles on the surface, rather than penetrates, and thus is evaporated rather than stored and made available to the crop. Ralph Jurgens of New Era Farm Services in Tulare has two photos of walnut orchards following a rain, one with bare ground the other with a ground cover.
_No-till doesn't mean Herbicides_ While most vineyards still french-plow, an increasing number of orchards have found that no-till without herbicides works best. A well-managed ground cover plays several important roles that bare soil can't match. To review: legumes fix nitrogen; these and other crops can "mine" other nutrients unavailable to the tree's root system; flowers attract beneficial insects; roots hold soil in place; increased organic matter increases water field capacity; chlorophyll absorbs sunlight and stores energy, while bare soil reflects it, causing sunburn; plants moderate both high and low temperatures. During the 1990 frost, weed scientists predicted that organic citrus and avocado groves would be damaged worse because the groud cover would raise the frostline. Instead, the ground cover moderated the day- and night-time swings in temperature, saving both fruit and whole trees. It was a disaster either way, but organic farmers didn't seem to have to replant as many trees, and certainly weren't any worse off, as weed scientists thought they'd be.
I know that this might be focusing too much on perennial crops, but growers are also using no-till without herbicides in field and row crops. Lundbergs, for example, have been experimenting with no-till rice without herbicides, and have made some progress. _No-Till Isn't Always Environmentally Sound_ The San Joaquin Valley has a mandatory plow-down for cotton to maintain a host-free period for pink bollworm and boll weevil control. The program has been very successful, but recall a few years ago, before the end of the drought, a dust-storm came and kicked up enough dust to cause a major pile-up on I-5, the main artery between LA & SF. Plow-down was generally acknowledged to be the culprit. More rotation into winter small grains and hay, with buffer strips of alfalfa (used for lygus control as well as for erosion) would have helped, trees planted as windbreaks even moreso, but choosing between no-till or no pink bollworm and no boll weevil is a trade-off California farmers have chosen to make in favor of plow-down, which IMHO was the far lesser of two evils.
_Toward a New Weed Science_ A stone-fruit and grape grower who still uses herbicides on johnsongrass once told me a story about weed science. He found an interesting-looking plant in his peach grove, and called extension to have them come out and identify it. They knew what it was, and immediately told him of about 20 herbicides he could use to kill it. But he didn't really want to kill it without first knowing, Why was it there? What was it doing? Was it competing? Would it attract beneficial insects? What kind of organisms co-exist within its root system? Are these organisms beneficial, pathogenic or neither? These questions the weed scientists could not answer, so the plant lived, with no apparent deleterious effect on the orchard. I recognize that there are a few really noxious weeds: bermudagrass, johnsongrass, field bindweed, purple nutsedge, for example. Expanding the list beyond that is possible, but debateable. Some might be noxious in some situations, but beneficial in others, like sandbur or lamb's quarters. Still others are almost never harmful to crops, like sweet alyssum, ageratum, or poppies. Some of these are actively sown to attract beneficial arthropods. Yet there is an imperative to show domination of nature through complete elimination of every non-crop species planted in what are almost always fields planted to monoculture. This does not always make economic sense. Giving up herbicides is often hard for a farmer who has been told that a measure of his or her management is how controlled the system is. At the risk of making a sexist generalization, I've found that women farmers have less of a "hang-up" about weeds than men. I talked to the daughter of a prominent organic farmer in North Dakota, who told me that herbicides were difficult for her father to give up, not because the economic damage of weeds or soil erosion was so great, but because _his_ father thought he was mismanaging the farm by the number of weeds in a field. Bankers have also been known to make snap judgements about the economic viability of a farm based on whether or not they see weeds, regardless of what the balance sheet shows.
But fathers and bankers aside, the people who seem hardest to change are the weed scientists. Weed scientists haven't learned the lesson that entomologists learned with IPM--that killing every weed is not economically efficient. After a certain threshold, weeds aren't worth killing. Anybody seen articles on economic thresholds of weeds? If so, I'd like to read them. If not, I would posit that just as insects and micro-organisms in farming system are not always pestiferous or pathogenic, non-crop species of higher plants do not always compete with crops at levels that justify control. When they are found at levels that justify control, herbicides are never the only and not always the "best" tool for the long-term management of the problem. When a plant becomes a "weed" it is the symptom of a larger problem. As Alan Savory points out in _Holistic Resource Management_, herbicides treat the symptom, not the problem. Cultural modification can be cheaper and more effective in the long-run, but herbicides offer a quick fix and dramatic results.
While one can argue whether one-in-a-million or one-in-five cancer risk is a more reasonable way to set public policy, the fact that any herbicides are found in water shows that they are not being used efficiently--they do a farmer no good to control weeds in his or her field if they're in the river or the well. My brief foray into weed science in grad school was pretty disappointing. I found that herbicides kill weeds pretty darn good, and that these cultural methods don't kill 'em nearly as well. End of story. The last time I looked, weed scientists seemed to all learn their craft from General Sheridan--"The only good weed is a dead weed," or Arlo Guthrie at the draft board in Alice's Restaurant, "I want to kill, Kill, KILL!" I haven't picked up a weed science journal in a few years. I hope the next time I do, I'll be pleasantly surprised to find at least some articles on the benefits of weeds in agro-ecosystems. Weed scientists are going to need to learn the roles that non-crop plants play, both positive and negative, in order to make a contribution to sustainable farming systems.