The Watchful Eye of the Master
Either you or a crop consultant should be walking your fields each summer. The frequency of trips depends on the crop grown, with corn and soybeans typically needing more attention than a field of continuous grass. There's nothing better than "the watchful eye of the master" - the farmer who walks his fields, especially during the crops' most rapid growth phases. In some cases you can detect problems before they become serious or unfixable, while in other cases a relatively uneventful trip simply gives you peace of mind. Following are some things to look for as you walk your fields.
Alfalfa and alfalfa-grass
Alfalfa is a tremendous user of potassium, and if too much is applied it can "lush consume" this nutrient, taking up more than it needs. Therefore, if your soil tests of several years ago showed adequate soil potassium levels and you've been harvesting good yields of alfalfa, don't assume that all is well. Examine alfalfa leaflets for white or yellow spots on the edges, a sign of potassium deficiency. With more serious deficiencies the edges turn yellow and die. We'll probably be seeing more potassium deficiencies than normal in alfalfa fields this summer, because in recent years high muriate of potash prices resulted in farmers reducing their potassium application rates and in some cases eliminating them entirely. A single application of manure seldom supplies enough potassium to replace that removed by several cuttings of alfalfa. Alfalfa-rass can be even more of a challenge because grass is an aggressive user of potassium, and, as with alfalfa, will take up much more than it needs. As the saying goes, "You can pay me now, or you can pay me later." A walk through your alfalfa and alfalfagrass fields may reveal that the "later" payment is now due! However, don't apply potassium fertilizer based solely on the appearance of spots on the edges of alfalfa leaflets; confirm the problem with a soil analysis. If the problem appears in only part of the field, take two samples, each consisting of a number of soil cores: Take one sample of the entire field and the other just of the affected areas. There's a little-known condition of alfalfa resulting in white spots on leaflets that appear much like those of potassium deficiency. I've seen this (rarely, though) in alfalfa fields, and sampling the soil around the plants in these fields revealed very good soil test potassium levels.
Photo courtesy of Charles Thompson/sxc.hu.
Also be on the lookout for alfalfa weevils and potato leafhoppers, both potentially devastating to alfalfa plants. A mild winter may have resulted in an unusually high survival rate of insect pests. Weevil larvae chew holes in the alfalfa leaflets but don't eat the leaf veins, resulting in a skeleton-like appearance. Potato leafhoppers insert their mouthparts into the plant tissue to extract the juices, resulting in yellowing (and sometimes reddening) of leaflets. Some people mistake "leafhopper burn" for boron deficiency, since both involve discoloration of the leaflets. However, leafhopper damage is in a triangular or "V" pattern, while boron deficiency occurs as a general discoloration. Boron deficiency is often more noticeable during dry periods. Soil analysis for boron is generally reliable and should be used to confirm a suspected deficiency. Boron should not be used during the seeding of alfalfa because of potential toxicity problems. Unfortunately, I had first-hand experience of what can happen when a boron-containing fertilizer is mistakenly used as a band-applied fertilizer during alfalfa seeding, and it wasn't pretty.
By midsummer any zinc deficiency symptoms that may have been there early in the season have disappeared. This doesn't mean there's no yield loss, but it's too late to do anything about it this season. The same goes for phosphorus. If you notice any nutrient deficiencies in corn this time of year, it's most likely potassium or nitrogen. Potassium deficiencies have become more common in corn for much the same reason as with alfalfa: reduced use of potassium fertilizers. Potassium deficiency starts as a yellowing of the leaf tip and progresses to yellow, necrotic (dead) areas along the leaf margin. In severe cases the entire leaf dies. The symptoms don't appear earlier in the growing season because it's only during rapid growth that the plant requires a large amount of potassium. As with alfalfa, potassium deficiencies should be confirmed by a soil analysis. Dry weather reduces nutrient uptake, and if roots are challenged when the corn plant needs a lot of potassium, deficiency symptoms can appear even if fertility is adequate. Rain or irrigation should correct the problem, but affected leaves won't recover. Early nitrogen deficiency symptoms are a yellowish green instead of the plant's normal color, but by now you may be seeing V-shaped yellowing of the bottom leaves of the plant. If affected leaves reach as high as the ear leaf, you're almost certainly losing yield potential.
The most noticeable midseason insect problem in corn is obvious when you see it, but because it typically affects only portions of a field, scouting is the best way to detect it. Armyworms love corn and will readily devour the leaves on plants, feeding mainly at night and leaving only the midribs. Severely affected portions of an armyworm-infested field look more like a pineapple plantation than a cornfield! When defoliation occurs this early in the season yields will be severely reduced. When the damage is spotty most farmers choose to ignore it (if indeed they detect it before harvest), but with serious infestations contact your crops consultant or cooperative extension personnel for control advice.
I've made the tongue-in-cheek comment that you can tell a "grass farmer" by the fact that if the jacket he's wearing wasn't given to him by his feed company, he probably either won it in a raffle or actually had to buy it. That's because, typically, farmers growing only grass - no alfalfa or row crops - seldom need to purchase seed, herbicides or insecticides. No purchases of these products usually means no free jacket. There are few insect pests that attack forage grasses, with the exception of an armyworm attack on reed canarygrass or a grasshopper infestation, and even these are few and far between. Nor are weeds usually a serious problem. Grasses need plenty of nitrogen for good yields and protein content, but beyond that an annual application of dairy manure will supply most nutrient needs. And, as long as you don't scalp grass fields during mowing, most grass species should survive and be productive for a long time.
Ev Thomas has worked as an agronomist in New York for 45 years, first with Cornell University Cooperative Extension, then with the William H. Miner Agricultural Research Institute in Chazy, N.Y., including managing its 680-acre crop operation. He continues to work part-time for Miner Institute and is now an agronomist at Oak Point Agronomics. He has a written our Forage column for over 13 years and has been an expert contributor on a number of other topics.