Tom's Take on the CRW Buzz
By Tom Hoegemeyer, Chief Technology Officer

I've been attending numerous field days and visited with a large number of producers this fall. There seems to be a widely held perception that Corn Rootworm hybrids (CRW), by definition, have better roots. Thus far, I haven't seen any data or heard anything that would seem to verify this. However, I thought I'd share my thoughts on the issue.

All of the transgenic genes have been introduced for a purpose. In all instances to date, these have been for specific insect or herbicide tolerance. The bulk of a hybrid's phenotype, what it looks like and does, is determined by the thousands of other genes. While there is a slight probability that the introduced gene impacts another agronomic trait, mostly it is neutral for other traits. For instance, a herbicide-resistant version of an inbred parent should look and perform like the same inbred before the herbicide resistance is introduced. Current CRW hybrids, as well as the ones soon to be introduced, produce a crystalline Bt toxin that "rips" a hole in the insect larvae's gut. Period.

When a CRW gene is added to an inbred or a hybrid with inherently poor roots and root strength, it still will be an inbred or hybrid with inherently poor roots, but it will be protected from significant root feeding by rootworms. An inbred or a hybrid with inherently good roots and root strength, when one adds a CRW gene, will be an inbred or hybrid with inherently good roots, but it also will be protected from significant root feeding by rootworms.

Consider two theoretical hybrids, X and X-crw. There are fairly frequent instances when X is planted on soybean ground in a normal rotation; it will suffer low levels of rootworm feeding. Rootworm larvae do exist in rotated soybean ground at relatively low levels, even without "extended diapause" or "soybean-field, egg-laying rootworms" in high numbers. In such situations, I would expect a strip test to show a slight advantage to X-crw, although not a significant one. What might be a more important factor is that current CRW hybrids all must be sold with either Cruiser® or Poncho® 250 applied to the seed.

In our own work, we have seen that there is often a yield advantage to Poncho 250 in protection from wireworms, cut worms, seed corn maggots, etc., that could show up as root differences in isolated instances. If any of these critters are at work, or if there are rootworms, root health can be compromised and it may show as drought in hybrid X, if the environment is relatively dry and hot.

As to the conversion issue, plant breeders, while not the sharpest knives in the drawer, are likely to make reasonably good choices for "donors" for genes to use in making conversions. Say you have a poor-rooted, high-yielding line you want to convert to the CRW trait. If a donor is available that has better than average roots, breeders likely would choose it as a donor rather than another poor-rooted line. The selection system, using both DNA markers and visual observation, will be largely focused on recovering the original line plus the CRW trait, while trying to minimize the amount of DNA you carry in from the donor. However, if you also have DNA markers for root strength genes in the donor, it is possible to drag some of those along during the process. (The same thing can be done in the field IF root lodging conditions occur.) However, this is not often the practice. I have to say that most of the conversions I have seen have very little effect on other traits.

In northwest Iowa, there is some possibility that there are "extended diapause rootworms" that could have impacted lodging on corn planted on soybean ground. However, if the field was treated with something like Aztec® or Force® and still had significant root lodging, it is more likely that the lodging was caused by wind and rain on hybrids with inherently poor roots. If it was untreated, then one would have to check the roots for root feeding, which should still be apparent.

If a field has lodged, the first thing I'd do is check for signs of root feeding. Spending an hour digging might pay BIG money. For example, a farmer with 300 acres of corn could potentially make $1,200 an hour if he dug, found little root feeding, and thus could save the premium of $12 per unit on 100 units of seed he needs for next year. If he really found rootworm feeding, purchasing a CRW hybrid for those 300 acres next year could save $6,000 (10 bu/A, or 3,000 bu of corn). A range of $1,200 to $6,000 for an hour's work is pretty good pay.

We have been conscious of root strength issues for several years. For instance, our 2661 and 2679 families of hybrids have exceptional root strength and solid root regrowth capabilitiesÑand the ratings show it.