Last week we received several samples of and inquiries about alfalfa plants that died shortly after breaking dormancy. The samples have come from north central and west central Iowa. These plants generally have signs of decay in the roots and/or crowns. Crown rot symptoms in these plants are typical of Fusarium species. In some cases, the crown rot extends a considerable distance down the taproot. The poor growing conditions last year predisposed plants to crown rots, and we are now seeing lingering effects of last years weather. Plants that developed crown and root rots last season may not have shown obvious symptoms at that time. These symptoms may be appearing now as the plants were weakened further by winter conditions.
In addition to crown rot symptoms, we have seen some samples with extensive root decay. This decay appears to be secondary in nature, often associated with plants that died during the winter. It is common to isolate root rot pathogens such as Fusarium and Pythium species from these roots. So far, Phytophthora root rot has not been found on these samples, but it probably will show up in plants infected last year. We suspect that we will continue to see poor growth and diseases in stands that were damaged by last years weather.
In general, it is best to rotate away from alfalfa when a stand has suffered severe root or crown rot or winter injury. Crown and root rot pathogens may be plentiful in such fields. If the field is to be seeded to alfalfa, be sure to use metalaxyl (Apron) - treated seed and a variety that has good resistance to Phytophthora root rot. Researchers in Wisconsin report better results when Phytophthora resistance is combined with Aphanomyces resistance. This might not be a bad idea in Iowa, although we dont know how extensive Aphanomyces is here. There have been reports that metalaxyl (Ridomil) soil treatment or double treating seed with Apron will improve stand establishment when reseeding alfalfa. There may be some benefit to these practices, but neither has been proven to be effective or, more importantly, cost-effective. Also, double treating seed will probably result in higher-than-label rates.
There are other factors to weigh when considering reseeding poor alfalfa stands. How much do you need forage from the first cutting this year? Are there other newly-planted or younger fields which are in good production? Are the fields uniformly thin, or is the damage localized to small areas? Consider these replant or emergency forage options:
If most of the field is normal and the injured areas are localized and in areas large enough to manage separately, consider tilling and reseeding the injured patches. Harvest the newly-seeded areas about 55-60 days following seeding, and try to bring the newly-seeded areas back into the cutting schedule of the remainder of the field by late summer.
If the field is uniformly thin and you must keep the field for this last growing season, consider sowing 2 bushels per acre of oats into the thin stand, and harvest the oats as forage along with the alfalfa. You will still have a thin, and likely an unprofitable, stand for the remainder of the summer. Plan to reseed another field this year if you have not already done so.
Seeding 6 to 8 pounds per acre of red clover into a thin alfalfa stand will extend the productivity of the field for about two years.
If you need forage and can use grass hay in winter rations, consider harvesting some of your oats for hay. Harvesting when the oats are just beginning to head, several weeks earlier than normal, will provide grass hay of average-to-good nutritive quality.
If you need to plant a more productive, emergency summer forage crop, consider planting one of the sudangrasses or sorghum-sudangrass hybrids in mid-May or early June. These crops can be grazed. Sudangrass can be cut for hay, if it is cut at 18-30 inches of height. Sudangrass and sorghum-sudangrass hybrids can be cut for silage. Sorghum-sudangrass hybrids do not dry uniformly, so they are not generally recommended for harvest as dry hay.
Other annual emergency forages such as foxtail millet and Japanese millet can also be planted in May and early June, but they often provide only one growth of forage. Hybrid pearl millet has many of the same growth characteristics of sorghum-sudangrass hybrids, but it generally is slower to regrow and produces poorly in cool wet seasons.