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Forages: Other Crop Problems
Excerpt from Agronomy Guide for Field Crops (Chapter 5) Order OMAFRA Publication 811: Agronomy Guide for Field CropsTable of Contents
Forage WinterkillWinterkill of forage stands can cause serious problems on livestock farms and can be a limiting factor in alfalfa production. It can result in lower quality feed, shortages of feed, disruption of the rotation and additional costs for reseeding lost stands. With forage production, it can sometimes be difficult to optimize the competing demands of quality, yield and persistence. Farmers need to determine how much forage persistence they need and then try to manage the risks accordingly. Causes of winterkill include:
Some forage species are hardier than others. Much of the concern over winterkill centres around alfalfa. The legumes birdsfoot trefoil, red clover, wild white clover and alsike will tolerate more adverse winter conditions than will alfalfa or ladino clover. The grasses timothy, reed canarygrass, bluegrass and bromegrass rarely winterkill. Thus their use in mixtures gives stand insurance. Orchardgrass and perennial ryegrass may be killed by icing or low temperatures. Hardening is the process of cold tolerance development initiated by shorter fall days and cooler temperatures. During the hardening process, plants store carbohydrates in crowns and taproots. The starch is converted to sugars, which gives the plants some protection from freezing. Plants also lose some cellular water to reduce freezing damage. Long fall periods with cool, dry, sunny conditions favour winter hardening. Factors That Affect Winter SurvivalCritical Fall Harvest Period for Alfalfa Harvesting before the "critical fall harvest period for alfalfa," also known as the "fall rest period,' allows the plants to regrow and build sufficient root energy reserves for overwintering. Adequate root reserves are necessary for winter survival and persistence, as well as vigorous spring growth and good first-cut yields. The critical fall harvest period is approximately 6 weeks long and varies with location. See Figure 5-4, Start of the Critical Fall Harvest Period for Alfalfa,for a map detailing the critical fall harvest period in your area. The risk of alfalfa winterkill increases by harvesting during this period and should be weighed against the immediate need for forage. Yield sacrificed by not harvesting during the critical fall harvest period is usually easily regained in first-cut yield the following year. To ensure persistence and succeeding yields, do not harvest alfalfa during 6-week period following the date shown on the map for each region.
Figure 5-4. Start of the Critical Fall Harvest Period for Alfalfa | Top of Page | Winter-Tolerant Alfalfa Varieties Refer to the Ontario Recommended Forage Variety Performance list available at OMAFRA Resource Centres or on the Internet at www.plant.uoguelph.ca/performance_recommendations/ofcc/ofcc.htm. High alfalfa yield indexes in the third and fourth year indicate high relative persistence. This is a composite measure of a number of production characteristics, including winterhardiness, cold tolerance, disease resistance and yield. In general, varieties that have "resistant" or "highly resistant" ratings for disease have a lower risk of winterkill. Adequate snow cover of at least 15 cm (6 in.) insulates the alfalfa crown and root at moderate temperatures. Soils with a lack of snow cover can expose alfalfa crowns to temperatures less than -15ºC (5ºF). This results in freezing damage to plant cells and eventual plant death. The insulation effect from snow also reduces soil temperature fluctuations and risk from heaving. Fluctuating winter temperatures with lows below freezing and highs greater than 5ºC (41ºF), without snow cover, can cause plants to break dormancy and become more susceptible to freezing. | Top of Page | The fast melting of snow followed by cold temperatures results in ice sheeting, which smothers the plants by restricting oxygen. Ice sheeting also causes freezing due to the poor insulating ability of ice. Leave Fall Growth to Hold Snow After a hard fall frost, alfalfa can be harvested without lowering root reserves, but there are disadvantages. Leaving the fall growth will aid in catching snow, which insulates the soil from cold temperatures. The stubble also helps alfalfa plants survive ice sheeting by protruding through the ice, allowing the movement of air for respiration. Unlike grasses, fall regrowth of alfalfa does not cause "smothering," but ice sheeting does. Both surface and subsurface drainage are essential to alfalfa winter survival. Heaving occurs on heavy soils with poor drainage since repeated freezing and thawing cause the taproot to be pushed out of the soil. Plants heaving more than 1 in. will dry out and have broken taproots and lateral roots, and elevated crowns. These plants will die or be severely stunted. Surface drainage is a bigger issue on flat land than on rolling land, because with frozen ground, the water cannot drain off the field, resulting in ice sheeting.
Plate 99. Alfalfa heaving is caused by freeze/thaw cycles of early spring, lifting the crown up. | Top of Page | Frequent harvests with short cutting intervals put stress on alfalfa that can reduce winter survival and first-cut yield the following year. Conversely, a long interval between cuttings will rebuild plant reserves and enhance winter survival. Some growers place more emphasis on high-yielding, frequently cut, high-quality stands that last for 3 years and are less concerned about alfalfa persistence. Cutting intervals of less than 35 days between cuts increase the risk of winterkill. Allowing the final cut to show some flower before harvest to ensure adequate root reserves can be a compromise between quality and winter survival. A low soil potassium level is a major factor leading to loss of alfalfa in a stand, particularly on loams and sandy loams. Adequate soil potassium levels allow alfalfa to store sufficient carbohydrates in the roots to resist low temperature injury and act as the energy source for vigorous spring growth. Forage fields should be soil sampled regularly and fertilized according to recommendations. The fertilizer should be applied before the start of the critical fall harvest period to allow for sufficient plant uptake. However, high soil potassium levels can result in luxury consumption of potassium by alfalfa and subsequent nutritional problems when fed to dairy cows prior to calving. Potassium applications on soils testing over 150 ppm will not significantly increase winter hardiness and are not recommended. Boron deficiency can also lead to stunted alfalfa growth during midsummer dry periods and thus weaker plants going into winter. For soils low in pH, lime should be added the year prior to seeding (see the section Fertility Management). | Top of Page | Management of Insect and Disease Severe infestations of disease and insects that result in reduced stand vigour, reduced root reserves, and slow regrowth will increase the risk of winterkill. Potato leafhopper control can be important in reducing winterkill, particularly in the seeding year (see the section Potato Leafhopper). Assessing an Alfalfa Stand for Winter Survival Future yield potential can be estimated by looking at plants or stems per square metre, but the health of crowns and roots is extremely important. Stem counts are more accurate than plant counts, but in earlier spring it may only be possible to count the number of crowns. Be prepared to replace an older stand if it has less than 43 plants/m (4 plants/ft2). See Table 5-16. Minimum Number of Healthy Plants per Square Foot Required for a Desirable Alfalfa Stand. Dig several plants to determine the health of the crown and root. Healthy crowns are large and symmetrical and have many shoots. Cut a root open lengthwise. Healthy roots will have a white or creamy colour inside, and are firm and resistant to pealing when scratched with your thumbnail. Dying plants will have a discoloured crown and root and a spongy texture. Check for bud or new shoot vigour. Plants with broken taproots from heaving may green up but die later. Slightly heaved plants can survive, but their longevity and productivity will be reduced. When alfalfa is about 15 cm (6 in.) in height, stems/m2 (stems/ft2) can be used as the density measure. Stem density of 590 stems/m2 (55 stems/ft2) has good yield potential (see Figure 5-5, Alfalfa Yield Potential at Various Stem Count Densities). There may be some yield loss with stem counts between 431-539 plants/m2 (40-50 plants/ft2). Consider replacing the stand if there are less than 40 stems/ft2 and the crown and root health is poor. Figure 5-5. Alfalfa Yield Potential at Various Stem Count Densities
Source : Undersander et Cosgrove, University of Wisconsin, 1992 Other considerations include forage inventories and requirements, alternate forage options, how much grass is left in the stand, rotational requirements and weed pressure. | Top of Page | Updates on Forages: Other ProblemsNo updates available at this time. Related links...| Top of Page | For more information:Toll Free: 1-877-424-1300 Local: (519) 826-4047 E-mail: ag.info.omafra@ontario.ca |
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