Nitrate Concentration of Cereal Forage Species at Three Stages of Maturity
by Dr. Tracy Brewer, Assistant Research Professor of Range Science, Joe Skeen Institute for Rangeland Restoration; and Dr. Jeff Mosley, Extension Range Management Specialist, Department of Animal and Range Sciences, Montana State University
"The three-punch combination has knocked the health out of many winter-spring calving pastures"
Have your winter calving pastures been showing increasing signs of stress over the past few years? Do plants seem smaller and more widely spaced now than in the past? Is there more bare ground now than you remember being present before? If so, you are not alone. Winter calving pastures in Montana have taken a hit in recent years from the combined effects of several years with warm spring temperatures, with less rainfall in May-June, and without much snow cover, which exposes plants to winter grazing. By itself, one of these stressors would not normally have much effect, but the three-punch combination has knocked the health out of many winter-spring calving pastures.
Open Winters, Warm Spring Temperatures, and Extended Drought
Limited snow cover enables cattle to graze plants closer to the ground in winter and
remove more of the residual stubble that protects the grass plants’ buds, the buds
needed to produce new grass shoots in spring and summer. Grass plants initiate growth
in the spring from buds that are housed in plant crowns, at ground level. Each dormant
bud in the plant crown has the potential to produce a new tiller during the growing
season if soil moisture and temperature conditions are suitable. Suitable environmental
conditions foster growth of abundant tillers from grass plant crowns, promote larger
plant size, and enhance plant productivity. When environmental conditions are not
ideal, such as during extended drought when soil moisture levels are minimal, a larger
number of buds in the plant crown may remain dormant for that growing season. In winter,
exposed buds are more vulnerable to freezing temperatures and to trampling damage
by cattle. Reduced bud survival in winter limits the potential for grass growth in
spring and summer.
Mild temperatures during winter and early spring cause soils to thaw earlier in the year, increasing the possibility of soil compaction from cattle trampling. Compaction limits the amount of moisture that can penetrate the soil surface to aid plant growth, and compacted soils make root growth more difficult.
Extended drought intensifies the negative effects caused by winter grazing. Low soil moisture levels in spring when plant growth begins increase the proportion of buds that remain dormant, which reduces tillering, plant size, and plant yield and creates bare ground where plants used to be. Inadequate green leaf material during the growing season prevents grass plants from manufacturing and storing sufficient energy to produce healthy, vigorous buds that will survive the winter. Extended drought in spring and summer also weakens the root systems of grass plants, reduces their stored energy reserves, and reduces the production of mulch, leaving grass buds less insulated in winter and soils less protected. The cumulative effects of winter grazing and drought are first seen when fewer tillers are produced per plant each year and ultimately when plant size decreases because of reduced root growth.
Adequate Stubble Is Key
The most effective way to protect plant health in calving pastures is to leave sufficient residual stubble on plants. Have you ever noticed two pastures across the fence from one another where one pasture has ample residual stubble and retains snow cover longer and the other has little to no residual stubble and very little snow cover? Residual stubble creates a mild microsite for buds to reside in over the winter, protecting them from harsh winds, freezing temperatures, and ice. In addition, stubble traps moisture that aids plant growth when spring arrives. At least two inches of residual stubble should remain on grass plants in winter calving pastures.