Understanding plant growth across seasons is key to grazing

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The month of August is full of transition and the anticipation of fall is just around the corner. In many of our pastures the grasses were dry, crisp and a shade of dull green to brown. But mid-month rain gave a needed boost to our drought-stressed landscape.

We share words of commiseration about the heat and how the grass won’t grow and the low yields that come from hay cuttings at this time, but this is a pattern we see many years and we should expect it.

So let’s remind ourselves what is happening out there in our pastures. The basic function of a plant is to take carbon dioxide, water and energy (sunlight) and convert it to carbohydrates and sugar.

This is photosynthesis, without which our very existence comes into question. However, there are different types of photosynthesis that affect how plants grow.

For example, in Ohio we predominately grow cool season grasses. These are typically characterized by C3 photosynthesis and are generally considered less efficient at gathering carbon dioxide and using water than C4 plants (warm season grasses).

Water loss

The greatest water loss due to respiration actually occurs during the peak photosynthetic activity in July and August. Examples of “cool season” grasses are bluegrass, orchardgrass, tall fescue, reed canary grass, brome and timothy.

Optimum air temperature for growth is in the 65-75 degree range. A C4 plant is more efficient and prefers warmer temperatures. Corn is a C4 plant in addition to other “warm season” grasses like sorghum, Bermuda grass, and other prairie grasses.

In managing our perennial grasses we take advantage of the plant’s ability to store carbohydrate reserves during periods of stress and dormancy so that they may initiate new growth when favorable conditions occur.

We expect high production from cool season grasses in the spring into early summer, with a slowdown during peak summer, and an increase in production in late summer into early fall.

By contrast, the warm season grasses are at peak production when the cool season grasses slump in the summer. As a manager of pasture, the main responsibility is to take advantage of a plant’s mission to reproduce a seed head to continue the existence of the species and take advantage of the livestock’s goal of eating the best tasting meal it can find.

Thus, a primary goal is to ensure that in its mission to eat well, the livestock do not consume so much of a plant as to inhibit the plant’s ability to store carbohydrate reserves for continued growth.

Leaf maintenance

A plant needs to maintain enough leaf area to collect sunlight for photosynthesis. This is why strategies that encourage rest periods and multiple paddock designs are strongly encouraged.

“Take half, leave half” is one simple rule of thumb for judging when to move livestock or make a cutting. Three-four inches of residual leaf area is standard for rapid recovery in cool season grasses.

We can take advantage of the late summer bump in forage growth. Tall fescue shows poor growth during the summer months and as a result may fall out of favor in some programs but it is one of the most prolific grasses in our area during the fall months and may be utilized to stockpile forage for winter grazing.

The time to make a final clipping or grazing of tall fescue for stockpiling use in November or December is actually about mid-July to early August. Extending the grazing season in this manner minimizes weather concerns, provides high quality forage, requires less labor and has the potential to lower expenses through less use of hay.

Understanding plant growth is vital to successful grazing management. Take the time to evaluate your management system. Consider your grass varieties and ability to utilize rotations.

Resources available

There are many resources out there to assist the pasture manager. An addition to all the fact sheets we have at our disposal, two resources I keep close are the Forage Crop Pocket Guide, available from the International Plant Nutrition Institute, and the Forage Field Guide, available from Purdue and Ohio State University Extensions.

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