Drought again affects forage legumes and grasses

Craig Sheaffer, Extension forage agronomist

Tall fescue regrowth in a horse pasture following early
June grazing. It's surrounded by Kentucky bluegrass, a
non-drought tolerant grass.

The backbone of Minnesota forage production systems are cool season legumes and grasses. These plants, including alfalfa, red clover, orchardgrass, Kentucky bluegrass, and tall fescue, grow best at air temperatures of 60-80 F and with adequate rainfall.

Cool season forages have less heat and drought tolerance than warm season plants, like sudangrass and corn, that grow best at temperatures of 80-95 F. Therefore cool-season forages typically undergo a “summer slump” while warm season forages produce best in summer.

Typical growth pattern of forage legumes and cool-season and warm season grasses.

Drought effects on forages

However, when weather patterns create high air temperatures and drought, yields of cool season legumes and grasses suffer. Cool season grasses like Kentucky bluegrass that have very shallow root systems suffer most during drought. Instead, warm season grasses like sudangrass and teff should be considered for use in a system to supply forage during the summer months. For more details see: Sorghum-sudangrass and teff as summer forages for livestock.

Average water use by forage crops is greatly affected by weather. Higher air temperatures, low dew points, and higher air temperatures increase water use. Some average water use estimates per ton of forage follow:

Alfalfa: 4-7 inches per ton
Cool season grasses 4-6 inches per ton
Warm season grasses: 2-4

As described in the most recent U.S. drought monitor for Minnesota, over half of the state has some level of drought with the warmest months ahead. While rainfall has occurred in some regions, our overall rainfall is below normal and this has affected growth of forage crops.

Water held in the soils acts as a buffer against drought with clay soils holding more water than sandy soils. However, when soil water is depleted, a significant recharge from rainfall is needed for normal forage growth. For example, a summer thunderstorm producing an inch of rainfall will not have much effect on regrowth of forages that can on average water use as much as .3 inch per day!

Alfalfa has the best drought tolerance

Figure 1. Forage yield of perennial legumes when grown
with and without a water deficit. Source: Peterson et al.,
1992. Agron J. 84:774-779

Because of its extensive root system, alfalfa will yield better than other commonly grown legumes such as red clover and birdsfoot trefoil (Figure 1). Alfalfa has greater yield potential than these legumes under normal rainfall conditions and its yield was reduced only 46% by severe drought compared to 80% or more for the other legumes.

In Alfalfa during drought, we described how new alfalfa seedings at least 8 weeks old and old stands might have yield reduction, but can recover rapidly following normal rainfall. The secret of alfalfa’s survival during drought cycles is its ability to have enhanced carbohydrate storage in its roots and its deep root system. While birdsfoot trefoil and red clover grow well on some soils that have a lower soil pH than is required for alfalfa, we recommend alfalfa for climates and soils where drought is a risk.

Grasses have less drought tolerance

Figure 2. Forage yield of perennial grasses when grown
with and without a water deficit. Results are total for 4
harvests over 3 years Source: Sheaffer et al., 1992.
J. Prod Agric. 5:556-561.

Perennial cool-season grasses than have fibrous and relatively shallow root systems that limit their capacity to reach water deep within the soil profile. As shown in Figure 2, grass yield reductions ranged from 61% for reed canarygrass to 53% for orchardgrass under drought conditions. Timothy and smooth bromegrass yield reduction averaged 56%.

Comments about grass species

Orchardgrass has relatively good yield and persistence under drought conditions. In addition, it is adapted to three to five harvests per year. It is an excellent pasture grass. Tall fescue and meadow fescue should be considered as high yielding options to orchardgrass.
Reed canarygrass has very good drought tolerance and is high yielding when grown, but is difficult to establish and seed of low alkaloid types may be in short supply.
Smooth bromegrass has very good drought tolerance but will not persist in 3 or 4 cut harvest systems when harvests occur before anthesis. Most of its yield will occur in spring harvests. It is best adapted to regions with two harvests per season.
Timothy has poor drought and heat tolerance and like smooth bromegrass will not persist in 3 and 4 cuts harvest systems in southern MN. It is well adapted to harvest systems and climates of northern Minnesota.

Alfalfa grass mixtures

Mixing alfalfa with cool season grasses provides many advantages, as discussed in
several forage mixture Crop News articles. In addition, having an alfalfa-cool season grass mixture during drought provides a diversity of water use efficiencies. If growth of shallower rooted cool season grasses shuts down during drought, deep rooted alfalfa can provide some forage.

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