As wheat greens up and begins growing with warmer temperatures in the coming weeks, producers should start thinking about when to pull cattle off pasture to protect grain yields. After greenup is underway and before the wheat has reached jointing, it is important to scout fields closely for signs of the “first hollow stem” (FHS) stage. This stage occurs as the wheat switches from the vegetative stage to the reproductive stage of growth.
When the leaf sheaths become erect, the developing growing point, which is below the soil surface, will soon begin to form a tiny head. Although the head is quite small at this point, it has already established some important yield components. At this stage, the maximum potential number of spikelets is determined. Sufficient nitrogen (N) should already be available in the root zone at this growth stage in order to have the maximum effect on the potential number of seeds per head.
Once the embryo head has developed, the first internode will begin to elongate pushing the head up through the leaf sheaths. This first internode will be hollow. This will be visible before you can actually feel the first node (joint, located just above the first internode). Prior to this stage the nodes are all formed but tightly packed together and hard to see.
FHS is the point at which a 1.5 cm (about half-inch) length of hollow stem can first be identified above the root system and below the developing head (Figure 1). This length is roughly equivalent to the diameter of a dime, which makes its identification in the field easier. FHS occurs when the developing head is still below the soil surface, which means that producers have to dig plants out of the ground to do the examination.
Figure 1. Comparison between wheat at jointing (plant on the left) and wheat at first hollow stem (plant on the right). Photo courtesy of Jeff Edwards, former Oklahoma State University Extension Wheat Specialist.
To look for FHS, start by digging up some plants from fields that have not been grazed, such as field corners or just outside the electric fence. Date of FHS is variety- and field-specific, so it is important to sample each individual field. Select the largest tillers to examine, and slice the stem open from the crown area up. Look for the developing head, which will be very small. Next, see if you can find any hollow stem between the developing head and the crown area. If there is any separation between the growing point and crown, the hollow stem is elongating. If that separation is 1.5 cm, the wheat plant is at FHS. FHS occurs between a few days to a week or more prior to jointing, depending on temperatures.
If the wheat has reached FHS, cattle should be removed to prevent grain yield loss. Yield losses from grazing after FHS can range from 1 to 5% per day, depending on grazing intensity and the weather following cattle removal. If cattle removal is followed by cool, moist weather, yield losses will often average about 1% per day grazed after FHS; if weather is hot, dry, and harsh, yield losses of 5% per day or more can be expected. In fact, as much as 1.25 bushels per day yield decrease can occur according to OSU data. It is easy for producers to be late by a few days in removing livestock as they wait for obvious nodes and hollow stems to appear, and even the first few days can be significant.
Two things are observed when wheat is grazed too long: 1) fewer heads per acre because the primary tiller has been removed and 2) smaller and lighter heads than expected because leaf area has been removed. As cattle continue grazing, the wheat plant is stressed and begins to lose some of the tillers that would produce grain. A little later, if there is not enough photosynthate, the plant begins aborting the lower spikelets (flowers where seed develops) or some of the florets on each head. Finally, if there is not enough photosynthate during grain filling, the seed size will be reduced and if the stress is severe enough, some seed will abort.
Romulo Lollato, Wheat and Forages Specialist