A grazing/mowing trial with smooth bromegrass and Ladino clover was designed to study the relative importance of two variables: the length of the grazing period and the length of the recover period. Twenty different grazing systems were studied. Grazing periods of 2, 4, 8, and 16 days were compared on paddocks designed with grazing intervals of 8, 16, 24, 32, and 40 days. The stocking rate of yearling ewes was adjusted daily to ensure the desired degree of defoliation. The area of each paddock was calculated on the basis of the predicted growth rate of the forage, and with the digestible nutrients required by two ewes for each of the assigned grazing periods.
On the first day of grazing a paddock, a companion plot in an adjacent area was mowed, leaving a 2-3" stubble. This intensity of defoliation was likewise the goal for grazed paddocks; however, it was seldom achieved. The mowing interval for the companion mowed plots equalled the sum of days grazed plus days rest.
The premise was taken that in any grazing system, the paddock most representative of the system would be the one grazed in the middle of the cycle. Thus, the prescribed treatments were deferred from "time zero," taken as April 30, for a period equal to one-half the total number of days in the cycle (grazing plus rest). This decision introduced an important variable; however, it was not considered important when the trial was designed. Imposing the initial grazing and mowing treatments at successively later dates provided the opportunity to study the effects of defoliation at different developmental phases of smooth bromegrass. Further, the sharp contrast in rates of grass recovery following grazing versus mowing, revealed that at certain growth stages, mowing destroyed vegetative meristem sustems that were spared in grazed plots.
* Early-spring management precautions
With many cool-season grasses, the main goal of offering livestock an "early bite," while grass shoots are still in the vegetative stage of shoot development, is to ensure prompt competitive recovery of the desired species. Prompt regrowth depends on the degree to which the shoot apex (active above-ground meristem system) is safeguarded.
At "early-jointing," corresponding closely with the transition stage, the shoot apex should be considered to be the chief mechanism for regrowth. It is easily found at the base of a shoot by splitting the shoot lenghtwise. The shoot apex (growing point) contains the rudimentary seedhead and undeveloped nodes and internodes of the central stem (culm). Nodal tissue includes meristematic zones which account for leaf sheath and leaf blade development. It is self-evident that the growing point, together with the associated meristem, represents a critical zone on the grass shoot.
Grasses which are prolific seed producers, having a high percentage of flowering shoots, are particulary vulnerable to mismanagement at the early-jointing stage of stem development because many of the shoots may be either denuded (leaves severed below the collar zone but above the shoot apex) or decapitated (shoot severed below the shoot apex). A denuded shoot will have a seed head but perhaps only a portion of the flag leaf, where as with decapitation, no vegetative system remains. The grass must recover from new shoots arising from basal buds in the crown system. Thus, it is wise to avoid close grazing during transition until basal sprouts in crown tissue appear to be ready to produce aftermath shoots.
In this experiment, initial treatments were imposed on certain paddocks during the first week in May. Internode elongation had commenced in bromegrass shoots. The ewes grazed slightly above the growing point but below the collar zone of may leaf blades. Consequently bromegrass recovery growth consisted chiefly of naked (denuded) flower stems with poorly developed seed heads. When ewes returned to paddocks having long rest intervals, they found the recovery growth to be unpalatable. Ladino cover was dominant because of minimal grass competition.
In sharp contrast, plots that were mowed, leaving a 2-3" stubble, recovered from new shoot initials in crown tissue since the growing point of essentially all of the shoots were destroyed. Recovery was severely delayed because, in early May, aftermath shoot initals are not well developed. Slow recovery allowed clover, broad-leaved weeds, and weedy grasses to flourish. Grazing would have produced a similar effect had the stocking rate been heavier, forcing the ewes to take a second bite of the remaining stubble.
When the comparative effects of mowing and grazing are evaluated, it is essential to take careful note of the condition and vulnerability of regrowth mechanisms so as to avoid the ill effects of untimely denuding and/or decapitation of grass shoots. The treatment effect imposed by undergrazing should be similar to the effect imposed by clipping.
* Mid-spring management precautions
When bromegrass (and other susceptible "jointed" grass) shoots approach the late-boot to early heading stage, livestock numbers should be increased to ensure that the available forage will be consumed in a timely manner. Once seedheads appear, forage quality and palatability decrease rapidly. Ungrazed seedheads should be clipped to encourage regrowth from shoots in the crown.
* Aftermath shoot management
With smooth bromegrass, aftermath shoots tend to produce culms with distinct internodes after approximately five weeks recovery. This growth habit raises the growing point of the aftermath shoot to a vulnerable position. Without exception, close grazing or clipping destroyed the growing point of a high percentage of the shoots resulting in varying degrees of summer dormancy and weed invasion. Longer rest periods provided for the development of new shoot initials in the crown zone. Summer dormancy was not a problem with these systems. Paddocks grazed prior to culm development (24-day rest interval) continued growth, however, subsequent grazing events were untimely with respect to the location of the growing point and summer dormancy followed.
This experiment exposed the risks involved when the grazing-rest intervals are fixed, as opposed to being flexible, where grazing pressure and/or degree of defoliation are adjusted according to the vulnerability of meristem systems which will account for recovery growth.