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Superintendent
David Davis
21262 Genoa Road
Linneus, MO 64653
Phone: 660 895-5121
FAX: 660 895=5122
Email: DavisDK@missouri.edu

Forage Systems Update
Vol 9, No. 3

Calendar-based vs. Need-based Rotational Grazing

Management flexibility is one of the keys to success in management-intensive grazing systems. Being able to change length of grazing periods and rest periods in response to growing conditions and animal demand is what produces the positive response that so many graziers are experiencing. Flexibility is much more difficult to maintain in a research environment where carefully defined systems and rules are needed to make the results of a study interpretable. The differences in results between two ongoing studies at FSRC have really brought home the different results from calendar-based and need-based rotations.

From a standpoint of statistical interpretation, fixed calendar rotations at constant stocking rates are much easier to compare and evaluate than are systems with flexible management. Flexible management is based on the physiological and nutritional needs of the pasture plants and animals. Some of the needs of pasture plants are opportunity to restore carbohydrates to the storage organs or produce seed for annual plants or short-lived perennials. Two studies which have been reported previously in Forage Systems Update are a stocking rate study (1998:v7:n2, 1999:v8:n3) and the diversity study (1999:v8:n2,2000:v9:n1). Both of these studies were initiated in 1995. Both birdsfoot trefoil and red clover were interseeded into grass swards in March of 1995 at similar seeding rates.

The stocking rate study uses yearling steers and evaluates four stocking rates with either continuous or rotational grazing. Stocking rates are .5, 1, 1.5, and 2 steers per acre with grazing from early April until early September. This study is a calendar-based rotation system with all stocking rates typically managed with 2-day grazing period and 22-day rest period. In each year, the first grazing cycle has consisted of daily moves and in two years, at least one midsummer cycle used 3-day grazing periods and corresponding 33-day rest period. The three day grazing period proved to be long to sustain steers at the highest stocking rate and for the past two years the two-day grazing period has been used. This inevitably leads to what Andre Voison termed "untoward acceleration" where each successive grazing period provides less forage and the rest period is shortened until the rotation collapses. In this study, the rotation is maintained at a constant rate and animal performance declines in response to the lessening forage supply.

The diversity study compares 16 different pasture mixtures across a range of species diversity levels. Mixtures include monocultures of tall fescue or smooth bromegrass with and without N fertilizer, binary mixtures with red clover and birdsfoot trefoil, tertiary mixtures with both legumes, and more complex mixtures with additional grass species also included in the sward. Individual plots are 2500 ft2 and each plot is individually grazed when a target height of 8 to 10 inches occurs. The grazing period is usually about 6 hours with about 5000 lb of animal liveweight per plot. Rest periods have ranged from as short as 9 days to as long as 70 days, depending upon the particular mixture and growing conditions. At some point in the season almost all of the legume mixtures will have a rest period in excess of 40 days. Both birdsfoot trefoil and red clover can produce adequate seed for stand maintenance with a 40-day mid-season rest period.

Figures for the diversity study use the following reference numbers to identify components of each mixture. With TF=tall fescue, BFT=birdsfoot trefoil, RC=red clover, A=alfalfa, OG=orchardgrass, T=timothy, SB=smooth bromegrass, and BB=big bluestem.

Tall Fescue Base ref. nos.             Smooth Bromegrass Base ref. nos.
--------------------------             --------------------------------
1)  TF + 120 lb N/acre                 9) SB + 120 lb N/acre
2)  TF + 0 N/acre                     10) SB + 0 N/acre
3)  TF + BFT                          11) SB + BFT
4)  TF + RC                           12) SB + RC
5)  TF + A +BFT                       13) SB + A + BFT
6)  TF + A + BFT + RC                 14) SB + A + BFT + RC
7)  TF + A + BFT + RC + OG + T        15) SB + A + BFT + RC + OG + T
8)  TF + SB + A + BFT + RC + OG + T   16) SB + A + BFT + RC + TF + OG 
                                                              + T + BB

Legume content of the diversity study pastures remained around 30% for most mixtures throughout 1999 (Figure 1). Smooth bromegrass-based pastures tended to have a higher legume presence than did tall fescue-based pastures, likely due to the less competitive nature of bromegrass. In contrast the legume component of the stocking rate study pastures, which had been near 20 to 30% in 1997 and 1998, declined significantly in 1999 (Figure 2). While some of this may be attributable to the dry conditions of 1999, we believe that most is due to lack of seed production and reestablishment of red clover and birdsfoot trefoil in this study. The lack of seed production, particularly at the higher stocking rates, is directly attributable to the inadequacy of a 22-day rest period to allow natural reseeding.

The real difference between the studies becomes apparent when numbers of legume seedlings present in the spring are compared. In both 1999 and 2000, total legume seedling per square foot was less than .5 across all stocking rate treatments. In contrast most legume mixtures in the diversity study had at least four legume seedlings per square foot in 2000 (Figure 3). Similar numbers were recorded in 1999 for both studies. Observation of other pastures both at FSRC and on commercial farms support the claim for higher legume content and greater seedling recruitment for pastures managed on flexible rotations.

Perennial grasses seem to be much more tolerant of calendar rotations than are legumes. In both of these studies the relative amounts of tall fescue, orchardgrass, and timothy are similar. The one exception is at very low and very high stocking rates where tall fescue seems to be better adapted. At moderate stocking rates, the common cool-season grasses appear to do equally well in calendar or flexible rotations.

In summary, mixed pastures are very dynamic in terms of botanical composition. Inflexible management will select for those species which are most adaptive to the particular grazing regime imposed. Maintenance of legumes in pastures by natural reseeding requires variable rest period length through the season. Short rest periods early in the season combined with aggressive grazing will reduce grass competition while longer lat summer rest periods facilitate seed production.