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Published in Crop Sci. 43:1789-1796 (2003).
© 2003 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

FORAGE & GRAZING LANDS

Developmental Morphology of Smooth Bromegrass Growth Following Spring Grazing

B. A. Brueland*, K. R. Harmoney, K. J. Moore, J. R. George and E. C. Brummer

Dep. of Agronomy, Iowa State Univ., Ames, IA 50011-1050

* Corresponding author (bbruelan{at}iastate.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The timing of initial spring grazing of smooth bromegrass (Bromus inermis Leyss.) pastures affects regrowth potential and subsequent forage yield and quality. This study was conducted to quantify the effects of the timing of the initiation of spring grazing on these factors. A field experiment was conducted at the Rhodes Research and Demonstration Farm in central Iowa from April through July in 1996 and 1997. Treatments were an undefoliated control and four initial dates of defoliation that were applied weekly, beginning when one fully collared leaf per tiller was present. Defoliation was done using two mature cattle (Bos taurus) to quickly (12–24 h) graze the sward to a height of 5 to 10 cm. Regrowth was sampled weekly for 8 wk, staged, and analyzed for forage quality. Early grazing of smooth bromegrass pastures did not delay sward development. Initiation of grazing at the later dates caused short delays in both development and the ability to regrow rapidly, but allowed utilization of a larger quantity of forage for the initial grazing event, with minimally decreased forage quality. Sward development differed between years. Tiller density and mean stage count did not differ among treatments during the 7 wk after defoliation in 1996 but did so in 1997. However, by the end of 8 wk, development was similar for all treatments. On the basis of this study, initiation of grazing when one fully collared leaf per tiller was present, followed by an adequate recovery period before beginning rotational grazing would give producers an early start on pasturing with limited impact on sward yield and health.

Abbreviations: CP, crude protein • GDD, growing degree days • IVDMD, in vitro dry matter digestibility • MSC, mean stage count • NDF, neutral detergent fiber • TNC, total nonstructural carbohydrates


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE OPTIMUM TIMING of initiation of grazing in the spring of each year is difficult to determine. Depending on environmental conditions and developmental stage of the sward, the timing of grazing initiation may alter many characteristics of a forage stand, including regrowth potential, forage yield, forage quality, and long-term stand health. Both temperature and precipitation must be considered when determining when to begin grazing. Environmental conditions following defoliation play a large role in the regrowth of grasses (Davidson and Milthorpe, 1965). For example, Sullivan and Sprague (1949) determined that high temperatures caused rapid usage of stored fructosans and actually slowed leaf growth in perennial ryegrass (Lolium perenne L.).

Regrowth of smooth bromegrass occurs from axillary buds and leaf meristems when apical meristems are removed (Paulsen and Smith, 1969; Casler and Carlson, 1995). Yield and tiller dynamics following defoliation are a function of the stage of development at the time of defoliation (Harrison and Romo, 1994). Regrowth following defoliation at early growth stages is much quicker than if the herbage is removed during internode elongation when axillary buds are slow to activate (Eastin et al., 1964; Paulsen and Smith, 1968, 1969; Carlson and Newell 1985). Removal of apical buds by early grazing may actually thicken a sward if previously inactive buds are activated, and new aerial tillers are formed from rhizomatous buds. Eastin et al. (1964) reported that smooth bromegrass ceased producing new tillers by the jointing stage and did not resume again until postanthesis unless seed heads and the associated apical dominance were removed. Early grazing would delay this cessation of tillering by keeping the sward in a prejointing stage for a longer period of time.

Parsons et al. (1988) and Frank et al. (1993) recommended defoliation based on sward development rather than on fixed time intervals as a method to maximize development of tillers. A producer must have an easy, reliable system available to accurately determine sward developmental stages and then be able to apply this information to sward management. Moore et al. (1991) described a system utilizing mean stage count (MSC) that describes five primary growth stages in perennial forage grass development: germination, vegetative, elongative, reproductive, and seed ripening. These categories are further classified into several substages. This system could benefit a producer when quantifying factors related to a forage sward and making management decisions, such as those involving grazing times.

George and Obermann (1989) used early season defoliation of switchgrass (Panicum virgatum L.) to utilize high quality early growth and to delay adverse maturity effects on forage quality. Their short harvest period of high quality forage was realized while improving forage quality later in the grazing season. Forage quality of smooth bromegrass stems and leaves has been shown to be equal in in vitro dry matter digestibility (IVDMD) during preheading stages, but stems decline in IVDMD at more than twice the rate of leaves postheading (Pritchard et al., 1963; Smith 1973; Buxton and Marten, 1989; Sanderson and Wedin, 1989). Grazing the early, leafy growth of smooth bromegrass may increase total seasonal forage quality utilization. Increasing plant maturity decreases plant protein concentration (Fick and Onstad, 1988; Minson, 1990) and increases neutral detergent fiber (NDF). Early grazing should provide a higher crude protein (CP) content and lower NDF in the grazed forage. Increases in lignin, cellulose, and hemicellulose also occur with maturity and are the primary reason for lower digestibility of more mature forages (Casler, 1987).

Smooth bromegrass stores primarily short-chain fructosans as the major carbohydrate reserve (Grotelueschen, 1966; Smith, 1981), and removal of top growth has been shown to increase utilization of these stored carbohydrates (Okajima and Smith, 1964). Removal of top growth early in the spring before sufficient photosynthetic area accumulates may have an impact on the regrowth of smooth bromegrass as it must utilize stored carbohydrates to initiate new shoot material. Available carbohydrate levels in crown tissue decrease as spring growth initiates and following defoliation (Smith 1962; Eastin et al., 1964). Buwai and Trlica (1977) reported significantly higher levels of carbohydrate reserves in undefoliated western wheatgrass [Agropyron smithii Rydb. (= Pascopyrum smithii (Rydb.) A. Love] than in plants that had been defoliated.

The objective of this study was to determine the effects of the timing of initial grazing of smooth bromegrass on subsequent growth and development, root total nonstructural carbohydrates (TNC), tiller density, and forage quality.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The experiment was conducted at the Rhodes Research and Demonstration Farm near Rhodes, IA (41°52' N, 93°10' W), in 1996 and 1997. The soil was a Downs (fine-silty, mixed, superactive, mesic Mollic Hapludalfs). Each year, 10 paddocks (5 by 20 m) were arranged in an established stand of smooth bromegrass within a larger, uniform pasture area. The experiment was located on a different but adjacent site in the second year. Ammonium nitrate was applied at a rate of 90 kg N ha-1 in early April before grazing both years.

Treatments consisted of an ungrazed control and four initial dates of grazing that were applied weekly beginning when one fully collared leaf per tiller was present (Table 1). Paddocks were defoliated using two mature cows per paddock to graze the sward to a height of 5 to 10 cm. Defoliation was accomplished within 24 h for all treatment dates. This grazing technique has been used for rapid defoliation where grazing of immediate regrowth is not desired (Mislevy et al., 1982; McCartney and Bittman, 1994) and recommended over clipping treatments by Sollenberger and Cherney (1995).


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  		Table 1. Initial grazing dates, mean stage count (MSC), accumulated growing degree days (GDD) for 1996 and 1997.

 
Regrowth was sampled at weekly intervals for 8 wk following defoliation. Samples were cut from two subplots (2.5 by 5 m) within each paddock by clipping all forage within a quadrat (0.3 by 0.6 m) to ground level. Samples were hand separated and staged according to the Nebraska method (Moore et al., 1991) for quantifying growth stages of perennial forage grasses. Samples were then recomposited, dried in a forced-air oven at 60°C, and ground in a cyclone mill through a 1-mm mesh screen (Udy Manufacturing Corporation, Fort Collins, CO).

Each sample was analyzed to determine concentrations of CP, NDF, and in vitro dry matter digestibility. Crude protein was determined using the Kjeldahl N value for each sample and multiplying N concentration by 6.25 as described by Bremmer and Breitenbeck (1983). In vitro dry matter digestibility was determined using the NC-64 direct acidification system (Marten and Barnes, 1980). Neutral detergent fiber was determined using the ANKOM (ANKOM Technology Corp., Fairport, NY) method described by Vogel et al. (1999).

Root samples were also collected at each sampling date by removing smooth bromegrass root material from the ground. Soil samples (100 cm2) were extracted to a depth of 15 cm, soil washed from the roots, and the upper 7.5 cm of root material was arbitrarily separated from the samples. These samples were oven dried, ground through a cyclone mill (1-mm screen), and analyzed for TNC using the method described by Guiragossian et al. (1977).

Weather data were obtained from the Iowa State University Agronomy Department meteorology division as recorded at Colo, IA. Accumulated growing degree days (GDD) were calculated using a base temperature of 5°C (Buxton and Marten, 1989); at temperatures below the base it was assumed that growth rates were very low. The GDD equation used was

Statistical analyses of treatment and sampling date comparisons were conducted using the General Linear Models (GLM) procedure of the Statistical Analysis System (SAS Institute, 1991). The experimental design was a randomized complete block design with two replications. Data were analyzed using a combined analysis with years and blocks within years random, and treatments fixed (McIntosh, 1983). Sample dates within years were analyzed using a split-plot univariate analysis (Steel and Torrie, 1997). Mean comparisons were made using an F-protected LSD (Steel and Torrie, 1997). All tests of significance were made at the 0.05 probability level unless otherwise specified. Gompertz predication equations were estimated using the nonlinear least squares algorithm of DataFit (Oakdale Engineering, 1999).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Environmental Conditions
Environmental conditions following defoliation play an important role in the regrowth of grasses (Davidson and Milthorpe, 1965), with temperature and precipitation being the main factors. Precipitation and temperatures that occurred during this study period were similar between years and were adequate for normal growth of smooth bromegrass treatment responses.

Rainfall throughout the study period was more evenly distributed in 1996 than in 1997 (Fig. 1). Precipitation events were larger but less frequent in 1997, resulting in a precipitation total 10% greater than in 1996. Total GDD accumulation did not differ between years (Fig. 2). However, total values for each study period were somewhat lower than the historical average (1951-1997) of 1838. Weekly GDD accumulations were virtually identical, with the exception of periods two (Days 99–106) and seven (Days 134–141), where 1997 was slightly lower. Thus, even though temperature, or GDD accumulation, has a large impact on development of festucoid grasses, it probably did not adversely influence treatment responses between years in this study.



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  		Fig. 1. Precipitation for 1996 and 1997.

 


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  		Fig. 2. Growing degree day accumulation for 1996 and 1997 with means from 1951-1997.

 
Developmental Morphology
Growth and Development
The earliest defoliation treatments delayed development slightly, but may also have increased tillering. Tillering increases, combined with the low total accumulation of GDD in the early weeks of the study (Table 1) as compared with the later weeks, and the effects of photoperiod on elongation, kept the MSC below 1.5 for the first 4 wk each year regardless of treatment (Fig. 3).



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  		Fig. 3. Mean stage count for 1996 and 1997. SE = 0.12

 
Yield
Control treatment yields increased during each of the first 7 wk in both years with a decline attributable to maturation effects in Week 8 of each year (Fig. 4). Dry matter yields were initially higher in 1996 than 1997, which is consistent with the higher tiller density during the first 4 wk of 1996 compared with 1997 (Fig. 5). Weeks 4 through 8 showed higher yields in 1997 than 1996, again consistent with higher number of tillers during that period in 1997. Control yields were used as the baseline for individual treatment yields which are not presented in this paper.



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  		Fig. 4. Yield of control treatments for 1996 and 1997 fit with a Gompertz prediction equation.

 


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  		Fig. 5. Tiller density and stage distribution for control treatments for 1996 and 1997. SE = 213.6.

 
Mean Stage Count
Rapid accumulation of GDD and frequent precipitation resulted in ideal conditions for rapid development (Harrison and Romo, 1994). Lack of precipitation before and following the fourth treatment in 1997 may have caused its lower MSC than the other treatments (Fig. 3). Dry weather in 1997 likely depressed regrowth and MSC in all treatments compared with 1996. Lack of environmental effects were likely the reason that MSC did not differ between treatments in 1996 (Table 2).


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  		Table 2. Analysis of variance with mean squares for crude protein (CP), neutral detergent fiber (NDF), in vitro dry matter digestibility (IVDMD), mean stage count (MSC), tiller density (TD), and total nonstructural root carbohydrates (TNC) for smooth bromegrass regrowth in 1996 and 1997.

 
Growing degree day accumulation was nearly the same in 1996 and 1997, but the MSC for 1997 was greater for most of the sampling period compared with 1996. This study has shown that although GDD accumulation was consistent between years (Table 1), there were differences in MSC. Figure 6 shows the relationship between changes in MSC and accumulation of GDD. The disparity in MSC between years may have been caused by factors not quantified in this study, such as tiller initiation the prior fall, potential winterkill of initiated tillers during the prior winter, or moisture conditions. The control (ungrazed) data shows that MSC was increasing at a faster rate in 1997 than in 1996 until a period of lower rainfall around 600 GDD. Fitting this data with a Gompertz prediction equation shows that because of the influence of other environmental factors, MSC cannot be completely predicted from accumulated GDD, although MSC may be used to predict forage quality (see below).



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  		Fig. 6. Mean stage count control treatment data for 1996 and 1997 fit with a Gompertz prediction equation.

 
Tiller Density
The initial number of vegetative tillers in the control treatment was higher in 1996 than 1997 (Fig. 5). A cool period during Week 1 of 1997 may have been the cause for the initial decline in tillers between Weeks 1 and 2. Elongating tillers appeared at Week 3 and reproductive tillers at Week 4 in both years. Tiller numbers were at the same density by Week 4 in both years due to a large loss of tillers in the first 4 wk of 1996. The trends were consistent between years when examining transitions from vegetative to elongative and into reproductive stages with both years ending with nearly the same distribution and frequency of tillers.

Total Nonstructural Carbohydrates
Removal of shoot material increases consumption of stored carbohydrates (Okajima and Smith, 1964) until shoot regrowth is able to supply sufficient photosynthate to meet the plant's needs. Adequate rest periods following grazing will give the sward time to accumulate additional carbohydrate reserves. If regrazed before adequate leaf area was regained, the root would eventually deplete its stored reserves, increasing plant mortality.

Total nonstructural carbohydrates were different between harvest dates but not different between treatments. Throughout this study adequate reserves were maintained and no effects on season-long growth were observed.

Forage Quality
Forage quality of grass species declines with maturity (Nelson and Moser, 1994) usually due to maturation effects attributed to both the postheading decline in the leaf:stem ratio (Ugherughe, 1986) and decreases in quality of the stem itself. Declines in forage quality may be estimated using the Nebraska MSC method (Moore et al., 1991), as demonstrated through the correlations shown below.

Crude Protein
Crude protein levels were not different between years but were different between treatments (Fig. 7). Protein levels declined linearly with day of year for all treatments. The decline in CP levels is attributable to increased forage maturity. Crude protein levels 8 wk after grazing for each treatment were comparable with the exception of Treatment 1 in 1997. Crude protein in Treatment 1 in 1996 and Treatments 2 and 3 in 1997 increased slightly the week following grazing, likely because new, immature regrowth appeared after defoliation.



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  		Fig. 7. Crude protein for 1996 and 1997. SE = 1.87.

 
Overall CP levels across both years was linearly related with MSC (R2 = 0.87) (Fig. 8). Initiation of grazing at the earliest dates, when swards had the lowest MSC, allowed utilization of the highest protein forage throughout the period evaluated in this study.



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  		Fig. 8. Crude protein as a function of mean stage count for 1996 and 1997.

 
In Vitro Dry Matter Digestibility
In vitro dry matter digestibility differed between years (P < 0.01) and decreased linearly with time in 1996 and 1997 (Fig. 9). Slower declines in IVDMD were shown in 1997 during the period from 1 to 3 wk posttreatment for Treatment 1, and 5 to 8 wk posttreatment for Treatment 4, than for other treatments.



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  		Fig. 9. In vitro dry matter digestibility for 1996 and 1997. SE = 1.43.

 
In vitro dry matter digestibility normally decreases with maturity and MSC. Leaves of cool season grasses decline an average of 6.6 g kg-1 in IVDMD for each degree Celsius rise in temperature (Wilson and Minson, 1980). Temperatures increased during the experimental period from early spring into summer. The linear relation of IVDMD and MSC was R2 = 0.83 (Fig. 10).



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  		Fig. 10. In vitro dry matter digestibility as a function of mean stage count for 1996 and 1997.

 
Neutral Detergent Fiber
Neutral detergent fiber is the fraction of a forage that contains most of the fiber, and consists mainly of cellulose, hemicellulose, lignin, and heat-damaged proteins. Neutral detergent fiber is positively correlated with gut fill in ruminants; providing low NDF forage should increase energy availability and rate of passage (Buxton and Mertens, 1995), increasing the rate of gain in grazing animals.

Neutral detergent fiber increased uniformly with time in 1996 with differences found between years (P < 0.01) and treatments (P < 0.01) (Fig. 11). Although weather was similar between years, I would hypothesize weather factors did indeed influence the year to year interactions, and the treatment differences were caused by increasing maturity of each subsequent treatment at harvest. Treatment 3 in 1997 had a 2-wk period in which NDF did not increase (between 3 and 5 wk post treatment). The linear relation of NDF and MSC was R2 = 0.92, showing a strong relationship between NDF and MSC (Fig. 12).



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  		Fig. 11. Neutral detergent fiber for 1996 and 1997. SE = 1.37.

 


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  		Fig. 12. Neutral detergent fiber as a function of mean stage count for 1996 and 1997.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Initiation of grazing of smooth bromegrass at early dates did not impart morphological or developmental delays on the sward as a whole. Initiation of grazing at later dates temporarily delayed development, but compensatory recovery was rapid and within a few weeks, no parameters measured were significantly different. Initiation of grazing at later dates increased the amount of forage initially available for consumption at turnout. However, livestock rate of gain might be enhanced by grazing higher quality growth found in the early initiation treatments.

On the basis of this study, initiation of grazing when one fully collared leaf is visible, approximately MSC 1.2, followed by a sward rest period before rotational grazing begins would be desirable. Regrazing should be initiated after a sward has had time to replace used stored carbohydrates, but before maturation effects cause a large decline in forage quality. However, data to determine the optimum timing to initiate regrazing was not collected in this study. Grazing earlier than usual could limit the amount of stored forages needed to be fed.

This experiment demonstrated that forage quality could be predicted using the MSC of the sward as quantified using the Nebraska method of staging (Moore et al., 1991). Long-term effects of early grazing on future seasons were not evaluated. Although the earliest grazing initiation dates may not produce the largest amounts of forage yield initially, data presented here has shown that the quality of forage harvested the remainder of the season should not be adversely impacted.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Journal Paper No. J-19543 of the Iowa Agric. Exp. Stn., Ames, IA. Project No. 2899. Supported by Hatch Act and State of Iowa.

Received for publication September 17, 2001.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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Crop Science 2003 43: 1581. [Full Text]  




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