Stocking Method Affects Plant Responses of Pensacola Bahiagrass Pastures

R. Lawton Stewart, Jr., Graduate Student, Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg 24061-0306; Jose C. B. Dubeux, Jr., Professor, Depto. de Zootecnia/UFRPE, Av. Dom Manoel de Medeiros, S/N, Dois Irmãos, 52171-900, Recife-PE, Brazil; Lynn E. Sollenberger, Professor, Department of Agronomy, University of Florida, Gainesville 32611-0300; João M. B. Vendramini, Assistant Professor, Department of Soil and Crop Science, Texas A&M University, Overton 75684; Sindy M. Interrante, Graduate Student, Department of Agronomy, University of Florida, Gainesville 32611-0300

Abstract

Stocking method is an important management tool that may affect plant responses, but there are few studies that have evaluated these responses under a wide range of stocking methods. The objective of this research was to determine the effect of different stocking methods on herbage accumulation and nutritive value. Treatments were four rotational stocking strategies differing in length of grazing period (1, 3, 7, and 21 days) but with the same resting period of 21 days, and one continuous stocking treatment of ‘Pensacola’ bahiagrass (Paspalum notatum Flügge) pastures. Herbage accumulation did not differ among rotational strategies, but rotational stocking lead to greater herbage accumulation than continuous stocking (62 versus 37 lb/acre of DM per day). Herbage crude protein (CP), P, and in vitro organic matter digestion (IVOMD) were not affected by grazing method (continuous versus rotational) or length of grazing period (rotational treatments) in more than 1 out of 3 years. The results suggest that rotational stocking, across a range of lengths of grazing period, promotes greater herbage accumulation than continuous stocking but there is little variation among grazing methods in herbage nutritive value.

Introduction

Stocking method may affect plant and animal responses. Blaser (2) summarized the results of several grazing experiments with cool-season grasses and legumes and concluded that individual animal performance was similar on rotationally and continuously stocked pastures, but animal output per unit of pasture area was greater with rotational stocking due to greater stocking rate. The author suggested that stocking rate was greater in the rotationally stocked pastures due to greater herbage growth, a function perhaps of greater leaf area or reserve carbohydrate status compared to continuously stocked pastures. Bertelsen et al. (1) observed that rotational stocking of alfalfa (Medicago sativa L.)-orchardgrass (Dactylis glomerata L.)-tall fescue (Festuca arundinacea Schreb.) pastures increased beef production per acre by increasing stocking rate without decreasing daily gain or diet quality compared with a continuous stocking system. In contrast, Grant et al. (5) concluded that rotational stocking of perennial ryegrass (Lolium perenne L.) offered no advantage over simpler continuous stocking systems, provided that sward conditions were controlled.

Ortega et al. (8) showed that rhizoma peanut (Arachis glabrata Benth) pastures grazed frequently, to simulate continuous stocking, yielded less than rotationally stocked pastures, but there are few studies that have evaluated the effect of a wide range of stocking methods on plant growth and nutrient concentration in grazed pastures. Thus, the objective of this study was to evaluate the effect of different stocking methods on ‘Pensacola’ bahiagrass (Paspalum notatum Flügge) herbage accumulation and herbage CP, P, and IVOMD.

Procedures for Assessing Effect of Stocking Methods on Herbage Response

A grazing experiment was performed at the Beef Research Unit, northeast of Gainesville, FL (29°43'N) on Pensacola bahiagrass pastures. Soils were mainly of the Pomona and Smyrna series of sandy Spodosols with average pH of 5.9. Mehlich-I extractable soil P, K, Ca, and Mg average concentrations at the beginning of the experiment were 5.3, 28, 553, and 98 ppm, respectively.

Treatments were imposed in 2001, 2002, and 2003, and consisted of continuous stocking and four rotational stocking strategies differing in length of the grazing period (1, 3, 7, and 21 days), all with a 21-day rest period between grazings. Treatments were replicated twice using a randomized complete block design. For all treatments, stocking rate was 1.7 animal units (AU; 1 AU = 1100 lb) per acre and N fertilization was 320 lb/acre per year. These high levels of stocking rate and N fertilization were used because the pastures were part of a larger study (3) evaluating the effect of a range of management intensities on performance of bahiagrass pastures. Because drought delayed the start of the grazing season in 2001, N at 240 lb/acre was applied that year. Nitrogen fertilization was split-applied in three equal applications (80 lb/acre) in 2001 (June, July, and August) and four equal applications in 2002 and 2003 (April, June, July, and August). Phosphorus (15 lb/acre) and K (60 lb/acre) were applied to all treatments in April each year, and in July 2002 there was a second application of the same amount of P and K. Sulfur was also applied in 2002 at a rate of 27 lb/acre.

Each experimental unit for the rotational treatments consisted of one paddock of a 2.5-acre system because only plant responses to grazing treatment were measured. Paddock sizes were approximately 0.11, 0.31, 0.62, and 1.24 acres for the 1-, 3-, 7-, and 21-day grazing periods, respectively. Thus, if all paddocks for these four grazing-period treatments were present, there would be 22, 8, 4, and 2 paddocks, respectively. At the beginning of each grazing season, crossbred (Angus × Brahman) yearling heifers were arranged in groups of five or six animals of equal total liveweight per group. A group was assigned to each rotational treatment for the designated length of grazing period. In the continuous stocking treatment (0.83-acre pastures), two heifers were assigned to each experimental unit and remained there during the length of the grazing season (112 days in 2001 and 168 days each in 2002 and 2003). The liveweight of the two heifers on continuous pastures was one-third that of an individual group assigned to a rotational treatment because continuous treatment pasture area was one-third the total area of each rotational system (0.83 versus 2.5 acres). Artificial shade (10 ft × 10 ft) was provided on each experimental unit, and cattle had free-choice access to water and a mineral mixture.

In the rotational treatments, herbage accumulation and herbage mass were estimated by taking 30 disk (387-inch² aluminum disk) settling heights in each pasture at the initiation and at the end of each grazing period (Fig. 1). Herbage mass at the initiation (pre-graze herbage mass) and at the end (post-graze herbage mass) of the grazing period was calculated using the average disk settling height for the 30 observations per pasture and regression equations obtained from double sampling (3). Daily herbage accumulation was calculated by subtracting the post-graze herbage mass of the previous grazing cycle from pre-graze herbage mass of the current cycle and dividing the result by the number of days between measurements (21 for all rotational treatments). In the continuous treatment, herbage mass was determined by taking 30 disk meter readings per experimental unit at each evaluation date (every 14 days). Because animals were present on the continuously stocked pastures during the entire grazing season, a cage technique (Fig. 2) was used to quantify herbage accumulation (3,11). Disk heights were measured at six sites per pasture, cages placed over the sites, and disk settling heights measured at the same sites 14 days later. Change in herbage mass divided by 14 gave daily herbage accumulation. Chemical analyses were conducted on hand-plucked samples (Fig. 3) taken pre-graze (rotational treatments) or biweekly (continuous treatment) and included CP, P, and IVOMD determinations (4,7). Samples from rotational treatments were severed at the post-graze stubble height of recently grazed pastures, while in continuous pastures the top 2 inches of forage was removed. Twenty sites per experimental unit were sampled and composited before drying and analysis.


Fig. 1. Double sampling procedure using the disk settling height as the indirect measurement for herbage mass determinations. A portable shade structure is in the background.		Fig. 2. Cages used to estimate herbage accumulation in continuously stocked pastures.

Fig. 3. Hand-plucked sampling of bahiagrass for determination of chemical composition.

Statistical analyses were performed using Proc Mixed of SAS (SAS Institute, Inc., Cary, IN). Data averaged across evaluations within each grazing season were used for analyses. Orthogonal polynomial contrasts were performed to compare rotational strategies, and single degree of freedom contrasts were used to compare the continuous treatment with the average of the four rotational treatments.

Herbage Mass and Accumulation Responses to Stocking Method

Pre-graze and post-graze herbage mass did not vary greatly among rotational treatments and averaged 3350 lb/acre (SE = 220) and 1880 lb/acre (SE = 103), respectively. Average herbage mass for the continuous treatment was 2480 lb/acre (SE = 297) across the three grazing seasons.

Rotationally stocked pastures had similar herbage accumulation rates among treatments, but across the 3 years accumulation rate for the four rotational treatments averaged 62 lb/acre of DM per day compared to 37 lb/acre of DM per day for the continuous treatment (P = 0.002) (Table 1). Defoliation interval likely played a role in the herbage accumulation response. Parsons and Penning (9) observed an increase in herbage growth when the duration of regrowth for perennial ryegrass was extended from 12 to 13 to 19 to 23 days. In the current study, the period between grazing bouts at a given patch in the continuous treatment likely was not long, and probably considerably less than the 21-day rest period for the rotational treatments. This difference likely accounted in part for the herbage accumulation response observed.

Table 1. Herbage accumulation rates on rotationally stocked
bahiagrass pastures with different grazing periods (rest period
of 21 days for all) and under continuous stocking during
2001-2003. Data are averages across 3 years.

Treatment	Herbage accumulation (lb of DM per acre per day)
Rotational
1 day	58
3 days	61
7 days	64
21 days	67
Polynomial contrasts^x	P > 0.10
Continuous	37
Contrast rotational vs. continuous (P value)	0.002
SE	3

^x Polynomial contrasts (linear, quadratic, and cubic) for length
of grazing period effect for rotational treatments.

Seasonal patterns in herbage accumulation were compared, and for ease of presentation only the data for the 1- and 7-day rotational treatments and the continuous treatment will be presented. Stocking method interacted with month for herbage accumulation (Fig. 4). Interaction occurred because in June, herbage accumulation was similar among stocking methods but as the grazing season progressed the continuous stocking treatment showed lesser herbage accumulation rate compared to the 7-day treatment. The 1-day treatment was similar to the 7-day treatment throughout the experiment, and showed greater herbage accumulation than the continuous treatment in August and September (Fig. 4). The continuous treatment showed greater herbage accumulation in June and July compared to the other months.

Fig. 4. Seasonal herbage accumulation of Pensacola bahiagrass pastures as affected by stocking methods. Treatments were rotational stocking with 1- and 7-day grazing periods (21-day rest period) and continuous stocking. Data are monthly averages across 3 years. Means within a month followed by the same letters are not different (P > 0.05).

Herbage accumulation rate increased yearly from 2001 to 2003 (46, 54, and 71 lb/acre/day, respectively) perhaps due in part to the increase in soil nutrient concentration associated with intensive management. Soil P, for example, averaged 5 ppm at the beginning of the experiment in 2001 and after 3 years of grazing it averaged 10 ppm across treatments (3). Soil K also increased from 28 ppm in 2001 to 108 ppm in 2003 at the 0- to 3-inch soil depth (3). Lower rainfall in 2001 (39.7 inches) when compared to the 30-year average (52.8 inches) and the other experimental years (48.7 inches in 2002 and 53 inches in 2003) also was important in the yearly trends observed.

Herbage Nutritive Value Responses to Stocking Method

Crude protein. There was a treatment × year interaction for CP, with values generally increasing after the first experimental year (Table 2). Lower N rate in 2001 than 2002 and 2003 for all treatments likely explains this response. In 2001, a linear increase in CP concentration occurred with increasing grazing period, but no significant effect was observed in the following years. Continuous stocking did not differ from rotational stocking in terms of plant CP concentration. Values observed in this experiment were above the average of 857 bahiagrass samples collected from producer pastures in nine counties throughout central Florida, which had an average of 10.9 ± 2.7% CP for low yielding bahiagrass and 9.8 ± 1.9% CP for high yielding bahiagrass (10). The high N fertilization used in the present experiment (320 lb of N per acre per year) likely explains that observation, as producers are most often applying less than 100 lb of N fertilizer per acre. Stocking method interacted with month affecting CP concentration (Fig. 5). Interaction occurred because CP was greater for the 7-day treatment than for the 1-day or continuous treatments in June but not in the other months.

Table 2. Crude protein concentration in hand-plucked herbage from
rotationally stocked bahiagrass pastures with different grazing periods (rest period of 21 days for all) and from continuously stocked pastures during
2001-2003.

Treatment	Year
Treatment	2001	2002	2003
Rotational	Crude protein concentration (%)
1 day	12.6 B^x	14.3 A	14.4 A
3 days	13.0 C	15.2 A	13.8 B
7 days	12.9 B	14.5 A	13.1 B
21 days	13.4 C	15.0 A	14.1 B
Polynomial contrasts^y	L, P = 0.03	P > 0.10	P > 0.10
Continuous	12.5 B	14.1 A	14.7 A
Contrast rotational vs. continuous (P value)	0.27	0.16	0.13
SE	0.4

^x Means followed by the same letter within a row do not differ (P > 0.05) by the SAS least square mean test (PDIFF).

^y Polynomial contrast for length of grazing period effect for rotational treatments; L = linear.

Fig. 5. Seasonal crude protein concentration in hand-plucked samples of Pensacola bahiagrass pastures as affected by stocking methods. Treatments were rotational stocking with 1- and 7-day grazing periods (21-day rest period) and continuous stocking. Data are monthly averages across 3 years. Means within a month followed by the same letters are not different (P > 0.05).

Phosphorus. There was a treatment × year interaction for herbage P. In 2002, P concentration decreased linearly with increasing grazing period, but there was no effect in 2001 and 2003. Herbage P concentration increased from 2001 to 2003 (Table 3). Phosphorus build up due to P fertilization and also P cycling to more available forms under high stocking rates could explain increasing herbage-P concentrations from the beginning to the end of the experiment. The increase from year to year was greatest from 2002 to 2003, and this may reflect the additional P application that occurred during July 2002. Herbage-P concentration in continuously stocked pastures was not different from rotationally stocked pastures during any of the 3 years.

Table 3. Phosphorus concentration in hand-plucked herbage from rotationally stocked bahiagrass pastures with different grazing periods (rest period of 21 days for all) and from continuously stocked pastures during 2001-2003.

Treatment	Year
Treatment	2001	2002	2003
Rotational	P concentration (%)
1 day	0.15 C^x	0.19 B	0.25 A
3 days	0.15 C	0.19 B	0.24 A
7 days	0.14 C	0.17 B	0.24 A
21 days	0.15 C	0.18 B	0.25 A
Polynomial contrasts^y	P > 0.10	L, P = 0.05	P > 0.10
Continuous	0.15 C	0.18 B	0.24 A
Contrast rotational vs. continuous (P value)	0.87	0.48	0.39
SE	0.004

^x Means followed by the same letter within a row do not differ (P > 0.05) by the SAS least square mean test (PDIFF).

^y Polynomial contrast for length of grazing period effect for rotational treatments; L = linear.

In vitro digestion. There was a treatment × year interaction for IVOMD (Table 4). In general, digestibility increased from 2001 to 2003, and treatment differences were more pronounced in 2003. In 2003, IVOMD decreased linearly with increasing grazing period, with no similar effect observed in 2001 and 2002. Higher stocking densities in the short grazing periods may have promoted a more uniform defoliation, and therefore, more uniform regrowth and less occurrence of mature, undefoliated herbage. The contrast between rotational treatments and continuous stocking showed higher IVOMD for the rotational treatments (P < 0.02) in 2001 but not in 2002 and 2003. Mathews et al. (6) compared rotational stocking with short- and long-grazing periods with continuous stocking of ‘Callie’ bermudagrass [Cynodon dactylon (L.) Pers.]. They did not observe differences in heifer daily weight gain and concluded that was partially due to little variation in IVOMD among stocking methods.

Table 4. In vitro organic matter digestion (IVOMD) in hand-plucked
herbage from rotationally stocked bahiagrass pastures with different
grazing periods (rest period of 21 days for all) and from continuously
stocked pastures during 2001-2003.

Treatment	Year
Treatment	2001	2002	2003
Rotational	IVOMD (%)
1 day	48.0 C^†	55.2 B	60.7 A
3 days	47.2 C	57.3 B	59.8 A
7 days	48.3 B	51.0 A	52.1 A
21 days	49.7 C	54.7 A	52.4 B
Polynomial contrast^y	P > 0.10	P > 0.10	L, P = 0.0001
Continuous	43.6^xC	52.9 B	55.8 A
Contrast rotational vs. continuous (P value)	0.02	0.13	0.83
SE	1.0

^x Means followed by the same letter within a row do not differ (P > 0.05) by the SAS least square mean test (PDIFF).

^y Polynomial contrast for length of grazing period effect for rotational treatments; L = linear.

Summary and Conclusions

Herbage accumulation was lower in continuously stocked pastures when compared to rotational ones, but there were no differences among rotational strategies. Greater herbage accumulation in the rotational treatments was probably due in part to longer intervals between defoliations than those in the continuously stocked pastures. Herbage nutritive value (CP, P, and IVOMD) increased after first experimental year, but it was affected by grazing method (continuous versus rotational) or length of grazing period (rotational treatments) in only 1 out of 3 years.

Under the conditions of this study, the rationale for using rotational versus continuous stocking is primarily to increase herbage production not nutritive value. This conclusion supports previous findings (1,2,6) of little effect of grazing method on individual animal performance but an advantage in average stocking rate for rotational versus continuous stocking.

Acknowledgment

This research was sponsored in part by USDA/CSREES Tropical and Subtropical Agricultural Research Program Grant 34135-12348.

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