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Univ. of Florida, Range Cattle Res. and Educ. Center, Ona, FL 33865. I.V. Ezenwa, current address: Univ. of Florida, Southwest Florida Res. and Educ. Center, Immokalee, FL 34142
* Corresponding author (ivezenwa{at}ifas.ufl.edu)
Received for publication April 2, 2006.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMETHODS AND MATERIALSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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50% and delayed grazing in 1 of 4 yr. Forage production, nutritive value, and livestock gains on signalgrass equaled or exceeded those of bahiagrass, but poor cold tolerance, limited growth before May, and excessive growth in July are potential problems with signalgrass pasture production.
Abbreviations: ADG, average daily gain • BCS, body condition score • BW, body weight • DM, dry matter • IVOMD, in vitro organic matter digestion
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMETHODS AND MATERIALSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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40 000 ha of bahiagrass in the mid-1990s to tawny mole cricket (Scapteriscus vicinus Scudder) highlighted the need to identify other grasses with qualities similar to bahiagrass (Adjei et al., 2001).
Urochloa spp. (syn. Brachiaria spp.) have greatly increased productivity of grazing lands on the infertile, acid soils that cover up to 70 million ha in Brazil (Miles et al., 1996). They are high-yielding grasses with reasonable nutritive value. Creeping signalgrass, a highly stoloniferous species, is sown on 3% of that area where low soil fertility, poor drainage, and extensive management predominate (Santos Filho, 1996). It shares many of the desirable characteristics of bahiagrass: moderate forage production with low soil fertility, establishes from seed, and persists with frequent, close grazing. Although creeping signalgrass does not tolerate the wide range of soil conditions and temperatures as well as bahiagrass does, it is adapted to the wet, infertile soils of the warmer central and southern Florida, where the majority of cattle in the state are produced.
Creeping signalgrass was tested in clipping trials at the Range Cattle Research and Education Center (REC) at Ona (Hodges and Martin, 1975), and further south at the Southwest Florida REC at Immokalee (Mislevy and Everett, 1981). More recent mob-grazing trials at the Range Cattle REC confirmed earlier clipping trial results that creeping signalgrass was persistent, produced relatively high forage yield, and had similar nutritive value as bahiagrass (Mislevy et al., 1996). However, grazing tolerance and livestock production measurements have not been documented for creeping signalgrass in Florida.
The purpose of this experiment was to compare calf gain and cow weight changes in the 90-d periods before and after weaning, respectively, on creeping signalgrass and bahiagrass. An additional objective was to compare forage accumulation, mass, and nutritive value of the grasses.
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METHODS AND MATERIALS |
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TOPNOTESABSTRACTINTRODUCTIONMETHODS AND MATERIALSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Both grasses were stocked in May for the start of comparative evaluation. The start was based on resumption of growth in creeping signalgrass, which was more dependent on the rainy season than bahiagrass. An exception was 2001 when pastures of both grasses were stocked in late June to allow for signalgrass recovery following a winter freeze. Before the start of the trial in May of each year, bahiagrass had been grazed for 28 d as described below.
Each pasture was stocked with a randomly assigned group of five, pregnant crossbred (Brahman x British) cows and their calves (2.5 cow-calf pairs ha–1). Cow weights were balanced to assure that pastures had similar mean cow weights in May. Cattle were rotated weekly among four, 0.5-ha paddocks in each of the six, 2-ha pastures. In 2002 and 2003, creeping signalgrass was variably stocked by incrementally adding four additional, mature nonlactating cows to each of the three signalgrass pastures at the onset of the rainy season in mid-June to utilize rapid growth. Extra cows were removed in July. We used various criteria (rainfall, grass phenology, visual estimate of forage mass, etc.) for adding and removing cows.
Cows and calves were weighed (not shrunk) in May and in the first week of August when calves were weaned. Each group of five cows returned to their previously assigned pastures where they remained until the end of October when they were reweighed. Excess grass remaining in pastures was cut, baled, and removed.
Calf weights were adjusted for sex and mean age at the respective weigh dates. At May, August, and October weigh dates, a body condition score (BCS) was determined by two observers for each cow. Scores were visual evaluations based on a range of 1 to 9, with 1 = very thin cows and 9 = very fat cows (Herd and Sprott, 1986).
Forage accumulation, as defined by the Forage Grazing Terminology Committee (1992), was determined every 28 d from May to October by cutting forage to a 5-cm stubble height in four, wire mesh exclusion-cages (1.2 by 1.2 by 1.2 m) in each 2-ha pasture. Forage was weighed, and a subsample (0.5 kg) was dried at 60°C to constant weight for dry matter (DM) determination. Cages were moved to adjacent areas that were staged to a 5-cm stubble on the day of sampling. Forage mass was measured weekly from May to October on the day cattle were rotated into fresh paddocks. Two strips (6 by 0.45 m) were cut to a 5-cm stubble in each of six paddocks, and DM was determined as described above. Hand-plucked samples of grass, which simulated what cattle were eating, were taken in each paddock at each of the two locations where forage mass was determined. These samples were dried, ground, and analyzed for N and in vitro organic matter digestion (IVOMD) at the Forage Evaluation Support Laboratory at the University of Florida (Sollenberger and Fethiere, 2000). Total-herbage samples of a 5-cm stubble were taken from each paddock in mid-June after rainfall initiated a flush of growth and in October when cattle were removed from pasture. These were dried, ground, and analyzed as described above. Rainfall and temperature were measured at the climatological station located 0.8 km from the pasture area.
Ground cover in creeping signalgrass pastures was evaluated in April 2001 following a freeze in January. Percentage ground cover of live signalgrass was visually estimated in 10-, 0.5- by 2.0-m quadrats located along a transect in each of the 12, 0.5-ha signalgrass paddocks. In April 2003, this procedure was repeated along the same transect for evaluation of ground cover of creeping signalgrass, bahiagrass, common bermudagrass (Cynodon dactylon L.), vaseygrass (Paspalum urvillei Steud.), and smutgrass [Sporobolus indicus var. indicus (L.) R. Br.].
The six pastures formed three, randomized complete blocks each containing two grasses. Response variables were analyzed with a mixed linear model using the MIXED procedure (SAS Institute, 1999). For cattle responses, pasture means over five cows or calves were used in the analysis. The model for cattle responses included blocks, grasses (whole plots), year (subplots) and grass x year interaction as fixed effects with the block x grass interaction (error term for grasses) as a random effect. Forage accumulation was additive, that is, forage accumulation for May represented 28-d growth, while June represented 28-d growth in June plus that measured in May, etc. Forage mass, crude protein (N x 6.25), and IVOMD of hand-plucked samples were averaged over the four paddocks in a 28-d rotation (months) to provide a mean for each of the six pastures. The model for forage responses was the same as that for cattle responses, but months were sub-subplots, which were fixed effects. Differences between years and grass x month interactions were investigated with the PDIFF option (SAS Institute, 1999).
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RESULTS |
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TOPNOTESABSTRACTINTRODUCTIONMETHODS AND MATERIALSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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1.6°C (frost) from 22 Nov. 2000 to 19 Apr. 2001 with a minimum –5°C on 5 January, and signalgrass was severely injured. May to October 2002 and 2003 had more rainfall than the 62-yr mean, but preceding winter temperatures were relatively normal.
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Cattle
Cows
There was no difference between grasses for cow weight or BCS at the start of grazing in May of each year (Table 2). Year affected these responses, but there was no interaction with grass. The highest cow weights were recorded in 2002. At weaning in August, cow weight and BCS tended to be greater on creeping signalgrass than bahiagrass pastures. At the end of grazing in October, there was a grass x year interaction for cow weight but not BCS (Table 2). Cows from signalgrass pastures weighed more than cows from bahiagrass, with the exception of 2001 when the grazing season was shortened to allow creeping signalgrass recovery after the freeze. For creeping signalgrass, cow weight in October was affected by year while there was no year effect for bahiagrass. Cows grazing creeping signalgrass had higher BCS than cows grazing bahiagrass (Table 2). Year had a significant effect on cow BCS (Table 3). There was no difference in cow weights in 2001 and 2003, while cow weight in 2000 was the least (Table 3).
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Forage Accumulation and Mass
While there were no differences in forage accumulation between grasses at any month, there was a large monthly increase in forage accumulation of creeping signalgrass at the beginning of the rainy season that resulted in a grass x month interaction (P = 0.0001) (Fig. 1a
). Specifically, there was a 3120 kg DM ha–1 increase in forage accumulation of creeping signalgrass from 1370 kg DM ha–1 in June to 4490 kg DM ha–1 in July. The increase in bahiagrass accumulation between the same months was 1260 kg DM ha–1. Both grasses flowered during this period, and much of the DM was due to reproductive growth, especially for creeping signalgrass with a visually estimated 35 to 40% of the DM attributable to reproductive growth. Between August and October, month-to-month increases in accumulation were similar for the grasses.
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After July, much of the of the creeping signalgrass forage mass was residual stem that formed a stubble layer. During each 7-d long grazing period, cattle ate mostly leaves that had regrown on the stubble layer during the 21-d rest periods. Bahiagrass was entirely leaf, but much of the leaf material was senescent or dead by October.
Nutritive Value
There were grass x month interactions (P = 0.0001) for crude protein and IVOMD in hand-plucked samples. Crude protein in bahiagrass was higher than creeping signalgrass at every month (Fig. 2a
). Crude protein in both grasses increased from May to June then declined through September and rose in October.
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Total herbage samples of creeping signalgrass and bahiagrass in early June contained 119 and 139 g crude protein kg–1 and 550 and 499 g IVOMD kg–1, respectively. These samples were from rapid-growing plants and were entirely leaf. In October at the termination of grazing, crude protein in creeping signalgrass and bahiagrass averaged 48 and 74 g kg–1, and IVOMD was 371 and 311 g kg–1, respectively. These samples consisted mainly of stems for signalgrass and senescent or dead leaves for bahiagrass.
Ground Cover and Insect Pests
The 90% ground cover found at the start of the fourth yr was indicative of the good ground cover signalgrass maintained throughout the trial (Table 4). However, following the January 2001 freeze, signalgrass live-plant cover in April 2001 ranged from 13 to 86% and averaged 52% in the creeping signalgrass paddocks (data not shown). By late June 2001, creeping signalgrass ground cover had increased to 85% due to spread from stolons. Bahiagrass was the major weed in creeping signalgrass pastures followed by common bermudagrass. While ground cover from these weeds was slight, they occurred in 34% of the quadrats sampled. Their presence was more obvious in dry spring months (when creeping signalgrass was still dormant), but following rain in June and the resumption of creeping signalgrass growth, the weeds contributed essentially nothing to forage mass.
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Spittlebug [Prosapia bicincta (Say)] larvae and their spittle masses were found from June to October on creeping signalgrass. Their occurrence was patchy, with no apparent plant damage. No insects pests were noted aboveground on bahiagrass, but mole crickets were found in insect traps in pastures of both grasses (M.B. Adjei, personal communication, 2001).
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DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMETHODS AND MATERIALSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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While creeping signalgrass resulted in comparatively good cow gain in our study, on Oxisols in South America, cattle liveweight on the grass was inferior to those on other Brachiaria species (Lascano and Euclides, 1996). The lower liveweight gain on creeping signalgrass was attributed to low CP content of the forage, and inhibition of nitrate accumulation in the pasture compared with plant-free plots (Sylvester-Bradley et al., 1988). Differences in soils may explain the disparity between the reputed low livestock weight gains on creeping signalgrass on Oxisols and the comparatively good gains reported here on Spodosols.
The absence of a significant difference between grasses for calf weaning weights may be due to the relatively short time calves were on trial. Also, a nursing calf is buffered by milk from the cow, so nutritional aspects of pasture before weaning may affect cows more than calves (Rouquette, 1988).
Considering that calf ADG was greater (P < 0.07) on creeping signalgrass (0.66 kg head–1 d–1) than bahiagrass (0.48 kg head–1 d–1), it may be beneficial to delay weaning on the pasture to take advantage of additional calf gain. Provided cows are in good body condition (BCS > 5), as were our cows on creeping signalgrass in August, fall-calving cows could nurse calves up to 8 wk beyond the standard weaning age of 7 to 8 mo to maximize production (Pate and Kunkle, 1986). In years when calf prices are high, keeping cows and calves on creeping signalgrass into September could be profitable. This assumes calf ADG would continue at the same rate after early August. However, decline in CP of creeping signalgrass could limit calf growth in August and September. Calf ADG may also be lower in years with high rainfall.
Forage Accumulation and Mass
Between May and October, forage accumulation of creeping signalgrass and bahiagrass from May to October was not different, which is supported by the results from other Florida trials (Hodges and Martin, 1975; Mislevy and Everett, 1981). However, there were differences between the grasses for composition of the sward, growth rate, and time of growth.
Composition
We did not separate DM into leaf and stem components, but we did observe that bahiagrass was mostly leaf from May to October with some reproductive growth in June and July. Cuomo et al. (1996) determined that summer growth of Pensacola was 81 and 89% leaf in 20- and 40-d clipping frequencies, respectively. In contrast, we observed that a large portion of creeping signalgrass vegetation in June and July was reproductive growth. We estimate that 65% of our annual creeping signalgrass accumulation was leaf, which is equivalent to 6400 kg DM ha–1. This is based on a creeping signalgrass leaf/stem ratio = 2, and annual leaf production of 6900 kg DM ha–1 (Valle et al., 1993).
Growth Rate
Hodges et al. (1976) asserted that the major problem with bahiagrass was "inefficient use of the rapidly-maturing forage." That problem may be greater in creeping signalgrass where 33% of the annual creeping signalgrass growth came in a 28-d period at the start of summer rain. Since much of this prodigious growth was reproductive, the pasture was more difficult to utilize efficiently under grazing. As a result, a stiff, residual strawlike stubble-layer formed and remained for the duration of the grazing season. Increasing the stocking density in June and July 2002 and 2003 helped to reduce this problem. However, the greater forage mass of creeping signalgrass than bahiagrass from July to October (Fig. 1b) was a result of the stubble layer. From the residual material that was baled and removed after October, creeping signalgrass stubble layer averaged 3000 kg DM ha–1, which was twice that removed from bahiagrass pastures.
The residual stubble layer probably has little effect on individual calf gains since cattle coming into a creeping signalgrass paddock after a 21-d rest eat mostly leaves growing above the stubble layer. However, minimizing the thickness of the stubble layer by temporarily increasing stocking density on creeping signalgrass at the start of the rainy season may increase animal output per hectare. Abramides et al. (1984) suggested increasing the stocking density on B. humidicola to 5.1 animal units ha–1 during the rainy season in Brazil to maintain a 20 to 25-cm sward grazing height. In 2002 and 2003, we increased density to 4.5 cows ha–1 in July and together with 2.5 calves ha–1, this was close to the suggested 5.1 animal units ha–1 (Abramides et al., 1984).
Time of Growth
Having sufficient forage for grazing in April and May in Florida is a challenge because this is a period of limited rain. If typical rain accumulation for April and May in central Florida occurs, then bahiagrass may produce 12 to 18% of annual production in these months (Blue, 1971), which is the equivalent of 1800 kg DM ha–1 in April and May (Sumner et al., 1991). Creeping signalgrass is essentially nonproductive without adequate rainfall (Grof et al., 1989), and the result is about 600 kg DM ha–1 in central Florida in April and May (Kalmbacher et al., 2005). If rain equivalent to that received in June in central Florida is needed to initiate creeping signalgrass growth, we can expect 1 yr in 16 for April and 1 yr in 10 for May to provide just 75% of the 62-yr mean rain for June. Temperature is less of a limitation than rain. Our mean (62 yr) daily minimum temperatures for April and May is 14.5 and 17.4°C, respectively, compared with a 7°C minimum required for growth of Brachiaria spp. (Fisher and Kerridge, 1966). Photoperiod length is not limiting for tropical grass growth in April and May in central Florida (Sinclair et al., 2001).
Nutritive Value
Creeping signalgrass holds a distinct advantage over bahiagrass in digestibility. Similar to our findings, Mislevy et al. (1996) found signalgrass IVOMD was 77 (June–July) and 80 g kg–1 (August–September) higher than Pensacola when grazed at a 21-d frequency. In a clipping trial, mean IVOMD of creeping signalgrass from April to September was 120 g kg–1 higher than Pensacola (Mislevy and Everett, 1981). We believe that the greater gain for cows and claves grazing creeping signalgrass largely resulted from greater digestibility of this grass.
Crude protein concentrations in hand-clipped signalgrass samples were always lower than bahiagrass especially from August to October when they averaged 80 g kg–1. In clipping trials, crude protein in creeping signalgrass was consistently lower than that in bahiagrass (Mislevy and Everett, 1981; Mislevy et al., 1996). However, in spite of lower crude protein in signalgrass, protein was apparently not limiting because we measured relatively good cow gains, especially in late summer after weaning. Signalgrass can contain high crude protein concentrations if fertilized heavily with N. Mean crude protein of B. humidicola averaged 128 g kg–1 when cut on 30-d intervals and 50 kg N ha–1 applied after each cut (Sotomayor-Rios et al., 1981). Such fertilization is neither economical nor warranted in commercial cow-calf ranching.
Persistence and Adaptability
The greatest impediment to signalgrass persistence will be winter freezes. Based on 62-yr means at the Range Cattle REC, the –5.0°C freeze we experienced in 2001 has occurred in 1 of 6 yr. While a winter freeze may not eliminate creeping signalgrass due to the strong stoloniferous habit of growth, a freeze will render a pasture unproductive and open to weed growth until mid-summer. We suggest that planting of creeping signalgrass pasture be restricted to the Florida peninsula south of Orlando. The first pasture sown to signalgrass was 135 ha at Deseret Cattle & Citrus (30 km south of Orlando) in 1996, and that pasture has persisted for 8 yr (Kalmbacher, unpublished data, 2002).
While signalgrass is noted to be tolerant of intermittent flooding (Fisher and Kerridge, 1966), we noted it had similar tolerance to flooding compared with bahiagrass. Signalgrass did not grow in ditches and depressions where water (5 cm) remained for several weeks.
Spittlebugs could always be found on signalgrass throughout the rainy season. While signalgrass is tolerant, it is not resistant to spittlebugs (Lapointe, 1993). The possibility remains that spittlebugs could weaken signalgrass just as it does limpograss [Hemarthria altissima (Poir.) Stapf & C.E. Hubb.] pasture in central Florida.
Weeds were not a problem in either grass. Bahiagrass was often found growing in signalgrass pastures that were formerly in bahiagrass for many years. Common bermudagrass, one of the most persistent invasive species in Florida pastures, was not problematic for either grass. Mislevy et al. (1996) noted that ground cover of common bermudagrass in signalgrass declined over 3 yr under mob-grazing.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMETHODS AND MATERIALSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMETHODS AND MATERIALSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMETHODS AND MATERIALSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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