Leucaena for weaned cattle in south Florida

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1 Tropical Grasslands (21) Volume 35, Leucaena for weaned cattle in south Florida R.S. KALMBACHER, 1 A.C. HAMMOND 2, F.G. MARTIN 3, F.M. PATE 1 AND M.J. ALLISON 4 1 University of Florida, Range Cattle Research and Education Centre, Ona 2 USDA-Subtropical Agricultural Research Station, Brooksville 3 University of Florida, Gainesville 4 USDA-ARS, National Animal Disease Centre, Ames, Iowa, USA DHP-degrading bacteria was demonstrated. Leucaena is a dependable legume and can provide a high level of nutrition for weaned cattle in Florida providing its use is deferred until late summer when bahiagrass nutritive value is low. Weaned cattle not accustomed to leucaena may require adaptation by grazing with cattle used to grazing leucaena in order to hasten acceptance and permit transfer of rumen bacteria with DHPdegrading ability. Abstract Two experiments using newly weaned heifers or steers compared leucaena (Leucaena leucocephala) (.4 ha) + bahiagrass (Paspalum notatum) (.4 ha) vs bahiagrass (.8 ha) (3 replicates). In Experiment I, 18 heifers were inoculated with dihydroxypyridine-degrading (DHP) bacteria and placed on treatments from June 1994 January Presentation yields of leucaena leaf in June were 18 kg/ha with 3% crude protein, and 62% IVOMD. Liveweight gains (LWG) for 1994 were greater for heifers grazing leucaena + bahiagrass (34 kg/hd) vs bahiagrass alone (12 kg/hd) even though heifers ate little leucaena until September. Experiment I continued with the same group of heifers grazing the same treatments from April December Leucaena leaf was 12 kg/ha in April and virtually nil by June. LWGs were similar for both groups. In Experiment II, two different groups of 18 steers each grazed from July November 1996 and Initial presentation yields of leucaena leaf in July 1996 and 1997 averaged 22 kg/ha and declined to 4 kg/ha in November each year. Mean LWG over 112 days in 1996 and 1997 was 53 kg/hd for steers grazing bahiagrass + leucaena and 8 kg/hd for steers grazing bahiagrass alone. Passive transfer of Correspondence: R.S. Kalmbacher, Range Cattle Research and Education Centre, 341 Experiment Station, Ona, FL 33865, USA. grassdr@gnv.ifas.ufl.edu Introduction Providing adequate nutrition for young cattle is a problem in the tropics. Weaned cattle in Florida would typically go on to pasture when nutritive value is declining (summer) or when both available forage and its nutritive value are declining (fall). Ranchers have overcome this problem for steer calves by selling and shipping them out of state at weaning. However, growing replacement heifers requires expensive nutritional inputs and a high level of management. In Florida, legumes, such as Aeschynomene americana and A. evenia, are undependable annuals that rely on rain for spring establishment and growth. Perennial legumes, like Desmodium heterocarpon, are dependent on rainfall for green-up in the typically dry spring months. Leucaena (Leucaena leucocephala) has shown promise as a long-lived, dependable, droughttolerant legume that is high in nutritive value (Shelton and Brewbaker 1994). Leucaena has shown good potential as a forage in clipping trials on Florida s well drained soils (Othman et al. 1985), but it has not been evaluated under grazing on Florida s imperfectly drained Spodosols which constitute the major soil group where livestock are produced. This investigation evaluated leucaena under grazing in combination with bahiagrass (Paspalum notatum), the major pasture grass in Florida.

2 2 R.S. Kalmbacher, A.C. Hammond, F.G. Martin, F.M. Pate and M.J. Allison Materials and methods Research was conducted at the University of Florida s Range Cattle Research and Education Centre (27 26 N, W). Soil was a Pomona fine sand (sandy, siliceous, hyperthermic Ultic Alaquod) with an initial ph 5.7. In March 1991, 4.5 t dolomite/ha was incorporated by ploughing and an additional 2.2 t/ha was surface applied and disked into the seedbed. Liming was essential for leucaena establishment on a Pomona soil with exchangeable Al averaging 16 mg/kg (Kalmbacher et al. 1991). Soil ph in the present study was 6.9 at 24 months after dolomite application. Leucaena (K34) was inoculated and drilled in May 1992 at 15 kg/ha seed in rows.75 m apart and fertilised with 25 kg/ha P and 93 kg /ha K after emergence. Plant density in September 1992 was 28 plants/ha. In February 1993, the paddock was mowed (25 cm stubble height), fertilised with 25 kg/ha P and 93 kg/ha K and was grazed from July September Leucaena was cut to a.5 m stubble height by machete in February 1994, 1996 and 1997 and was fertilised in late February or early March with 25 kg/ha P and 93 kg/ha K. In 1994, 1996, and 1997, fertilisation included 2.8 kg/ha Cu, Fe, Mn and Zn as sulphates and 5.6 kg/ha elemental S. Bahiagrass did not receive dolomite (soil ph 5.5) and was fertilised annually in March with 56 kg /ha N. Grazed bahiagrass pastures in Florida have shown little economic response to fertiliser P and K and micronutrient deficiencies are rare. Bahiagrass was grazed as needed by other cattle until experimental cattle were placed on pasture. Cattle had free access to NaCl and a mineral mix containing Ca (12%), P (7.5%), Fe (.95%), Cu (.17%), Co (.1%) and F (.18%). Two experiments were conducted: I) heifers grazing from June 1994 November 1995 (same cattle); and II) steers grazing from July November 1996 and 1997 (different cattle each year) (Table 1). Paddocks were set-stocked with 3 head always grazing each of 3 replicates of 2 treatments (18 head/year):.4 ha bahiagrass +.4 ha leucaena vs.8 ha bahiagrass alone at a stocking rate of 3.75 head/ha. Heifers: 1994 and 1995 In June 1994, 18 newly weaned heifers were inoculated by orally drenching with about 25 ml of strained ruminal fluid obtained from a ruminally cannulated steer (from Brooksville, FL) known to have dihydroxypridine (DHP) derivative-degrading ability due to previous inoculation with strain 78-1 of Synergistes jonesii (Allison et al. 1992; Hammond 1995). Rumen fluid samples were taken by oesophageal tube from each heifer to check for DHP-degrading ability at 1 week before and 8, 16 and 48 weeks after inoculation. Cultural tests for the presence of DHP-degrading bacteria in rumen contents were made by inoculating rumen fluid (2 vials per animal,.15 ml each) into Fe-2 medium and examination for loss of DHP during incubation at 37 C for up to 16 weeks (Allison 1991). Heifers were introduced to treatments on June 2, 1994 and until January 2, 1995 those heifers on the bahiagrass + leucaena treatment had continuous access to leucaena (Table 1). All heifers were fed 2.3 kg/hd/d of molasses. On January 2, 1995, leucaena paddocks were closed, heifers were pooled as a single group and given access to the entire 3.6 ha bahiagrass and were fed 2.3 kg/head/day of a liquid supplement containing 85.5% molasses, 13% feather meal and 1.5% urea. They were given 6.8 kg/hd/d of ammoniated Paspalum atratum hay, which was 15% crude protein and 55% total digestible nutrients. On April 24, 1995, heifers were reassigned to their original treatments, and they grazed until December 4, Table 1. Timetable for grazing leucaena + bahiagrass vs bahiagrass alone at Ona, FL. Year Sex Wt (kg) 1 Cattle (shrunk wt) introduced to bahiagrass as one group (18 head). Leucaena paddocks closed. 2 Cattle divided by treatment:.8 ha bahiagrass alone vs.4 ha leucaena +.4 ha bahiagrass heifers were over-wintered on bahiagrass and the same group grazed in Different groups of June-weaned steers were used in 1996 and Steers: 1996 and 1997 Date on trial 1 Date on leucaena 2 Date off leucaena 1994 Heifer Jun 13 Jun 2 Jan Heifer 287 Apr 24 Apr 24 Dec Steer 239 Jul 3 Jul 24 Nov Steer 255 Jul 7 Jul 24 Nov 13 June-weaned steers were placed on pasture on July 3, 1996 and July 7, 1997 (Table 1) and grazed as one group on 3.6 ha of bahiagrass and.1 ha of leucaena (not part of the study area) for

3 Leucaena for weaned cattle in south Florida 3 a 2 3 week adjustment period. Leucaena paddocks were closed. During the 1996 adjustment period, 2 heifers that had grazed leucaena in grazed with the 1996 steers to enable transfer of DHP-degrading bacteria. Two steers that had grazed leucaena in 1996 grazed with the 1997 steers for their adjustment period. Steers were segregated into treatments and leucaena paddocks were opened on July 24, 1996 and Steers grazed either bahiagrass alone or bahiagrass + leucaena until November 13, 1996 and Ruminal fluid was collected from the 18 steers in each year and tested for DHPdegrading activity on October 16, 1996 and October 17, Cattle weights, presentation yields and nutritive value Unshrunk and shrunk weights (food and water withheld for 15 h) were obtained at the start and end of each grazing season (Table 1). Cattle were weighed (unshrunk) every 28 days beginning on the date that they were assigned to treatments. Liveweight gains (LWG) were calculated from shrunk weights, and average daily gains (ADG) from unshrunk weights. When cattle were introduced to treatments and at 28-day intervals thereafter, forage in all paddocks was sampled. A 6.48 m strip was cut to a 5 cm stubble in each of 6 bahiagrass paddocks (except November 1996). Total bahiagrass and a.5 kg subsample were weighed in the pasture, and the subsample was dried at 6 C and reweighed to estimate presentation yield. A composite sample of hand-plucked bahiagrass was gathered in each paddock and used to estimate nutritive value of consumed bahiagrass. In each of the 3 leucaena paddocks, sections of 2 adjacent rows, each 1 m long were cut from 3 representative locations. Compound leaves including the rachis were removed, dried and weighed, then stems up to 5 6 mm in diameter were clipped and dried as an estimate of grazeable leucaena on offer. Cynodon dactylon, which volunteered under leucaena, was sampled May October 1995 by clipping three,.25 m 2 quadrats in each leucaena paddock. Forage samples were analysed for crude protein (Gallaher et al. 1975; Hambleton 1977) and in vitro organic matter digestibility (IVOMD) (Moore and Mott 1974). Statistical analysis The response variable at a given sample date was the mean over the weights of the 3 heifers or steers in each replicate of each treatment (paddock) or forage subsamples within each paddock. Heifer and steer experiments were analysed separately as completely randomised designs using a general linear model (SAS 1985). Treatments were whole plots, years were subplots and sample dates were sub-subplots. For cattle responses, weigh date was examined by year using the repeated measures option of SAS (1985). Polynomial contrasts were used to determine if responses were linear or quadratic over sample dates. Treatments within a year and sample dates in the year treatment sample date interaction for total presentation yield were compared with the p-diff option (SAS 1985). Results Climate Total rainfall for June 1994 January 1995 was 149 mm compared with the 56-year mean of 156 mm (Figure 1). Between April October 1995, 1461 mm rain was measured compared with a 56-year mean of 162 mm. In contrast, rainfall totaled 39 mm for July November 1996 and 114 mm for July November 1997 compared with the 56-year mean of 732 mm for this period. Frost and freezes on December 25 and 26, 1996, and January 9 and February 5, 1997 affected growth in the 1997 growing season by killing leaves and some stems. There were only two other incidences of C or less in the 4-year period. There have been 4.2 days/year (56-year average) with temperature C or less at the Range Cattle Research Centre. DHP-degrading activity After 16 weeks incubation, none of the heifers had the ability to degrade DHP when sampled 1 week before heifer inoculation with S. jonesii. At 8, 16 and 48 weeks after inoculation, there was DHP degradation in all samples collected from the 18 heifers, indicating that DHPdegrading bacteria had been successfully introduced. Media incubation times required for DHP degradation averaged over the 3 post-inoculation sampling dates were 2 and 2.7 weeks for bahiagrass + leucaena and bahiagrass alone treatments,

4 4 R.S. Kalmbacher, A.C. Hammond, F.G. Martin, F.M. Pate and M.J. Allison Monthly 56-yr 5 Rainfall (mm) J J A S O N D J A M J J A S O J A S O N J A S O N Month Figure 1. Observed monthly rainfall for June 1994 November 1997 compared with the 56-year mean rainfall at the Range Cattle Research Centre near Ona, Florida. respectively. All 18 steers in 1996 and 8 of 9 steers on the 1997 bahiagrass + leucaena treatment and 5 of 9 steers on the 1997 bahiagrass alone treatment had the ability to degrade DHP, indicating the extent to which DHP-degrading bacteria had been passively transferred during the commingling periods. Degradation time in October 1996 (after exposure to heifers in June 1996) was 1 week for steers with access to leucaena and 2.2 weeks for steers grazing bahiagrass alone. In October 1997 (after exposure to 1996 steers in June 1997), incubation time was 2 weeks for steers on a diet containing leucaena and 5.3 weeks for steers on bahiagrass alone. Heifer weight changes For average daily gains, there was a treatment weigh date interaction (P =.5) in 1994 (Table 2). There was no difference between treatments except in October and January when heifers grazing leucaena had greater ADG than heifers grazing bahiagrass alone. Liveweight gain was slightly in favour (P =.8) of heifers grazing bahiagrass + leucaena. From January April 1995, heifer ADG was.49 kg/hd/d while wintered on hay and supplement. In the 1995 growing season, there were no differences between treatments for ADG or LWG, but there were differences among weigh dates (P =.1). Steer weight changes For average daily gain, there was a year treatment sample date interaction (P =.3) (Table 2). In 1996, there was no difference in ADG between treatments in August, but steers grazing bahiagrass + leucaena had higher ADG than steers grazing bahiagrass alone from September November. In 1997, steers grazing bahiagrass + leucaena had higher ADG than steers grazing bahiagrass alone at all weigh dates. Liveweight gains over 112 days depended on year (P=.3) with 38 kg/hd in 1996 and 1 kg/hd in Liveweight gains depended on treatment (P =.1) with 53 kg/hd from steers grazing bahiagrass + leucaena and 8 kg/hd from steers grazing bahiagrass alone.

5 Leucaena for weaned cattle in south Florida 5 Table 2. Average daily gains (ADG) and liveweight gains (LWG) for cattle grazing bahiagrass + leucaena vs bahiagrass alone. Year Treatment May Jun Jul Aug Sep Oct Nov Dec Jan LWG (kg/hd) ADG (kg/h/d) Heifers 1994 bahia+leucaena bahia alone P diff Mean Steers 1996 bahia+leucanea bahia alone P diff bahia+leucaena bahia alone P diff Presentation yields Heifer experiment. Leucaena leaf and stem yields showed a year sample date interaction (P =.1). Available leaf yield exceeded 2 kg/ha between July and September 1994 with stem less than 2 kg/ha (Figure 2a). During this time, heifers ate little leucaena and used the leucaena paddock for shade to avoid mid-day heat. The amount of available leaf increased between June and July 1994, then levelled off through September. After September, heifers began to eat leucaena and by January 1995, there was little leaf and stem available. This utilisation led to a quadratic (P =.1) pattern of leucaena leaf and stem presentation yields over 1994 sample dates. In 1995, heifers relished leucaena, and yield of leaf declined quadratically (P =.1) from 122 kg/ha at the start of grazing in April to about 25 kg/ha by June. Leaf yields declined slowly over the remainder of the growing season. Yield of grazeable stem was nil after June Mean bahiagrass presentation yield (259 kg/ha) over 1994 sampling dates was not affected (P =.18) by treatments, but in 1995 it was lower (165 kg/ha) in the bahiagrass + leucaena vs bahiagrass alone (214 kg/ha). Yield showed a year sample date interaction (P =.1) (Figure 2a), as bahiagrass yields in June and July 1994 were greater than yields in these months in Bahiagrass yields in August October 1994 were similar to those in When leucaena was overgrazed in 1995 and there was less shade, C. dactylon became more prominent in leucaena paddocks. Presentation yields were 255 kg/ha in May, 134 kg/ha in June, 133 kg/ha in July, 235 kg/ha in August, 214 kg/ha in September and 24 kg/ha in October. Heifers ate this grass in June and July after leucaena was diminished, but there was limited use in August October when C. dactylon became mature and fibrous. Presentation yields for the sum of bahiagrass + leucaena vs bahiagrass alone (mean over sample dates), were not different (P =.87) in 1994 (mean = 275 kg/ha), but in 1995 there was twice as much forage in the bahiagrass alone treatment (218 kg/ha) compared with the bahiagrass + leucaena treatment (17 kg/ha). This was due to diminished leucaena yields in 1995 (Figure 2a) and greater pressure on bahiagrass as a source of forage in the bahiagrass + leucaena treatment. Steer experiment. Leucaena leaf and stem yields showed a year sample date interaction (P =.1). Leucaena leaf yields were relatively low (168 kg/ha) when grazing began in July 1996 (Figure 2b) because of a reduction in leucaena vigour resulting from overgrazing in Winter freezes caused leucaena to regrow from shoots at the base of the plant in 1996 instead of upper branches. Steers did not utilise leucaena heavily in early July, but from August November 1996, leucaena leaf yield was diminished. In 1997, there were 275 kg/ha of leaf and 13 kg/ha of stem at the start of grazing. Steers consumed leucaena readily in July, and by November only about 5 kg/ha of stem remained. Leucaena leaf and stem yields increased then declined quadratically over 1996 (P =.1) and 1997 (P =.4) sample dates. Stem yield declined linearly (P =.4) in 1997.

6 6 R.S. Kalmbacher, A.C. Hammond, F.G. Martin, F.M. Pate and M.J. Allison Bahiagrass yield (2-year mean over sample dates) was lower (P =.5) in the bahiagrass + leucaena treatment (247 kg/ha) than in the bahiagrass alone treatment (295 kg/ha). Bahiagrass yield showed a year sample date interaction (Figure 2b) as a result of lower yields in August, October and November 1996 compared with these months in There was a year treatment sample date interaction (P =.5) for sum of bahiagrass + leucaena vs bahiagrass alone (data not shown). In the bahiagrass + leucaena treatment, reduced vigour from overgrazing in 1995 resulted in low leucaena yields in July 1996 (Figure 2b). Less bahiagrass in this treatment meant less overall forage compared with bahiagrass alone. In both years, total available dry matter generally favoured the bahiagrass alone treatment, indicating quality, not quantity of forage was the factor affecting performance differences. Presentation yields (kg/ha) (a) Leucaena leaf Leucaena stem Bahiagrass J J A S O N D J A M J J A S O (b) Presentation yields (kg/ha) J A S O N 1996 J A S O N 1997 Figure 2. Presentation dry matter yields in the heifer (a) and steer experiments (b). Bahiagrass yields are means over bahiagrass + leucaena and bahiagrass alone treatments for each experiment.

7 Leucaena for weaned cattle in south Florida 7 Nutritive value In the heifer and steer experiments, crude protein and IVOMD in leucaena leaves and stems showed year sample date interactions (P =.1 for both responses). However, in all years, crude protein concentrations in leaves always exceeded 2% and were much higher than those in stems or bahiagrass (Figure 3). In the heifer experiment, crude protein concentrations in leucaena leaves followed a quadratic (P =.1) pattern by decreasing from June October then increasing through January 1994 (Figure 3a). In 1995, crude protein concentrations decreased linearly (P =.1) from April October. In the steer experiment, crude protein concentrations in leucaena leaves did not change (P =.7) over 1996 sample dates, and increased linearly (P =.4) over 1997 dates (Figure 3b). 4 (a) Leucaena leaf 4 Crude protein (%) Leucaena stem Bahiagrass J J A S O N D J A M J J A S O (b) 4 35 Crude protein (%) J A S O N J A S O N Figure 3. Crude protein in leucaena leaf and stem (< 5 6 mm diameter) and hand-plucked bahiagrass in the heifer (a) and steer (b) experiments. Bahiagrass crude protein percentages are means over bahiagrass + leucaena and bahiagrass alone treatments for each experiment.

8 8 R.S. Kalmbacher, A.C. Hammond, F.G. Martin, F.M. Pate and M.J. Allison Leucaena stem, with a third or less of the crude protein contained in leaves, declined linearly (P =.1) over sample dates in both years of the heifer experiment (Figure 3a). In the steer experiment, crude protein concentration in leucaena stems did not change (P =.19) over 1996 or 1997 dates (Figure 3b). In vitro organic matter digestibility in leucaena leaves from the 1994 heifer experiment changed quadratically (P =.1) by decreasing from July October, then increasing in January (Figure 4a). In 1995, leaf IVOMD declined linearly (P =.1) from April October. In the steer experiment, leaf IVOMD changed quadratically (P =.7) over 1996 sample dates by declining from July September then increasing through November (Figure 4b). In 1997, leaf IVOMD increased linearly (P =.1) over sample dates. Stem IVOMD, which was about half that of leaf IVOMD, decreased linearly 7 (a) Leucaena leaf Leucaena stem Bahiagrass IVOMD (%) J J A S O N D J A M J J A S O (b) 7 6 IVOMD (%) J A S O N J A S O N Figure 4. In vitro organic matter digestibility (IVOMD) in leucaena leaf and stem (< 5 6 mm diameter) and handplucked bahiagrass in the heifer (a) and steer (b) experiments. Bahiagrass IVOMD is the mean over bahiagrass + leucaena and bahiagrass alone treatments for each experiment.

9 Leucaena for weaned cattle in south Florida 9 (P =.1) over sample dates in 1994 and did not change in 1995 (Figure 4a). Leucaena stem IVOMD decreased linearly (P =.2) over 1996 sample dates and did not change (P =.1) over 1997 dates (Figure 4b). Bahiagrass crude protein was not affected (P =.36) by treatment in the heifer and steer experiments, but was affected by year (P =.1) and sample date (P =.1). Crude protein means for years were 8.6% (1994), 9.7% (1995), 1.7% (1996) and 1.5% (1997). Over sample dates, crude protein declined over the summer months, levelled off, then increased in September or October (Figure 3). Bahiagrass IVOMD was not affected by treatment (P =.18), but showed a year sample date interaction (P =.1)(Figure 4). In 1994, 1996 and 1997, IVOMD changed quadratically (P =.3) over sample dates, declining from early to late summer, then increasing in October. In 1995, IVOMD declined linearly (P =.1) from April October. Discussion These experiments have shown that access to leucaena can play a key role in enhancing performance of weaners on bahiagrass in late summer and autumn in Florida. Two important management issues emerged which will impact on the success of such a strategy. First, weaned cattle unaccustomed to grazing leucaena apparently needed to learn to eat it. Leucaena was an unfamiliar forage for heifers in 1994, and they avoided eating much of it for nearly 9 days. Certainly, stress associated with weaning was a factor, but learning also affects what and how much livestock eat. Young animals prefer familiar foods and are reluctant to eat novel foods (Burritt and Provenza 1995). Experience with the steers in 1996 and 1997 suggested that grazing weaners with cattle accustomed to eating leucaena may reduce the duration of the learning phase. Second, heifers were placed on leucaena too early in They put so much grazing pressure on leucaena in April and May that its growth during the remainder of the growing season was limited and leucaena forage was not available in August October when bahiagrass nutritive value was relatively low. Sollenberger et al. (1988) found that crude protein concentration in extrusa from esophageally-fistulated steers grazing bahiagrass fell below 7% from mid-july to mid- August. Providing legumes has been shown to improve livestock weight gains while grazing in mid- to late summer. Steers grazing rhizoma peanut (Arachis glabrata) and bahiagrass had 54% greater ADG than steers grazing bahiagrass alone (Williams et al. 1991) during this period. Available forage containing as little as 7% of A. americana resulted in improved steer ADG in late summer (Pitman et al. 1992). Bahiagrass-D. heterocarpon mixtures have resulted in improved steer ADG compared with bahiagrass alone (Aiken et al. 1991a) primarily due to relatively high (about 17%) leaf crude protein (Aiken et al. 1991b). Deferring grazing from April to July would be a reasonable management alternative for leucaena because it maintains a high leaf:stem ratio for up to 12 days (Osman 1986). Lack of dependability has been the major shortcoming of pasture legumes in central and south Florida. Leucaena could provide a dependable and useful legume for grazing in late summer in Florida. It is known to be very productive and persistent on sites where it is adapted. Jones and Jones (1982) reported working with a 22-year-old stand in coastal Queensland. An important issue for Florida is high seasonal rainfall and periodic flooding, but leucaena has been relatively productive in the wet tropics of Queensland with 4 mm annual rainfall (Ferraris 1979), and it appears to be persistent on the wet flatwoods in south Florida. There are 12-year- old stands of leucaena at the Range Cattle Research Centre (R.S. Kalmbacher, unpublished data). Heifers were able to passively transfer DHPdegrading bacteria to steers in 1996, and these steers were able to pass the bacteria to other steers in Our relatively short incubation time suggests concentrations of bacteria were relatively high in the rumen (Hammond et al. 1989). Steers that were not on a diet of leucaena had longer incubation times after about a 12- week absence of leucaena from the diet. Heifers inoculated in June 1994 maintained viable populations of DHP-degrading bacteria in the rumen from January April 1995 without leucaena in the diet. Passive transfer of bacteria throughout a herd from a few animals and maintaining their presence in the absence of leucaena in the diet have been reported (Hammond et al. 1989; Hammond 1995).

10 1 R.S. Kalmbacher, A.C. Hammond, F.G. Martin, F.M. Pate and M.J. Allison Acknowledgements The authors appreciate the assistance of Mr Ed Bowers, USDA-ARS, for conducting DHP assays and Mr Art Luke, Range Cattle REC, for carrying out the field work in this trial. This is Florida Agricultural Experiment Station Journal Series R References AIKEN, G.E., PITMAN, W.D., CHAMBLISS, C.G. and PORTIER, K.M. (1991a) Plant responses to stocking rate in a subtropical grass-legume pasture. Agronomy Journal, 83, AIKEN, G.E., PITMAN, W.D., CHAMBLISS, C.G. and PORTIER, K.M. (1991b) Responses of yearling steers to different stocking rates on a subtropical grass-legume pasture. Journal of Animal Science, 69, ALLISON, M.J. (1991) Simplified cultural test for ruminal bacteria that degrade toxic dihydroxypyridine derivatives of mimosine. Leucaena Research Reports, 12, ALLISON, M.J., MAYBERRY, W.R., MCSWEENEY, C.S. and STAHL, D.A. (1992) Synergistes jonesii, gen. nov., sp. nov.: A rumen bacteria that degrades toxic pyridinediols. Systematic Applied Microbiology, 15, 522. BURRITT, E.A. and PROVENZA, F.D. (1995) How learning affects what and how much livestock eat with emphasis on the grazing animal. In: Proceedings of the 51st Southern Pasture and Forage Crop Improvement Conference. Blacksburg, Virginia. pp FERRARIS, R. (1979) Productivity of Leucaena leucocephala in the wet tropics of north Queensland. Tropical Grasslands, 13, GALLAHER, R.N., WELDON, C.O. and FUTRAL, J.G. (1975) An aluminum block digester for plant and soil analysis. Soil Science Society of America Proceedings, 39, HAMBLETON, L.G. (1977) Semiautomated method for simultaneous determination of phosphorus, calcium and crude protein in animal feeds. Journal of the Association of Official Analytical Chemists, 6, HAMMOND, A.C. (1995) Leucaena toxicosis and its control in ruminants. Journal of Animal Science, 73, HAMMOND, A.C., ALLISON, M.J., WILLIAMS, M.J., PRINE, G.M. and BATES, D.B. (1989) Prevention of leucaena toxicosis of cattle in Florida by ruminal inoculation with 3-hydroxy-4-(1H)-pyridone-degrading bacteria. Journal of Veterinary Research, 5, JONES, R.J. and JONES, R.M. (1982) Observations on the persistence and potential for beef production of pastures based on Trifolium semipilosum and Leucaena leucocephala in subtropical coastal Queensland. Tropical Grasslands, 16, KALMBACHER, R.S., RECHCIGL, J.E. and MARTIN, F.G. (1991) Effect of dolomite rate and placement on leucaena forage and seed yield. Soil and Crop Science Society of Florida Proceedings, 51, MOORE, J.E. and MOTT, G.O. (1974) Recovery of residual organic matter from in vitro digestion of forages. Journal of Dairy Science, 57, OSMAN, A.M. (1986) Leaf:stem ratio in leucaena a final report. Leucaena Research Reports, 7, OTHMAN, A.B., SOTO, M.A., PRINE, G.M. and OCUMPAUGH, W.R. (1985) Forage productivity of leucaena in the humid tropics. Soil and Crop Science Society of Florida Proceedings, 44, PITMAN, W.D., PORTIER, K.M., CHAMBLISS, C.G. and KRET- SCHMER, A.E., Jr (1992) Performance of yearling steers grazing bahiagrass pastures with summer annual legumes or nitrogen fertiliser in subtropical Florida. Tropical Grasslands, 26, SAS INSTITUTE, INC. (1985) SAS/STAT Guide for Personal Computers. Version 6. (SAS Institute, Inc.: Cary, North Carolina). SHELTON, H.M. and BREWBAKER, J.L. (1994) Leucaena leucocephala the most widely used forage tree legume. In: Gutteridge, R.C. and Shelton, H.M. (eds) Forage Tree Legumes in Tropical Agriculture. pp (CAB International: Wallingford, UK). SOLLENBERGER, L.E., OCUMPAUGH, W.R., EUCLIDES, V.P.B., MOORE, J.E., QUESENBERRY, K.H. and JONES, C.S., Jr (1988) Animal performance on continuously stocked Pensacola bahiagrass and Floralta limpograss pastures. Journal of Production Agriculture, 1, WILLIAMS, M.J., HAMMOND, A.C., KUNKLE, W.E. and SPREEN, T.H. (1991) Stocker performance on continuously grazed mixed grass-rhizoma peanut and bahiagrass pastures. Journal of Production Agriculture, 4, (Received for publication December 8, 1999; accepted June 23, 2)