Enhancing the Sustainability of Tall Fescue Forage Systems for Beef Cattle Production with Non-Toxic Endophyte Technology

Experiment 1

At the Northwest Georgia Branch Station near Calhoun, GA (latitude 34.5577 degrees N; longitude 84.8158 degrees W; elevation 209 m), four Kentucky-31 (KY-31) pasture treatments: (i) AR542-infected tall fescue, (ii) AR502-infected tall fescue, (iii) E- tall fescue, and (iv) E+ tall fescue were compared for animal toxicity and growth performance for three yr. A completely randomized design was used with two replications of each paddock treatment. The 0.81-ha paddocks were tall fescue monocultures established in October 1998. Seed supplied by J. H. Bouton was precision drilled into well-prepared seedbeds of Sequatchie loam and Pope fine sandy loam soil at a seeding rate of 33.6 kg/ha. Pastures were fertilized uniformly with 67 kg N/ha and P and K according to soil tests at establishment and in February and September of each subsequent year.

Angus crossbred cattle (mean BW = 227 kg) were randomly assigned to the treatment paddocks. Heifers were used during spring 1999, autumn 1999, and spring 2000, while steers were in the subsequent grazing seasons. The animals were supplied at all times with fresh water, free choice mineral blocks, and shade in each paddock. Cattle were treated for internal and external parasites at the initiation of each trial. In an attempt to maintain similar forage availability among paddocks, put-and-take grazing management was used. Based on forage availability, stocking rate was adjusted by removing or adding grazer animals with tester animals remaining on the paddocks for the duration of the experiment. Cattle were taken off of the experimental pastures at the conclusion of each autumn grazing period, grazed on E- tall fescue, and fed bermudagrass hay until they were reallocated to treatment pastures at the beginning of the following spring grazing period. Paddocks were restocked with new animals at the beginning of the autumn grazing seasons. Grazing was initiated when there was adequate forage available, approximately 1800 kg DM/ha. Grazing continued until forage availability dropped below approximately 1300 kg DM/ha. Precipitation and soil water holding capacity played a role in decisions regarding the management of available forage levels.

Paddocks were sampled for available forage on dates corresponding to animal data collection dates by clipping herbage within a 0.09 m2 quadrat from 10 randomly selected sites within each paddock. The material was dried in a forced-air oven at 60 degrees C, weighed, and kg DM/ha calculated. Rate of endophyte infection was assayed on tall fescue tillers near the onset and conclusion of each grazing season using an immunoblot procedure of Hiatt et al. (1998). Total ergot alkaloid concentration was determined using an ELISA procedure described by Adcock et al. (1992). Animal weights, blood samples, and rectal temperatures were collected at the onset of each grazing season, d 14, d 28, and then at 28-d intervals, thereafter. Initial and final animal weight were collected on two consecutive days and averaged. Approximately 7 ml of blood was collected from each animal in the caudal vein at the base of the tail. Blood samples were centrifuged at 3000 g to separate and harvest serum that was then frozen (0 degrees C). Analysis was then performed to determine serum prolactin (PRL) levels according to the RIA procedure of Mizinga et al. (1992). The intraassay CV was 9.0%, and there was no interassay CV since there was only one assay performed.

Animal days for each paddock were calculated as the sum of the days each animal, tester or grazer, spent grazing the paddock during a given grazing season. Animal ADG was computed by dividing mean tester animal gain in a particular paddock by the number of days in the grazing season. Gain/ha was calculated as the number of animal days multiplied by tester animal ADG. Mean stocking rate was computed by dividing animal days by the duration of the grazing season in days.

Experiment 2

Five pasture treatments: (i) AR542-infected Georgia-5 tall fescue, (ii) AR542-infected Jesup tall fescue, (iii) AR502-infected tall Jesup fescue, (iv) E- Jesup tall fescue, and (v) E+ Jesup tall fescue were compared for beef cattle toxicity and growth performance for three yr at the Central Georgia Branch Station near Eatonton, GA (latitude 33.3972 degrees N; longitude 83.4883 degrees W; elevation 167 m). A randomized complete block design was used with two replications of each paddock treatment. The 0.81-ha paddocks were tall fescue monocultures established in October 1998. Seed supplied by J. H. Bouton was precision drilled into well-prepared seedbeds of Pacolet sandy loam soil at a seeding rate of 33.6 kg/ha. Pastures were fertilized uniformly with 67 kg N/ha and P and K according to soil tests at establishment and in February and September of each subsequent year. During spring 1999 paddocks were stocked with cattle for 49 d.

Hereford crossbred steers (mean BW = 254 kg) were randomly assigned to the treatment paddocks. The animals were managed as described for Experiment 1. Grazing was initiated when there was adequate forage available, approximately 2700 kg DM/ha. Grazing continued until forage availability dropped below approximately 2300 kg DM/ha. Available forage, endophyte infection rate, ergot alkaloid levels, animal weights, rectal temperatures, and serum prolactin levels were sampled according to the procedures described for Experiment 1.

Experiment 3

Steer grazing behavior during spring and autumn 2001 was compared on three pasture treatments: (i) AR542 Jesup tall fescue, (ii) E- Jesup tall fescue, and (iii) E+ Jesup tall fescue. The replicated (n = 2) 0.81-ha tall fescue pastures were at a subset of the pastures used in Experiment 2. Eighteen Hereford steers (mean BW = 377 kg) were stocked on tall fescue pastures at an initial stocking rate of three steers/paddock in early March 2001. In September 2001, the pastures were restocked with a new set of eighteen Hereford steers (mean BW = 280 kg). Two steers in each paddock were designated prior to the start of each grazing season as testers, while a third steer was used as a grazer in a put-and-take grazing management system. The 12 tester steers were halter broken for ease of handling prior to the start of the trial and were a subset of the animals used in Experiment 2.

Behavioral measurements were taken on tester steers over four 5-d collection periods during spring 2001 and three 5-d collection periods during autumn 2001 (Table 3). The onset of the March and September behavioral data collection periods coincided with d 0 of the grazing trials. Prior to beginning grazing on the treatment pastures, steers were grazed on E- tall fescue and fed bermudagrass hay. Automatic jaw movement sensors (Rutter et al., 1997), leg movement sensors (Champion et al., 1997), and data recorders (Ultra Sound Advice, London, United Kingdom) (Figure 1) were used to measure grazing time, ruminating time, number of jaw movements, number of steps taken, and lying time. During each 5-d collection period behavioral data were collected for five 24 h periods that were staggered to allow time to handle animals to change recorder data cards and batteries. Data were downloaded from recorder data cards to a portable laptop computer after every 24 h of behavioral data collection. In addition, in-line water flow meters attached to automatic watering tanks measured paddock water usage. Meter readings were recorded daily during behavioral data collection periods. Paddock water usage was converted to a steer BW water usage basis by adjusting for stocking rate and steer BW.

In order to measure forage quality, two esophageally cannulated Jersey steers were fasted overnight, fitted with plastic collection bags, and grazed in each paddock until bags were filled to collect esophageal samples of forage harvested by these steers. Esophageal extrusa samples were frozen in liquid N, lyophilized, and ground through a 1 mm screen in a Wiley mill (Arthur A. Thomas, Philadelphia, PA). Samples were analyzed for DM by drying in a forced air oven at 135 degrees C for 2 h. Crude protein was determined for extrusa samples by combustion in a Leco FP-528 Nitrogen/Protein Determinator (Leco Corp., St. Joseph, MI) and then multiplying N content by 6.25. Determination of IVDMD was performed in a DaisyII incubator (ANKOM Technology Corp., Fairport, NY) with a modification (Holden, 1999) of the ANKOM procedure. NDF and ADF were sequentially determined using the ANKOM200 Fiber Analyzer (ANKOM Technology Corp, Fairport, NY). Indigestible ADF (IADF) was determined according to the procedure of Bernard and McNeill (1991).

Controlled-release chromic oxide boli (Captec Ind., New Zealand) were orally administered as an external marker to tester steers one wk prior to each behavioral data collection period. After an 8-d equilibration period, fecal grab samples were collected for five consecutive days corresponding with behavioral data collection periods. Fecal grab samples were dried in a forced air oven at 50oC for 96 h, composited over 5-d periods by steer, ground to pass through a 1 mm screen, and analyzed for DM and IADF content according the procedures detailed for esophageal extrusa samples. Chromium content was determined using the procedure of Fenton and Fenton (1979). Daily fecal output was estimated by dividing the quantity of chromium released daily from the chromic oxide boli as supplied by the manufacturer by the concentration of fecal chromium. Forage indigestibility was calculated using IADF as an internal marker by dividing forage IADF content from esophageal samples by fecal IADF content. DMI was computed by dividing fecal output by forage indigestibility. Steer DMI was then adjusted for steer BW. Mean bite size in kg/prehension was determined by dividing mean daily DMI by mean daily prehensions. Biting rate was calculated by dividing prehensions by grazing time.

Statistical Analysis

PROC GLM/LSMEANS of SAS (SAS Inst. Inc., Cary, NC) was used to separately analyze the data from each experiment. For Experiment 1, a completely randomized experimental design was used with paddock as the experimental unit. There were two replications of each experimental unit. Main effects were endophyte treatment, season, and grazing year. For Experiment 2, a randomized complete block experimental design was used with paddock as the experimental unit and paddock exposure to deer as the blocking factor. Each treatment occurred once in each of the two blocks. Main effects were endophyte treatment, season, year, and block. Each grazing year in Experiment 1 included a spring period and the subsequent autumn period, while each grazing year in Experiment 2 included a spring period and the preceeding autumn period. Each model included main effects and their interactions. Ergot alkaloid concentration and serum PRL means showed nonhomogeneity among their variances due to some treatments having near-zero values and others possessing values in the hundreds or even thousands. Thus, these data were subjected to square root transformations to address data non-normality prior to statistical analysis.

For Experiment 3, GRAZE (Ultra Sound Advice, London, United Kingdom), a software program designed to analyze the behavior data files (Rutter, 2000), was used to identify periods of grazing, ruminating, and lying, and to count prehensions, mastications, and steps. Forage availability, forage quality, steer performance, grazing behavior, and water intake data were analyzed with PROC GLM/LSMEANS in SAS (SAS Inst. Inc., Cary, NC) as a randomized complete block design. Paddock was the experimental unit. Main effects were endophyte treatment, period, and block. The model included main effects and their interaction.