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Range Cattle Research and Education Center, University of Florida, Institute of Food and Agricultural Sciences, Ona 33865
3 Correspondence:
3401 Experiment Station (phone: 863-735-1314; fax: 863-735-1930; E-mail:
jdarthington{at}mail.ifas.ufl.edu).
| Abstract |
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Key Words: Beef Cattle Calves Lolium perenne Pregnancy Weaning
| Introduction |
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4) have been shown to have more than a 30% reduction in pregnancy rate (Rae et al., 1993). Early weaning has been shown to be an effective means of improving cow reproduction. Younger cows (2 and 3 yr of age) tend to have longer postpartum intervals compared with mature cows. When weaned before the breeding season, 2-yr-old heifers may experience an increase in overall conception by 25.9% compared with a 7.9% increase in cows 4 yr of age and older (Laster et al., 1973). Additionally, EW cows have been shown to have a 24-d shorter postpartum anestrous period compared with contemporaries nursing calves (Houghton et al., 1990). This is achieved by removing the nutrient requirements associated with lactation (Wyatt et al., 1976). Typical cowherds in the subtropics are of Brahman (Bos indicus) lineage. Brahman crossbred heifers have been shown to have lower calving rates when bred to calve at 24 mo of age (DeRouen and Franke, 1989) compared with heifers of traditional English breeding (Bos taurus). Therefore, many producers in the subtropics wait to breed their Brahman crossbred heifers to calve at 3 yr of age. The mild winters associated with the subtropics, coupled with the ability to provide limited winter grazing, support the investigation of early weaning as a tool to manage young cows. The objective of this study was to investigate the effect of early weaning 3-yr-old, first-calf heifers on heifer and calf performance in the subtropics. | Materials and Methods |
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The animals used in these experiments were cared for by acceptable practices (FASS, 1999), and the protocol was approved by the University of Florida, Institutional Animal Care and Use Committee (#A755). All pastures were located and animals derived from the University of FloridaIFAS, Range Cattle Research and Education Center, Ona. This location in south central Florida (82°55'W and 27°26'N) is considered subtropical. The average historical (1942 to 2000) precipitation for the months of this experiment was 4.9, 4.9, 5.7, 6.5, 8.1, and 6.4 cm for November, December, January, February, March, and April, respectively, with an average low air temperature of 11.9°C.
Over two consecutive years (beginning January 2000), fall-calving, 3-yr-old, Brahman x Angus and Braford heifers were randomly assigned to one of two weaning treatments consisting of early weaning (EW; January) or normal weaning (NW; August). Calves were similar in age (average age = 84 ± 11 and 79 ± 7 d for yr 1 and 2, respectively) and BW at the time of early weaning (average BW = 88 ± 2.2 and 95 ± 11.9 kg for yr 1 and 2, respectively). The study began on the day of EW (January 23 and 3 for yr 1 and 2, respectively) and ended at the time of normal weaning (August 1, both years). The dates for early weaning were selected to correspond with the start of the breeding season. Early-weaned calves (n = 20 and 30 for yr 1 and 2, respectively) were placed onto annual ryegrass (Lolium multiflorum) pastures at rates of 8.2 and 10.7 calves/ha for yr 1 and 2, respectively, and were provided a supplemental grain-based mixture (13.8, 65.0, 1.12, and 0.72% CP, TDN, Ca, and P; as fed) at a targeted rate of 1.0% of BW daily. Calves received no vaccinations and were not implanted. In yr 2, the number of calves available from 3-yr-old heifers was not enough to achieve the desired stocking rate; therefore, stocking rates were increased by including five to six additional, nonstudy EW calves in each of the pasture replications. These additional calves were derived from 2-yr-old, first-calf heifers and were similar in age and BW to the study calves at the time of EW (average age = 87 ± 25 d, average BW = 98 ± 16.8 kg). Data from the extra calves were not included in the statistical model. Normal-weaned calves (n = 20 and 38 for yr 1 and 2, respectively) remained with their dams, and after early weaning, calves were returned into one of 12 groups of mature cows and maintained on bahiagrass (Paspalum notatum) pastures. Both EW and NW heifers were represented in each of the groups. Supplemental winter energy was provided uniformly to each cow group (grass hay and sugarcane molasses, fortified with 3.9% urea, as fed) at rates to support an average group body condition of 5.0 (1 to 9 scale; 1 = emaciated and 9 = obese). First-calf heifers, in combination with mature cows, were exposed to bulls for 90 d from early January to early April. The bull:cow ratio was approximately 1:22, and all bulls were
3 yr of age. Bulls were rotated through pasture groups at
10-d intervals. Pregnancy was determined by rectal palpation in July of both years. Individual heifer BW and BCS (Kunkle et al., 1999) and BW of EW and NW calves were collected at the time of EW (January), at the end of the breeding season (April), and at the time of NW (August 1). Heifer BCS was determined by visual appraisal and the final value was derived from the mean of two technicians scores. Calving interval was calculated by determining the time between the birth of the first and second calf.
Ryegrass Paddocks
In yr 1, two ryegrass pastures (1.2 ha each) were established in a depressional soil area consisting of a Popash mucky fine sand (loamy, siliceous, hyperthermic Typic Umbraqualfs) surrounded by a higher, dryer Pomona fine sand (sandy, siliceous, hyperthermic, Arenic Alaquods). In yr 2, a comparison was made between ryegrass cultivated on pastures containing the depressional areas (Popash soils) and drier Pomona soils. The pastures containing depressional areas in yr 2 were the same pastures used in yr 1. For the areas containing Pomona fine sand, two additional pastures (1.2 ha each) were established. All pastures were bordered by a 0.2-ha paddock of bahiagrass, where water, feed, and mineral were offered. Woven-wire fencing surrounded each pasture to prevent entry of wildlife. "Jumbo" ryegrass was seeded in a prepared seedbed at a rate of 16.8 kg/ha on November 28 and 21 for yr 1 and 2, respectively. In yr 1, a complete fertilizer (56-5-46 kg/ha of N-P-K, respectively) was applied at emergence followed by an additional N application (56 kg/ha) at 108 d following seeding. In yr 2, N was applied (56 kg/ha) at emergence followed by an application of complete fertilizer (84-8-42 kg/ha of N-P-K, respectively) 84 d after seeding. Pastures were not irrigated. Total precipitation for the establishment periods in yr 1 and 2 was 2.9 and 1.2 cm, respectively. Total precipitation during the ryegrass-grazing period was 19.4 and 28.0 cm for yr 1 and 2, respectively.
Beginning on January 31 and 4 for yr 1 and 2, respectively, and continuing at 28-d intervals, ryegrass was sampled (six random, 1-m2 quadrats in each pasture) by clipping whole plants at ground level. Forage samples were dried for 4 to 5 d in a forced-air oven at 50°C, and ground to pass a 1-mm screen. Forage N (Gallaher et al., 1975; Hambleton, 1977) and in vitro organic matter digestibility (IVOMD) analyses (Moore and Mott, 1974) were conducted on each sample plot at the Forage Evaluation Support Laboratory of the University of Florida in Gainesville.
Statistical Analysis
Statistical analysis for monthly changes in available ryegrass and ryegrass quality was achieved by ANOVA for a repeated-measures experiment within a completely randomized design using the PROC MIXED procedure of SAS (SAS Inst., Inc., Cary, NC). Analysis was conducted separately; the effect of year was assessed for the lower Popash mucky soils, which were used in both 2000 and 2001, and in which the model statement contained the effects of year, month, and the interaction of year x month, and the two soil types (Popash muck and Pamona fine sand soil) were compared in 2001, in which the model statement contained the effects of month, soil type, and the interaction of month x soil type. Data were analyzed using the year x pasture and the soil type x pasture interaction as random effects for the first and second analysis, respectively.
Analysis for calf BW and ADG was achieved by ANOVA for a completely randomized design using the PROC MIXED procedure of SAS. The model statement included the effects of year, treatment, breed, sex, and all possible interactions. Data were analyzed using the pasture (year x treatment) and the breed x sex x pasture (year x treatment) interactions as random effects.
Analysis of heifer BW, BCS, and calving interval was also achieved by ANOVA for a completely randomized design using the PROC MIXED procedure of SAS. Calving interval was calculated only from heifers that had become pregnant and calved in the following year. Analysis of calving interval was performed only for yr 2 due to a large number of missing cells for NW heifers in yr 1 (those that did not become pregnant). The model statement contained the effects of breed, treatment, and the interaction for breed x treatment. Pasture was the experimental unit. Data were analyzed using the pasture x breed (treatment) interaction as a random effect. The model statement for analysis of cow BW and BCS contained the effects of breed, treatment, year, and all possible interactions. Data were analyzed using the pasture x breed (treatment x year) interaction as a random effect.
Analysis of heifer pregnancy rate was analyzed by comparing heifer breed and treatment with pregnancy rate using PROC FREQ of SAS. Differences in pregnancy rate were compared with Fishers Exact Test.
| Results and Discussion |
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In yr 2, a comparison was made between ryegrass cultivated on Pomona-Popash vs. Pamona soils. Available ryegrass was initially higher (P < 0.05) on the Pomona-Popash vs. Pomona site in January and February (Table 2
). This response is presumably due to higher soil moisture content in the lower Popash soil.
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4, 5, or
6 had pregnancy rates of 61, 79, and 90%, respectively. In the current studies, all of the EW heifers achieved a BCS
5 by April. This improvement in BCS continued throughout the summer, and by August, EW heifers had a BCS that was 1.59 points higher (P < 0.001) than NW heifers (Table 5
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4 yr of age.
Cow BCS is also important for maintaining a 365-d calving interval. To achieve this production target, cows must conceive by approximately 80 d after calving. Herd and Sprott (1986) found that only 62% of beef cows with a BCS
4 were in estrus by d 80 postcalving. Analysis of calving interval was conducted only during yr 2 because of the high amount of missing data from nonpregnant NW heifers in yr 1 (calving interval not calculated for nonpregnant heifers). Calving interval was lower (P < 0.03) for EW heifers during yr 2 (384 vs. 404 d; SEM = 6.0). Similar to these results, Houghton et al. (1990) reported a 24-d shorter postpartum anestrous period for EW cows compared to contemporaries nursing calves.
Considering the costs of ryegrass establishment, fertilizer, and calf feed, the overall cost of gain for 112 d of ryegrass grazing averaged $0.87/kg over both years. In southern Florida, ryegrass pastures decline to minimal production by late April or early May. Typically, April and May calf markets are the best of the year. Also, these calves were weaned for more than 3 mo, which should provide further value to buyers.
| Implications |
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| Footnotes |
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2 Appreciation is expressed to C. Piacitelli and T. Wood for their technical assistance during the conduct of these experiments. ![]()
Received for publication October 21, 2002. Accepted for publication January 22, 2003.
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