INTRODUCTION
Early weaning is an effective management practice to increase pregnancy rates of young cows and first-calf heifers in the Gulf Coast region (Arthington and Kalmbacher, 2003); however, producers in the southeastern United States are often reluctant to adopt this management practice because of limited information on optimum management of early-weaned calves.
In the South, fall-born calves can be weaned at 3 mo of age and raised on annual cool-season pastures. Annual cool-season grasses, which have an elevated nutritive value, are well-suited for early-weaned calves with high nutrient requirements. However, Vendramini et al. (2006) verified that even on highly nutritious annual cool-season pastures, early-weaned calves still respond favorably to supplementation with concentrate. Further research on supplementation composition is needed to optimize supplement costs and better understand the requirements of early-weaned calves grazing pastures.
Annual cool-season grasses have significant concentrations of RDP (Poppi and McLennan, 1995); therefore, it is unlikely that additional CP supplementation will increase performance of early-weaned calves (Vendramini et al., 2006). On the other hand, warm-season grasses have a high concentration of CP fractions B (slowly degradable in the rumen) and C (undegradable in the rumen); therefore, early-weaned calves grazing warm-season grasses may be deficient in RDP (Vendramini et al., 2006). Increasing the level of CP supplementation may be a viable option to increase performance of early-weaned calves raised on warm-season pastures.
Cultivation of annual cool-season grasses is a common practice in the southeastern United States. Establishment success is highly dependable on weather conditions, and ryegrass establishment failure is not uncommon among producers. There is a necessity to evaluate an alternative forage-based system for early-weaned fall born calves that will not compromise calf performance and the economical feasibility of this management practice.
The objectives of this research were to 1) verify the effects of protein supplementation on the performance of early-weaned calves grazing annual cool-season grass during the winter and spring and warm-season perennial grasses in the summer, and 2) compare the performance of early-weaned calves grazing annual cool-season grasses receiving 1% BW supplementation or grazing warm-season perennial pastures and receiving 2% BW supplementation in the winter-spring period.
MATERIALS AND METHODS
The study was conducted at the University of Florida, Range Cattle Research and Education Center, Ona, FL (27.4° N) from January to August 2004 (Exp. 1) and from January to April 2005 (Exp. 2). The animals were cared for within the parameters of acceptable practices described by FASS (1999).
Exp. 1
Forty calves (crossbred primiparous cows sired by Angus bulls) were weaned on January 5, 2004, at an average age of 92 ± 15 d and an average initial BW of 66 ± 15 kg at weaning. Following weaning, calves were contained in a dry lot for 4 d with free choice access to stargrass (Cynodon nlemfuensis) hay and a pre-conditioning medicated concentrate feed (Preconditioning/Receiving Chow LW; Purina Mills, St. Louis, MO). On d 5 post-weaning, the unshrunk weight of the calves was recorded at 0900 h. Calves were stratified by BW and randomly allotted into 1 of 8 established ryegrass (Lolium multiflorum) pastures. Two supplement treatments were randomly allotted to pastures (4 pastures/treatment), consisting of 1) soybean hulls at 1% BW, or 2) soybean hulls at 0.8% BW + cottonseed meal at 0.2% BW. The supplement offered was adjusted for the targeted rate of 1.0% of BW on 28-d intervals 4 times during the experimental period. The compositions of soybean hulls and soybean hulls + cottonseed meal concentrates were 12 and 80%, and 20 and 79% for CP and TDN, respectively. The calves grazed ryegrass from January 9 to April 30 (112 d) and stargrass from April 30 to August 23 (112 d). The ryegrass was overseeded on dormant bahiagrass pastures in late November using a seeding rate of 22.4 kg/ ha of 'Jumbo' ryegrass. Ryegrass was fertilized in early December and early January with 56, 14, and 56 kg/ ha of N, P^sub 2^O^sub 5^, and K^sub 2^O, respectively, and again in early February with the same rates. Established pastures of stargrass were mowed at 5-cm stubble height in March and were provided with an initial fertilizer application of 50, 33.6, and 67.3 kg/ ha of N, P^sub 2^O^sub 5^, and K^sub 2^O, respectively. Additional application of 56 kg/ha of N was made to stargrass pastures in June. Ryegrass and stargrass pastures were 0.30 and 0.25 ha, respectively, and were continuously stocked with 5 calves/pasture.
Herbage mass was determined by clipping 6 random, 1-m^sup 2^ quadrats in each pasture at ground level. Forage samples were dried in a forced-air oven at 60°C for 3 d, and ground to pass a 1-mm screen. Average herbage allowance was computed as average herbage mass divided by calf BW on the experimental unit during the grazing period (Sollenberger et al., 2005). Nitrogen concentration was measured using a modification of the aluminum block digestion technique (Gallaher et al., 1975). Concentrations of CP in herbage DM was calculated as %N × 6.25. In vitro OM digestibility (IVOMD) concentration was determined by the 2-stage procedure of Tilley and Terry (1963) modified by Moore and Moot (1974).
Individual calf BW was collected on 28-d intervals at 0900 h. The change in unshrunk weight was used to calculate ADG.
Exp. 2
Twenty-four calves (crossbred primiparous cows sired by Angus bulls) were weaned on January 5, 2005, at an average age of 89 ± 20 d. The average weaning BW was 100 ±21 kg. Calves were held in a dry lot for 4 d with free choice access to stargrass hay and with the same preconditioning medicated concentrate feed used in Exp 1.
On d 5 post-weaning, the unshrunk weight of the calves was recorded at 0900 h. Calves were stratified by BW and randomly allotted into 1 of 2 treatments. Treatments for earlyweaned calves consisted of 1) grazing bahiagrass (Paspalum notatum) and receiving 2% BW supplementation, or 2) grazing ryegrass with 1% BW supplementation. The supplement consisted of a mixture containing 80% soybean hulls and 20% cotton seed meal for both treatments. The nutrient composition of the supplement was 20% CP and 79% TDN. Each treatment was replicated 3 times in a completely randomized design.
Pasture size was 0.3 ha (3 pastures/treatment; 4 calves/pasture). Calves were allocated to pastures using fixed and continuous stocking rates. Ryegrass establishment procedures were the same as those used in Exp. 1. Ryegrass was fertilized in late December and late February with 56, 14, 56 kg/ha of N, P^sub 2^O^sub 5^, and K^sub 2^O, respectively. Bahiagrass pastures were fertilized in March with 56 kg N/ha.
The procedures used to measure herbage mass, allowance, nutritive value, and calves' performance were the same described in Exp. 1.
Statistic Analyses
All responses were analyzed by fitting mixed-effects models using PROC MIXED procedure of SAS (SAS, 1996). Replicates and their interactions were considered random effects. Grazing periods were analyzed as repeated measures, and means were compared using PDIFF (SAS, 1996). Treatments were considered different when P ? 0.05. Interactions not mentioned in the text were not significant (P > 0.05). The means reported are least squares means.
RESULTS AND DISCUSSION
Exp. 1
There was no difference in annual ryegrass herbage mass (2,340 ± 120 kg/ha) and allowance (1.51 ± 0.07 kg DM/kg BW) between treatments. Favorable and well-distributed rainfall (Figure 1) resulted in superior ryegrass herbage mass accumulation during the experimental period. There was a period effect for herbage mass and allowance (P < 0.0001; Table 1). The decreased herbage mass in May reflects the end of the annual ryegrass life cycle. In south Florida, the reproductive stage of annual ryegrass starts in early April followed by reduced growth and finally death of the plants. Vendramini and Arthington (2007) reported annual ryegrass herbage mass and herbage allowance ranging from 480 to 1600 kg/ha and 0.58 to 1.02 kg DM/kg BW, respectively, from December to April 2003. In that study, forage was not limiting to animal performance. It is expected that herbage allowance of approximately 0.5 kg DM/kg BW is sufficient to provide adequate forage quantity to early-weaned calves grazing annual ryegrass and receiving 1% BW supplementation (Vendramini et al., 2006).
There was no treatment effect on forage IVOMD (75 ± 0.7%) and CP (14.5 ± 0.5%); however, there was a period effect on IVOMD (Table 1) and a period × treatment interaction (P < 0.04) on annual ryegrass CP concentrations (Table 2). The interaction occurred because the forage CP concentration of pastures with calves receiving soybean hulls increased from March to April, whereas pastures with calves receiving soybean hulls + cottonseed meal experienced a numerical decline in forage CP over the same period of time.
The inclusion of cottonseed meal in the supplement did not affect ADG of early-weaned calves grazing annual ryegrass pastures (Table 3). According to NRC (1996), the RDP requirement for 120-kg calves is about 300 g/d. The estimated annual ryegrass forage intake is about 1.8% BW and total DM intake about 2.6 kg DM/d (Vendramini et al., 2006). The forage and soybean hulls treatment provides about 304 g/d that would be adequate to supply the RDP requirements of the calves. Therefore, the inclusion of additional CP from the cottonseed meal treatment did not increase ADG.
In the summer, there was no difference in herbage mass (2,700 kg DM/ha), allowance (1.2 ± 0.1 kg DM/ kg BW), CP (10 ± 0.2%), and IVOMD (57.1 ± 0.6%) of stargrass pastures grazed by early-weaned calves receiving cottonseed meal or control treatments. There was a period effect for all evaluated forage parameters (Table 4). The oscillating herbage mass from May to August 2003 reflects a less-than-normal rainfall (Figure 1), which likely impacted herbage allowance. Nonetheless, average herbage allowance was 1.0 kg DM/kg BW, which is considered adequate for growing animals grazing warm-season grasses (Fike et al., 2003). The herbage had greater CP and IVOMD in May and declined thereafter. The greater CP and IVOMD in May was due to the combination of new regrowth after initial staging and the fertilizer applied in late March. The average CP and IVOMD are comparable with values presented by Vendramini and Arthington (2007) for stargrass forage (10 and 52% for CP and IVOMD, respectively).
There was a significant effect of cottonseed meal supplementation on ADG of calves grazing stargrass pastures (Table 3). Even with adequate herbage CP concentrations, RDP may be deficient in calves grazing warm-season grasses. Warm-season grasses have a lesser concentration of CP fraction A (readily degradable in the rumen) than cool-season forages because warm-season forages tend to allocate protein compounds to the chloroplast of the plant, and a greater proportion of the chloroplasts are located in the bundle sheath cell wall, which is slowly degraded in the rumen. This morphologic characteristic favors the larger proportions of CP fractions B and C in warm-season grasses (Redfearn et al., 1995). The CP provided by the cottonseed meal treatment contributed to the ruminal microbial requirements, which likely lead to increased forage intake and digestibility, resulting in greater ADG compared with calves supplemented with soybean hulls alone.
There was a period effect on ADG of calves grazing ryegrass and stargrass pastures (Table 5). There was no difference in ADG of calves grazing ryegrass from January to March, but a significant decrease followed in April. Calves grazing stargrass had greater ADG during May than other months of the experimental period. The greater ADG observed in May was possibly the result of differences in gut fill during the transition of the calves from cool-season to warm-season pastures.
Exp. 2
There was a treatment × period interaction on herbage mass (P < 0.0001), allowance (P < 0.02), CP (P < 0.0001), and IVOMD (P < 0.0001) for bahiagrass and annual ryegrass pastures (Table 6). There was no difference in herbage mass between ryegrass and bahiagrass pastures during the experimental period with the exception of January. The interaction occurred because ryegrass increased herbage mass from January to February, whereas bahiagrass herbage mass was unchanged during the same period. Herbage mass of bahiagrass was greater than ryegrass pastures in January because bahiagrass herbage accumulated in the summer and fall of the previous year, and ryegrass was only overseeded in late November. According to Gates et al. (2001), bahiagrass is a warmseason grass with limited growth during the winter; therefore, the observed decline in herbage mass was expected. Even with reduced forage growth during the winter, bahiagrass pastures had adequate herbage allowance during the experimental period, ranging from 1.1 to 3.4 kg DM/ kg BW. Calves grazing bahiagrass pastures were expected to have less forage intake because they were supplemented with 2% BW concentrate, which supplied approximately 75% of the calves' DM requirements. Bahiagrass CP and IVOMD increased from January to April (Table 6). Nutritive value was least in the early months as a result of a long regrowth period and one freezing event on December 15. The freezing event disrupted the tissue integrity and reduced the concentration of highly digestible cell contents. The increasing nutritive value over time occurred because the new regrowth likely contained a greater nutritive value compared with the residual forage remaining from the winter months.
Annual ryegrass had higher herbage mass in February and March and decreased growth in April because of the physiological change to reproductive life cycle, stimulated by increased day length. Herbage allowance of annual ryegrass pastures ranged from 0.7 to 2.3 kg DM/kg BW and did not impact performance of early-weaned calves. There was no correlation (r^sup 2^ = 0.07) between herbage allowance and calves' ADG from January to April, implying that herbage was in adequate supply. Annual ryegrass CP and IVOMD declined from January to April because of the increase in stem-to-leaf proportion associated with the appearance of reproductive tillers. Despite the decrease in nutritive value, the minimum concentrations of CP and IVOMD for annual ryegrass were still greater than the maximum concentrations observed in bahiagrass during the experimental period (Table 6).
Early-weaned calves grazing annual ryegrass and receiving 1% BW concentrate had greater (P < 0.02, SE = 0.05) ADG than calves grazing bahiagrass pastures receiving 2% BW concentrate (0.97 vs. 0.76 kg/d). Annual ryegrass had greater nutritive value than bahiagrass, which resulted in calves consuming a greater concentration of nutrients that yielded improved BW gain. Early-weaned calves have a limited rumen capacity and high nutrient requirements, making high-nutritive-value forages favorable for the development of these calves on pasture. Annual ryegrass should not be the sole source of nutrient to early-weaned calves because annual ryegrass has low DM concentrations and it is improbable that early-weaned calves, with limited ruminal capacity, can consume enough DM to meet their nutritional requirements (Vendramini et al., 2006).
IMPLICATIONS
These data indicate that additional CP supplementation is not effective for increasing performance of early-weaned calves grazing annual ryegrass and receiving 1% BW soybean hulls supplement. On the other hand, CP supplementation does increase performance of early-weaned calves grazing stargrass (perennial pasture) and receiving 1% BW soybean hulls supplement. Establishment of annual ryegrass can be costly and success is highly dependent upon climatic conditions; however, data from this study indicate that early-weaned calves grazing fall-cultivated annual ryegrass and receiving 1% BW concentrate supplementation have greater ADG than calves grazing perennial bahiagrass pastures and receiving 2% BW concentrate supplementation. The choice to raise early-weaned calves on annual ryegrass pastures is dependent upon the availability of land and resources to plant annual ryegrass and the willingness of the producer to take the risk of cultivating an annual cool-season crop.
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