CASE STUDY: Evaluation of Milk Production and Energy Partitioning in Primiparous Braford Heifers Calving at Two- vs. Three-Years of Age

Introduction

Beef heifers in the tropics and subtropics are traditionally bred to calve for the first time at 3 yr of age. This management process is in contrast to more temperate regions of the world that commonly breed heifers as yearlings (13 to 15 mo of age) and calve at 2 yr of age. Several researchers consider the practice of breeding yearlings a good strategy to begin the recovery of the costs associated with development and to increase lifetime productivity. McMillan et al. (1992) reported that the proportion of nonproductive cows within the herd might be reduced by managing heifers to calve for their first time at 2 yr of age. Morris (1980) reported that heifers calving as 2-yr-olds have the opportunity to produce almost one extra calf through their productive lifetime. Beef production efficiency has been linked to breed type (Marshall et al., 1976; Kress et al., 1990), animal size (Holloway and Butts, 1983; Kress et al., 1990), age (Coleman and Evans, 1986; Goetsch et al., 1991), and milk yield (Neville, 1962; Rutledge et al., 1971; Clutter and Nielson, 1987). However, milk production and BW tend to be considered the most important components for efficiency evaluation of beef cows (Dickerson, 1970; McMorris and Wilton, 1986; Montano-Bermudez et al., 1990; Arango and Van Vleck, 2002). The objectives of this study were to determine differences in energy utilization as well as performance of Braford heifers calving for the first time at 2 vs. 3 yr of age at a similar level of nutrition. Mature multiparous Braford cows were included for comparison purposes.

Materials and Methods

Animals, Care, and Diet. The animals utilized in these experiments were cared for by acceptable practices as outlined in the Guide for the Care and Use of Agricultural Animals in Agricultural Research and Teaching (PASS, 1999). The study was conducted for 85 d at the University of Florida Range Cattle Research and Education Center, Ona. One week prior to the start of the trial, 4 Braford cows and their calves were randomly selected from 1 of 3 age groups (n = 12; 4 pairs/age group), consisting of 2- and 3-yr-old and mature cows. Mature cows were multiparous, and 2and 3-yr-old cows were primiparous. The average number of days in milk did not differ (P = 0.16) among cow age groups (86

Sample Collection and Analyses. Milk was collected from all cows on d 0, 42, and 84 using an Alfa-Laval single-unit vacuum milking machine (Alfa-Laval, Kansas City, MO). Approximately 10 min before milking, the cows received 2 cc of acepromazine maleate i.m. (PromAce, Fort Dodge Animal Health, Fort Dodge, IA), and immediately before milking, each cow received 20 IU of oxytocin i.v. (Pro Labs, Ltd., St. Joseph, MO). On each sampling day an initial milking was performed at 0700 h to empty the mammary glands. The milk collected at that time was discarded. The second milking was performed approximately 6 h later and the collected milk was weighed. Duplicate samples of milk were sent to a commercial laboratory (Southeast Milk, Inc., Belleview, FL) for determination of fat and protein concentration and somatic cell count (SCC). A 24-h unadjusted milk production value was calculated from the amount of milk produced over the actual time interval between each milking. This value was then converted to 4% fat corrected milk (FCM). Body weight and body condition score (BCS; 1 to 9 scale; Kunkle et al., 1999) data were collected from all cows on d 0, 42, and 84 and ADG was calculated for each cow. Fat thickness was measured via ultrasound between the 12th and 13th ribs, three-fourths of the length ventrally over the longissimus. The measurements were taken with an Aloka 500-V ultrasound with a 17.2, 3.5-MHz linear probe (Aloka, Wallingford, CT).

Statistical analyses of SCC, FCM, BW, BCS, fat thickness, and ADG were achieved by ANOVA for a factorial experiment within a completely randomized design using the MIXED procedure of the SAS (SAS Institute, Inc., Gary, NC). Pen (cow) was the experimental unit and age group was the treatment. The model statement contained the fixed effects of age group, day, and age group x day interaction. The random statement contained the effect for cow (pen). Differences among treatments at a fixed day of the trial were separated by LSD obtained from the LS Means option.

Net energy utilization was assessed using actual recorded data for d 0, 42, and 84, and its variation over time estimated uniformly between these days. The response variable was NE^sub g^ and was conditioned to the availability of energy after satisfying NE^sub m^, and NE^sub l^. The regression procedure for a general linear model for SAS was performed to compare the estimated NE^sub g^ values with recorded changes in BW with the cow as the experimental unit. Body composition was estimated from BCS (Ferrell and Jenkins, 1984) to calculate changes in stored energy and to derive the amount of energy from fat (9.37 Meal/kg) and protein (5.49 Meal/kg) mobilized in animals that were in a negative energy balance. The simulation worked under the assumption that the energy mobilized from body tissue was used firstly for milk production. The formulas used to determine NE^sub m^, NE^sub l^, and body composition were taken from NRC (1996); and those used for NE^sub g^ came from NRC (1984). According to NRC (1996), the amount of energy that is derived from BW loss is equivalent to the NE^sub g^ that was required to achieve that BW initially. Therefore, this energy was calculated from the live BW gain formula presented in NRC (1984).

Results and Discussion

BW, BCS, and Ultrasound Data. Although 2-yr-old cows weighed less (P < 0.05) than mature cows at d 0, there were no differences in BW at d 42 and 84 among the groups. Overall, the BW of mature cows decreased (P < 0.01), and the BW of the other groups increased (-0.42, 0.04, and 0.17 kg/d for mature, 2-, and 3-yr-old cows, respectively; SEM = 0.08; Table 3). Tennant et al. (2002), studying 367 Angus cows over 14 yr, reported that cow BW was directly influenced by BCS. The current study showed that 2-yr-old cows had a greater (P < 0.05) BCS when compared with the other groups on d 42 and when compared to the 3-yr-old cows (P < 0.05) on d 84. Treatment differences were not detected when comparing fat thickness; however, fat thickness was positively correlated to BCS (P < 0.05; r = 0.90) and negatively correlated to FCM production (P < 0.05; r = 0.88) among all cows.

Milk Yield and Constituents. Production of FCM tended (P < 0.09) to be less at d O for 2-yr-old than mature cows, and by d 42, FCM production of 2-yr-old cows was less (P < 0.05) than both 3-yr-old and mature cows. Averaged over all 3 collection times, FCM production tended to be 25% less for 2-yr-old than 3-yr-old (P = 0.08) and mature (P = 0.13) cows (4.49, 6.17, and 5.90 kg/d for mature, 3-, and 2-yr-old cows, respectively; SEM = 0.62; Table 3). Using cattle of similar genotype to the present study (Brahman x British), Johnson et al. (2003) reported that multiparous Brangus cows produced 66 and 84% more (P < 0.001) milk than primiparous cows during early and late lactation, respectively. Similarly, Van Oijen et al. (1993) reported that cow age had significant effects on milk production when comparing 2-, 3and 4-yr-old dams.

Milk protein and fat content did not differ (P > 0.31) among the cow age groups (2.55 ± 0.10 and 4.29 ± 0.30%, respectively). Similarly, no differences (P = 0.30) were found in milk SCC among cow age groups (271,000 ± 175,000 cells/mL). Similar to these results, Eberhart et al. (1979) and Sheldrake et al. (1983) reported that age and parity had very little effect on SCC changes.

Net Energy. The energy utilization simulation compared favorably between actual and calculated values (Table 4). All age groups allocated approximately the same proportion of energy for maintenance (65%). This estimation differs slightly from Ferrell and Jenkins (1985) who reported that approximately 70 to 75% of the total energy requirement for beef production was used for maintenance. Energy efficiency should be a criterion used to select or evaluate a particular breed or its crosses, particularly the energy for maintenance requirements, because they account for approximately half of the feed consumed in beef production (Prichard and Marshall, 1993). In the current study, the variation in energy utilization was likely related to milk production because there were no age-related differences in BW on d 42. Whereas 3-yr-old and mature cows allocated almost 35% of the available energy (including energy derived from tissue mobilization) to support milk production, 2-yr-old cows used only 25% to satisfy this requirement (Table 4). The mobilization of body tissue as a source of energy has an approximate efficiency of use of 80% for maintenance or milk production (Flatt et al., 1965; Russel and Wright, 1983). Further, Montano-Bermudez et al. (1990) reported that almost 23% of the variation in maintenance requirements was due to differences in milk production.

In the current study, 3-yr-old and mature cows could apparently mobilize energy from tissue to support lactation. In contrast, the energy partitioning in 2-yr-old cows appeared to place equivalent priority on both lactation and gain. Although the number of cows utilized in this case study was small, the differences found in milk production are meaningful. When cows are provided similar levels of nutrition, these data suggest that first-calf Braford heifers calving at 2 yr of age tend to produce approximately 25% less milk and direct more energy for gain than first-calf heifers calving at 3 yr of age. This finding suggests that 3-yr-old, primiparous cows produce a similar amount of milk compared to mature cows; however, it is unlikely these cows have reached mature BW by 3 yr of age. This direct comparison is confounded in the current study by diet because mature cows were provided less forage and concentrate compared with primiparous cows. This is best illustrated by the greater loss of BW in mature vs. 3-yr-old cows, even though milk production did not differ.

Implications

Breeding heifers as yearlings may increase overall herd productivity. In the current study, Braford heifers calving for the first time at 2 yr of age were shown to produce approximately 25% less milk and to direct more energy for gain than first-calf heifers calving at 3 yr of age. This lower level of milk production is a widely observed production outcome, and calves from 2-yr-old primiparous heifers typically have less BW at weaning compared to calves from older cows. Management systems focused on the alleviation of the nutritional pressure of lactation, especially on primiparous heifers, may leave more energy available for gain and reproduction.

Acknowledgements

Appreciation is expressed to T. Wood for technical assistance during the conduct of this experiment.

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