Influence of Calving Season and Stocking Rate on Birth Weight and ...

Influence of calving season and stocking rate on birth weight and weaning weight of Simmental-sired calves from Brahman-Hereford F1 dams S. J. Gaertner, F. M. Rouquette, Jr, C. R. Long and J. W. Turner J ANIM SCI 1992, 70:2296-2303.

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Influence of Calving Season and Stocking Rate on Birth WeigKt and Weaning Weigh of Simmental-Sired Calves from Brahman-Hereford Dams' S. J. Gaertner, F. M. Rouquette, Jr.2, C. R. Long, and J. W. Turner3 Texas A&M University Agricultural Research and Extension Center, Overton 75684

ABSTRACT: Brahman-Hereford F1 dams have been used to evaluate the influence of grazing pressure on forage attributes and animal performance at the Texas A&M University Agricultural Research Center at Overton. Data for this study were compiled from 1,909 records of Simmentalsired calves born to Brahman-Hereford F1 cows from 1975 to 1990. Birth weight and weaning weight were analyzed independently to estimate the influence of year, season of birth, dam age, weaning age, and sex of calf. The effect of stocking rate as represented by levels of forage availability on weaning weights and subsequent birth weights was measured. Within the fall and winter calving seasons, lactating dams grazing at a high stocking rate produced calves with the lowest subsequent birth weights. Lactating dams assigned to creepfed treatments had calves with the heaviest subsequent birth weights. Although dams that were e 3.5 yr of age had calves with the lightest

birth weights, there was no apparent decline in birth weight of calves from dams 12 to 17 yr old. Year, sex of calf, age of dam, stocking rate, season of birth, age at weaning, and birth weight were significant factors affecting weaning weight (P c .O1). Fall-born calves grazing cool-season annual pastures were heavier a t weaning (267.6 kg) than either winter- (252.0 kg) or spring-born calves (240.9 kg). A stocking rate x season-of-birth interaction was observed for birth weight and weaning weight (P < .051. Differences in weaning weight from lowvs high-stocked pastures were greater for fall-born calves (61.6 kg) than for winter-born calves (48.7). Sex x season of birth and sex x stocking rate were significant interactions for weaning weight ( P c .05). The difference between weaning weights of steer and heifer calves increased in response to low stocking rates and high-quality forage provided by cool-season, annual pastures.

Key Words: Beef Cattle, Stocking Rate, Pastures, Season, Weaning Weight, Birth Weight J. Anim. Sci. 1992. 70:2296-2303

Introduction The efficient utilization of forages by beef cattle is essential for optimum economic production in a cow-calf operation. Seasonal forage availability and quality should influence the calving season in these operations. In the humid southeastern United States, calving in the late spring requires extensive use of moderate- to low-quality, warm-

'TAES Technical Article no. 29007. 2To whom correspondence should be addressed. 3Present address: Dept. of Anim. Sci., Texas A&M Univ., College Station 77843. Received September 27, 1991. Accepted March 11, 1992.

season, perennial grasses. Climatic conditions in this region are conducive to the production of high-quality, cool-season, annual grasses and legumes for fall and winter grazing (McCartor and Rouquette, 1976). Utilizing cool-season annual forages can improve cow and calf performance; thus, a fall calving season may be more economically advantageous to cow-calf producers in the southeastern United States (Bagley et al., 1987b). Forage availability as determined by stocking rate can also have a significant effect on cow and calf performance (Rouquette et al. 1983). Forage availability is one of the primary variables that can be managed by cowcalf producers. With knowledge of the magnitude of the decline in animal performance created by decreasing levels of available forage, cow-calf producers can make

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CALVING SEASON AND STOCKING RATE OF BEEF COWS

sound economic and biological management decisions regarding time of weaning, pasture rotation schedule, and so on. Several nongenetic factors that affect preweaning performance include grazing management, age of dam, sex, age at weaning, and year (Ahunu and Makarechian, 1086; Bagley et al., 1987b1. The effect these factors have on preweaning performance tends to be breed-related and may be influenced by heterosis. Although preweaning characteristics have been evaluated for the Brahman and Hereford breeds independently (CartWright et al., 1964; Ellis et al., 19651 and direct and maternal heterotic effect when crossing them have been quantified (McElhenney et al., 1985, 1986; Roberson et al., 19861, little has been reported on the performance of Simmental-sired calves with Brahman-Hereford F1 dams. Considering the widespread importance and influence of the Brahman-Hereford F1 dam in the beef cattle industry of the southeastern United States, this study was designed to evaluate records from these dams and their progeny used in multiyear grazing trials. The primary objective of this study were to ascertain the influence of year, season of birth, dam age, weaning age, and sex of calf and to measure the effects of forage availability on the preweaning performance of Simmentalsired calves from Brahman-Hereford F1 dams.

Materials and Methods Data for this study were compiled from records of Brahman-Hereford F1cows and their progeny a t the Texas A&M University Agricultural Research and Extension Center at Overton from 1975 to 1990. Cows and their calves were used to evaluate the influence of grazing pressure on forage attributes and animal performance. Using cows that calved in fall and in late winter facilitated the utilization and evaluation of cool-season and warm-season forages, respectively. Data CoZZation. Records of 1,909 Simmental-sired calves born to these F1 dams in 1975 and from 1977 to 1990 provided a consistent, comprehensive data set from which birth and weaning weight could be analyzed. All calves included in the data set were sired by 7/8 or purebred Simmental bulls. Data originally recorded for each calf included dam identification, year of birth, sex, breed of sire, age of dam, date of birth, birth weight, weaning date, weaning weight, stocking rate code, and grazing trial status. A data set for analysis was derived from the primary data set according to several assumptions. First, calves were classified with regard to age of their dam. Because the exact birthdate of each darn was not available, ages were calculated assuming a March 1 birthdate.

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Dams were grouped as follows: 1)2.5 to 3.5 yr,2 ) 3.5 to 12 yr, and 3) 12 to 17 yr. Second, calf birth dates were used to designate their season of birth as follows: fall, September 1 to December 15; winter, December 16 to March 15; and spring, March 16 to May 31. Spring-born calves were managed in the same postcalving regimen as the winter-born calves. Third, weaning date was recorded as the day the calf was physically separated from its dam, even though some calves on high-stocked pastures stopped suckling earlier. Duration of herd average lactation was the basis on which fall-born calves were weaned in June and July, and forage availability and climatic conditions were the basis on which to wean winter- and spring-born calves in September and October. Fourth, a stocking rate code was assigned to every calf based on its pasture assignment. Stocking rate codes included high, medium, low, and grazer. Calves assigned a stocking rate code of creep-fed grazed at low stocking rates and were allowed to consume creep feed a d libitum. Grazers that remained in a specific test pasture for at least 60 d before weaning were assigned the same stocking rate code as the testers in that pasture. Fifth, every calf was assigned a previous stocking rate code based on the stocking rate code assigned to its dam’s calf in the previous year. If the dam did not have a previous calving record because she was a first-calf heifer, was dry, or had been recently purchased, a previous stocking rate code of grazer was assigned to the calf. Two separate files were created from these records so that birth weights and weaning weights could be analyzed independently. Analyses were completed across and within seasons of birth. Analyses across seasons of birth, using the total data files, were completed to evaluate the effect of season of birth and interactions with season of birth on birth weight and weaning weight. Effects on birth weight and weaning weight unique to specific seasons were evaluated by the withinseason analyses by using the fall, winter, and spring data files independently. For birth weight, calves that were assigned to a previous stocking rate of creep-fed were represented only in the independent analyses of fall and winter. Similarly, bull calves represented in the spring, and creepfed calves represented in the fall and winter, were included only in the respective within-season analysis of weaning weight. Pasture Management. Forage availability on test pastures for fall and winter calvers was controlled using a variable, put-and-take stocking rate method whereby additional animals C‘grazers”) were moved on and off of the pastures to maintain the desired level of forage availability. Grazers were contemporaries of the “tester” cow-calf pairs.


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On high-stocked pastures, grazing pressure was sufficient to prevent the accumulation of forage at dung and urine spots. Forage availability as measured to ground on these high-stocked pastures was < 800 kg/ha. On the mediumstocked pastures, forage availability ranged from 1,600 to 2,000 kg/ha and selective grazing was readily apparent. Forage availability was > 2,400 kg/ha on low-stocked pastures (Roth et al., 1990). Fertilizer was split-applied to provide total annual rates of 200-100-100 kg/ha of N-P205-K20. Before initiation of grazing on test pastures, cow and calf pairs were stratified into testers and grazers based on cow and calf characteristics. When grazers were not assigned to a test pasture, they were maintained on pastures at medium to medium-high stocking rates. Animal Management. In mid- to late September, cows that calved in fall were placed on actively growing Coastal bermudagrass pastures. Bermudagrass hay was offered when forage availability limited ad libitum intake. From the first of December through mid-February, cows and their calves received hay and were allowed intermittent grazing of rye-ryegrass for 2 to 3 h/d. During February, fall-calving cows were assigned as test pairs to ryegrass-clover-bermudagrasspastures that were subsequently maintained at three levels of forage availability, characterized by stocking rates of high, medium, and low. Generally, bermudagrass was present in the test pastures by the beginning of May and was the exclusive species in these pastures by the first of June. Cows and their calves remained on test pastures until midJune to mid-July, at which time all the calves were weaned. From mid-June to mid-July, cows calving in the late winter were assigned as test pairs to the bermudagrass pastures, which were maintained at three levels of forage availability. They remained on test pastures until the calves were weaned in early to mid-October. In mid-November, cows were provided ad libitum access to bermudagrass hay. Cows received a daily protein supplement if merited by cow condition score or climatic conditions. Target condition scores were to be 2 6 a t calving. Protein supplementation, if offered, was discontinued by mid-February when the cows and their calves were permitted intermittent grazing of rye-ryegrass pastures for 2 to 3 h/d. As forage availability increased, winter-calving cows and their calves were allowed full-time grazing of the rye-ryegrass pastures by April and until they were assigned to the bermudagrass test pastures in June. Statistical AnaZysis. Analyses of birth weight and weaning weight were completed using the GLM procedure of SAS (1985). Least squares means for birth weight and weaning weight were calculated

to compare the data because unequal subclass numbers were present. Birth weight and age at weaning were included as covariates in the models for analyses of weaning. Pearson correlation coefficients GAS, 1985) were calculated by stocking rate within season of birth to determine the correlation between these covariates and weaning weight. The statistical analyses of birth weight and weaning weight were not comprehensive. Not only were effects associated with individual dam and sire ignored, but also interactions by year and age of dam were not included in the models. These interactions were excluded because they were partially confounded within the data. Although the results from the tests of significance should be considered conservative, the collective analyses of this data set should appropriately classify the main effects and interactions affecting birth weight and weaning weight of Simmental-sired calves from Brahman-Hereford F1 dams.

Results and Discussion Birth Weight. Analyses of variance (Table 1) indicated that all main effects, including sex of calf, age of dam, year of birth, dam’s previous stocking rate, and season of birth contributed to variance in birth weight within the fall and winter (P e .05). In the fall, an interaction between sex of calf and dam’s previous stocking rate was present ( P < .05). Main effects were not different (P > .05) among spring-born calves, which represented only 166 calves. All main effects were also significant across seasons of birth, which is denoted as total in Table 1. An interaction was present between season of birth and previous stocking rate (P < .05).

Estimates of least squares means and standard errors for birth weight by sex of calf, age of dam, and previous stocking rate within each season are presented in Table 2. In the fall and winter calving season, respectively, bull calves weighed 1.2 and 1.9 kg more than heifer calves a t birth. Differences in birth weight due to sex were less than those reported by Roberson et al. (1986) for Brahman, Hereford, and Brahman-Hereford calves and by Bradley et al. (19661for Hereford and Hereford-Red Poll calves. In accordance with results reported by Ahunu and Makarechian (19861, first-calf Brahman-Hereford F1 heifers represented by the 2.5- and 3-yr-oldshad the lowest Simmental-sired calf birth weights at 31.6 and 34.0 kg for fall and winter, respectively. Ahunu and Makarechian (1986) also found some evidence that preweaning performance began to decline in dams 2 9 yr old. In our study, birth weights of the 3.5- to 12-yr-olds(35.9 and 34.8 kg) were not different ( P >


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Table 1. Analysis of variance by calving season for birth weight of Simmental-sired calves Falla

Source

df

Year Sex of calf (SEX) Age of dam Previous stocking rate (PSR) SEX x PSR Season of birth S S N ) SSN x SEX SSN x PSR Error

13 1 2 4 4

Type I11 sums of squares

14 1 2 4 4

668

*

402** 293*

-

-

-

-

12 1 2

-

-

743

21,493 10.9

-

574 28 28 137 24

3 3 -

-

Totald


df

1,467** 244** 198* 526** 81

-

25,035 17.0

-

Spring


df

2,780** 158*

951

R2

Winterb

-

-

-

-

142

4,722 14.6

-

df 14 1 2 3 3 2 2 6 1,825

-

Type I11 sums of squares 2,520** 194** 435** 301*

178 872** 31 436* 52,794 9.6

*Fall includes calves born from September 1 to December 15. bWinter includes calves born from December 16 to March 15. CSpring includes calves born from March 16 to May 31 dTotal represents the analysis across seasons of birth. *P .OS. **P < .01.

.05)from those of the 12-to 17-yr-olds(35.2and 36.2 kg) for the fall and winter, respectively. Although differences in birth weight due to sex of calf and age of dam are well documented 1985; Ahunu and (McElhenney et al., Makarechian, 1986; Roberson et al., 1986; Burfening et al., 19871, the effect of the dam’s previous stocking rate on birth weight has not been fully ascertained. Lactating dams assigned to a high stocking rate bore calves with the lowest (P e .051 birth weights in the fall and winter calving seasons, respectively (33.3,32.5 kg), whereas dams

assigned to a creep-fed treatment bore calves with the heaviest (P e .051 birth weights (35.6,37.9 kg; Table 2). Godfrey et al. (1988) found that dam’s weight loss due to forage availability during the fiist two trimesters of pregnancy did not affect calf birth weight. Although data reported by Godfrey et al. (1988) were also from the Texas A&M University Overton cow herd, their data included several breeds of sires, and records were limited to fall-born calves. Additionally, Godfrey et al. (1988) sorted records on dam weight change during early to mid-gestation rather than on forage availability

Table 2. Least squares means and standard errors by sex of calf, age of dam, and previous stocking rate for birth weight of Simmental-sired calves with Brahman-Hereford F1 dams Falla Item Sex of calf Male Female Age of dam c3.5 3.5 to 12.0 12.0 to 17.0

Previous stocking rate Creep-fed Low Medium High Grazer

n

Winterb

Birth wt, kg

n

SpringC

Birth wt, kg

n

Birth wt, kg

n

Birth wt, kg

35.59 f 33.70 f

.61i .62k

88 76

36.78 f 1.2Ele 35.63 f 1.3Se

938 921

34.99 f .35i 34.04 f .36k

32 125 7

37.55 f 2.21e 36.70 f .8Be 34.36 f 2.60e

114 1594 151

33.44 f .EOe 35.51 f .23f 34.59 f .52f

f 1.61e f .6gf f .65g f .Wh f .46g

14 19 34 97

505 471

34.83 f 33.64 f

.51e .5Sf

376 393

40 907 29

31.57 f .90i 35.92 f .26k 35.22 f l.Olk

606 121

32.94 f 1.07e 34.78 f .4Sf 36.22 f .77f

34 139 148 129 526

35.63 34.77 34.29 33.29 33.20

f 1.05e f .61e f .elf f .63g f .46g

16 131 124 132 366

37.91 34.66 33.98 32.49 34.17

42

Totald

36.17 38.25 35.32 35.08

f 1.9ge f 1.74e f 1.42e f 1.07e

&Fall includes calves born from September 1 to December 15. bWinter includes calves born from December 16 to March 15. cSpring includes calves born from March 16 to May 31. dTotal represents the analysis across seasons of birth. e*f,gbhSubclass means followed by different superscripts are significantly different P < ,051. lsk Subclass means followed by different superscripts are significantly different (P < .01). Downloaded from jas.fass.org by guest on July 10, 2011

284 291 295 989

34.96 35.20 33.44 34.45

f f f f

.58: .53l .45k .2Q1

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GAERTNER ET AL.

Table 3. Analysis of variance by calving season for weaning weight of Simmental-sired calves Falla

Source

df

Winterb

Type 111 sums of squares

df

SpringC

Type 111 sums of squares

df


Totald

df


~

Year Sex of calf (SEN Age of dam Age at weaning Birth wt Stocking rate (SRT) SEX x SRT Season of birth (SSNI SSN x SEX SSN x SRT Error R2

13 1 2 1 1 4 4

-

-

189,589* *

53,235** 3,354 84,140** 100,834** 481,050** 6,771*

-

-

-

897

528,412 71.0

-

13 1 2 1 1 4 4

-

-

508

-

101,075" 15,598** 651 98,279** 79,908** 214,482** 2,754 969,545 58.6

11 2 2 1 1 3 6

-

143

-

8,311* 3,048* 752 13,535** 11,508** 17,857** 1,377 149,295 58.5

13 1 2 1 1 3 3 2 2 6 1,570

-

225,717** 22,943** 5,129* 228,550** 209,524** 228,845** 4,909 39,574** 5,629** 24,423** 1,020,371 71.7

&Fall includes calves born from September 1 to December 15. bWinter includes calves born from December 15 to March 15. %pring includes calves born from March 16 to May 31. dTotal represents the analysis across seasons of birth.

* P < .05. **P < .01.

or stocking rate. Previous studies conducted by Tudor (19721, Laster (19741, and Bellows and Short (1978) have indicated that decreasing precalving feed level during the last trimester resulted in decreased birth weights. Birth weights were higher ( P c .01) for springborn calves than for either fall- or winter-born Simmental-sired calves (Table 2). These results reflect the utilization of high-quality, cool-season, annual forages by spring calvers during the last trimester of gestation. Bourdon and Brinks (1982) and McElhenney et al. (1985) reported that springborn calves were heavier than their fall-born counterparts. Even though there were several (n = 911 Simmental-sired calves that exceeded 44 kg at birth, there was almost no dystocia in this cow herd. Records of 1,909 calves indicated that only five Flcows giving birth to Simmental-sired calves required assistance from 1975 to 1990, and abnormal presentation, rather than weight, was the primary reason for assistance. The incidence of dystocia may have been low because few first-calf heifers were used in the multiyear grazing trials composing these records. In general, these results agree with those of Rohrer et al. (19881 and indicate that Brahman crossbred cows have a n inherently easy calving ability. Weaning Weight. The analyses of variance and tests of significance for weaning weight are presented in Table 3. Within the fall, winter, and spring calving seasons, stocking rate and the regression for age a t weaning and for birth weight contributed significant variance to weaning weight ( P c .011. Year and sex of calf were

significant ( P c .05), whereas age of dam was not significant (P > .05)within any season of birth. In the fall calving season, a sex-of-calf x stocking rate interaction was present ( P c .05).In the analysis of weaning weight across seasons of birth, denoted as total in Table 3, all main effects, including year, sex of calf, age of dam, stocking rate, and season of birth, were significant ( P c .05). Significant interactions of season of birth x sex and season of birth x stocking rate were observed (P c .05). Least squares means and standard errors for weaning weight by sex of calf, age of dam, and stocking rate are presented in Table 4. Age of dam groupings were quite disproportionate with regard to number of dams in each age class and may have contributed to the lack of significance of age of dam on weaning weight. First-calf heifers weaned the lightest calves across seasons of birth (246.3 kgl. Ahunu and Makarechian (1986) concluded that the weaning weight of Hereford and crossbred calves increased with increasing age of dam for 2- to 4-yr-olds ( P c .(Ill, and they also found that calf weaning weight began to decline in dams 2 9 yr old. In our study, there was no apparent difference in weaning weights of calves from 3.5- to 12-yr-old (256.0 kg) and 12- to 17-yr-old(258.2 kgl Brahman-Hereford cows. These results provide further evidence of the long, productive life span of Brahman crossbred cows (Rohrer et al., 1988; Sacco et al., 1989). In the fall and winter calving season, steer calves weighed 20.8 and 15.2 kg more a t weaning than did heifer calves. Numerous other studies have documented the superiority of male calves over female calves with regard to weaning weight (Bair et al., 1972;


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Table 4. Least squares means and standard errors by sex of calf, age of dam, and stocking rate for weaning weight of Simmental-sired calves with Brahman-Hereford F1 dams Fall* Item

n

Sex of calf Steer Heifer Bull Age of dam

474 450