Chlortetracycline

Abstract

Cow weights, hip heights, and reproductive data on 82 Angus and 78 Brahman cows grazing common bermudagrass or endophyte-infected tall fescue forage systems were used to evaluate relationships of breed, forage system, and reproductive status to seasonal weight changes and preparturn body condition. Cow BW gains were computed for winter, spring, and summer for each of these seasons before the calving season the next year.

Weight:height ratios (kg/cm) were computed from BW and hip height data taken in February of each year of the study to estimate body condition before calving and breeding. Thus, BW gain and condition data corresponded to seasons before the subsequent calf crop. Cows in each breed-forage system subclass were classified into four groups based on all possible combinations of previous and subsequent year calving data. Winter BW gain was not associated with pregnancy in the following breeding season (P>0.57). Spring BW gain in open heifers or cows did not affect subsequent pregnancy, but Angus cows that calved in consecutive spring seasons had greater spring BW changes compared with contemporaries that calved but did not rebreed (P0.31) Increased summer ADG during the breeding season was a useful indicator of subsequent calving rates in primiparous heifers and in cows that had calved the previous spring (P0.84). Consequently, these data indicate that a suitable predictor of subsequent reproduction in spring-calving cows is summer BW change.

(Key Words: Brahman, Angus, Tall Fescue, Bermudagrass, Cow Weight Change.)

Introduction

Nutrition derived from forages by beef cows plays a major role in the efficiency of cow-calf production. In heifers, winter and spring gains before the first breeding season could influence reproductive performance. Winter BW gain in spring calving beef cows directly influences condition at calving, which has a well-documented impact on rebreeding rates (Richards et al., 1986; Selk et al., 1988; Spitzer et al., 1995; Lalman et al., 1997). Postpartum BW changes can have an additive effect on subsequent rebreeding performance in conjunction with body condition at calving. However, little information exists on the impact of breed and forage type on seasonal BW gains and prepartum body condition in beef females. Consequently, the objectives of this research were to evaluate the effects of breed, production system, and production status on seasonal BW changes and prepartum body condition in Angus and Brahman heifers and cows grazing common bermudagrass or endophyte-infected tall fescue.

Materials and Methods

Five years of data from 82 Angus and 78 Brahman females born in the spring of 1985 were used in this study. Cows were purchased from approximately 20 different ranches per breed, and 32 and 30 sires were represented in the Angus and Brahman females, respectively. In the winter of 1985 and the early spring of 1986, heifers were assigned at random to one of four 16-ha common bermudagrass pastures [Cynodon dactylon (L.) Pers.] or one of four 16-ha endophyte-infected tall fescue pastures (Festuca arundinacea Schreb.) so that each pasture had approximately equal numbers of heifers from each source (ranch) and of each breed. Endophyte-infected pastures were 100% infected with estimates of ergovaline in March and April of 1995, averaging 115 and 201 ppb, respectively (M. A. Brown, unpublished data). Fertilizer N was applied to tall fescue pastures in February and September and to bermudagrass pastures in April and July to furnish 60 kg N/ha for each date. Soil P and K were applied as needed to avoid deficiencies in these nutrients. Tall fescue pastures were shredded in early summer to remove seed heads, and bermudagrass pastures were shredded in late summer to control broadleaf weeds. Because cattle on each forage type were managed in accordance with needs predicated by estimates of forage DM availability and quality (including toxicity in the tall fescue), forage type hereafter will be more accurately referred to as forage system and also may be abbreviated as "forage." Stocking rates for each pasture within the forage system ranged from 19 to 24 females with approximately equal numbers of Brahman and Angus females per pasture. Before the first breeding season in 1987, heifers were managed to gain approximately 0.35 kg/d by supplementing with approximately 0.85 kg/d grain (50% cottonseed meal, 50% chopped corn), and tall fescue, or bermudagrass hay according to visual estimates of forage DM availability and expected changes in quality as a function of time in the growing season for the respective pastures. Normally, supplemental feed [50% corn, 50% cottonseed meal (CSM), 0.85 kg/d] was provided from late November to late April in both forages. The reason for provision of supplemental grain to cattle in the tall fescue forage system in the fall and spring was to moderate potential toxicity from the forage. Minerals [Ca (15.5% min.), P (8% min.), NaCl (18.5% min.), trace minerals (Na, Mg, K, Cu, I, Se, Zn) and vitamins A, D, and E (Cargill Animal Nutrition, Minneapolis, MN)] were fed free-choice throughout the late fall, winter, and early spring. A medicated mineral mix [Ca (13% min.), P (8% min.), NaCl (19% min.), trace minerals (Na, Mg, K, Cu, I, Se, Zn), vitamins A, D, and E and chlortetracycline (3.1 g/kg) (Cargill Animal Nutrition)] were fed free-choice in the late spring, summer, and early fall with the antibiotic intended for control of anaplasmosis during fly season.

Heifers were managed to calve as 3-yr-olds to preclude parity differences between breeds and were bred during 75-d breeding seasons starting in early June for the first breeding season in 1987 and late May for the subsequent four breeding seasons. Four Brahman and four Angus sires were rotated among breeding pastures in both forage systems each year to prevent confounding of sire and forage system effects. Breed of sire was alternated in a breeding pasture to facilitate sire of calf identification. Cows were culled only on the basis of structural unsoundness and, infrequently, temperament until the fall of 1990. After pregnancy check in the fall of 1990, 18 Angus (7 from bermudagrass and 11 from fescue) and 11 Brahman (6 from bermudagrass and 5 from fescue) cows were culled on the basis of reproductive performance or structural unsoundness.

Calves were born from late February to late May each year and were weighed and tagged within 24 h of birth. A cow was credited with having a calf if she had a full-term calf, alive or dead. The exceptions to this were three Angus cows from tall fescue and one Brahman cow from bermudagrass that were palpated as pregnant in the fall of 1990, culled subsequently, and credited with a full-term calf for the 1991 calf crop. The 25 cows palpated as open and culled in the fall of 1990 were counted as open for the 1991 calf crop. Cows remaining for the 1992 calf crop had at least one calf born from 1988 to 1991 and were exposed to four Beefmaster and four Brahman sires during the 1991 breeding season. Further reproductive data on this cow herd are available in Brown et al. (1992).

Cows in the study were weighed annually in late October or early November (weaning); February (precalving); and in late May or early June (postcalving). Cow gains were computed for winter (October/ November to February), spring (February to May/June), and summer (June to October) grazing seasons before the subsequent calving season. Weight:height ratios (kg/cm) were computed from hip height data taken in February of each year of the study to estimate body condition before calving and breeding. Weight:height ratios have been used as a predictor of body composition in mature cows and are useful in describing the condition of cows that may vary widely in type and size (Klosterman et al., 1968). Gain and condition data corresponded to seasons before the subsequent calf crop (e.g., data for the 1989 calf crop were taken in the fall of 1987 and spring and summer of 1988). Dates for each season and calf crop are given in Table 1.

Cows in each breed-forage system subclass were classified into groups based on previous year and subsequent year calving data. Class 1 cows were open during the winter, spring, and summer before the subject calf crop year and open for the subject calf crop year (1988 to 1992). Class 2 cows were open during the winter, spring, and summer before the subject calf crop year and calved during the subject calf crop year. Class 3 cows were pregnant during the winter and calved in the spring before the subject calf crop year and were open the subject calf crop year. Class 4 cows were pregnant during the winter and calved in the spring before the subject calf crop year and calved during the subject calf crop year. Thus, Class 1 and 2 females were open for the winter, spring, and summer gain periods, and Class 3 and 4 were pregnant during winter and calved in the spring period. Because the first-calf heifers calving in 1988 did not contain Class 3 or 4 data, the 1988 calf crop data (primiparous heifers) were analyzed separately from the 1989 through 1992 calf crops (multiparous cows). Numbers of observations for each parity, class, breed, and forage subclass are given in Table 2.

Data were analyzed by methods of mixed model least squares. Linear models for winter, spring, and summer ADG and precalving condition for the 1988 calf crop included the fixed effects of breed, forage, class (Class 1 or 2) and appropriate interactions among these effects. Linear models for the 1989 through 1992 calf crops included the fixed effects of breed, forage, class (Class 1, 2, 3, 4), calf crop, and appropriate interactions among these effects. Calf crop was analyzed as a repeated measure with the individual cow as the subject. Tests of hypothesis concerning breed, forage, class, or interaction effects were done using t-tests and associated observed significance levels. Cows that failed to calve during the course of the study were excluded from the analyses.

Results and Discussion

Winter ADG. Least-squares means and SE for winter ADG for each parity, class, breed, and forage system are given in Table 3. There was little evidence of breed x forage x parity, breed x forage, breed x parity, or forage x parity interactions in winter ADG for primiparous heifers. Winter ADG for primiparous heifers was greater on tall fescue than bermudagrass (0.28 vs 0.19 kg/d, P0.77).

In multiparous cows, there was some evidence (P

Spring ADG. Least squares means and SE for spring ADG for each parity, class, breed, and forage are given in Table 4. There was some evidence that forage differences in spring ADG differed between Angus and Brahman primiparous heifers (P0.99). In multiparous cows, class differences depended on breed (P0.51 and P>0.32, respectively). This suggests that cows that were open during the winter and spring gained similarly, whether or not they calved in the subsequent year. However, spring ADG in Angus cows that calved in the spring and did not calve the next year differed from Angus cows that calved in the spring and calved the next year (Class 3 vs Class 4, P0.96), suggesting spring ADG had little relationship to subsequent pregnancy in Brahman cows. Spitzer et al. (1995) suggested that postpartum BW change to the start of breeding could influence pregnancy rates. Ellis et al. (1983) reported that conception rates were increased by feeding supplemental energy to cows on fescue pastures from the start of calving (mid February) to the end of the winter season (mid April). However, Marston et al. (1995) concluded that feeding supplemental energy postpartum did not enhance reproductive performance. Similar to results in winter ADG in this study, spring gains in open heifers or cows did not appear to influence reproduction, but Angus cows that calved in the spring and calved the next year either lost less BW during calving and/or managed to recover a greater proportion of BW after calving compared with contemporaries that calved in the spring but did not rebreed. A similar response was not found in Brahman multiparous cows, but it is possible that the lesser birth weights in calves from Brahman cows (Brown et al., 1993a, 1997, 2000) may have an influence on subsequent recovery and reproduction. In this study, spring ADG means in primiparous and multiparous cows reflected the more moderate spring weather with similar gains of Angus and Brahman on tall fescue. Results also suggested that the Brahman were better able to utilize the bermudagrass pastures during this period, resulting in greater gains (or lesser BW losses) of Brahman females on bermudagrass compared with Angus females, similar to results of Koger (1963). Scarbrough et al. (2001) reported that the nutritive value of bermudagrass is very poor after mid-December in Northern Arkansas.

Summer ADG. Least squares means and SE for summer ADG for each parity, class, breed, and forage are given in Table 5. While there was little evidence of a breed x forage x class interaction in primiparous heifers (P>0.58), breed x forage interaction in summer ADG was evident (P0.44), but there was evidence of a breed x forage interaction in multiparous cows (P

Prepartum Weight:Height Ratio. Least squares means and SE for prepartum BW:height ratio for each parity, class, breed, and forage are given in Table 6. This estimate of condition reflects the early spring condition in primiparous heifers before their first breeding season and the prepartum condition in multiparous cows before the second or later breeding seasons. There was evidence of a breed x forage x class interaction in primiparous heifers with a class difference in Brahman heifers on bermudagrass (P

There was also evidence of a breed x forage interaction (P0.54), while BW:height ratios of Brahman heifers on bermudagrass were larger than those of Brahman heifers on tall fescue (2.99 vs 2.48; P0.27), whereas BW:height ratios of Brahman cows on bermudagrass were larger than Brahman cows on tall fescue (3.49 vs 3.30; P

Implications

Cow-calf management systems should be designed to take advantage of a previous knowledge of breed characteristics, reproductive status, body condition, and BW changes resulting from forage growth and quality during each season of the year. Forage growth and quality will partly determine the appropriate season to make body condition and BW changes in spring-calving cows. This is especially true in the upper mid-south, where production systems often involve bermudagrass and endophyte-infected tall fescue.

Literature Cited

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Brown, M. A., A. H. Brown, Jr., J. R. Miesner, and W. G. Jackson. 1993b. Genotype x environment interactions in postweaning performance to yearling in Angus, Brahman, and reciprocal cross calves. J. Anim. Sci. 71:3273.

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Wettemann, R. P., K. S. Lusby, and E. J. Turman. 1982. Relationship between changes in prepartum body weight and condition and reproductive performance of range cows. Oklahoma Agric. Exp. Stn. Res. Rep. MP-112. p 12.

M. A. BROWN1*, PAS, A. H. BROWN, JR.[dagger], PAS, and B. A. SANDELIN[dagger] *USDA-ARS, Grazinglands Research Laboratory, El Reno, OK 73036 and [dagger]University of Arkansas, Fayetteville, AR 72701

1 To whom correspondence should be addressed: mbrown@grl.ars.usda.gov

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