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Arquivo Brasileiro de Medicina Veterinária e Zootecnia

versão impressa ISSN 0102-0935versão On-line ISSN 1678-4162

Arq. Bras. Med. Vet. Zootec. v.55 n.1 Belo Horizonte fev. 2003 

Age at weaning and the endocrine response to stress


[Idade à desmama e respostas endócrinas ao estresse]



A.R. BuenoI; R. RasbyII; E.T. ClemensII

ICurso de Medicina Veterinária da Universidade do Grande ABC, UniABC 09080-511, Santo André, SP
IIDepartment of Animal and Veterinary Sciences, University of Nebraska-Lincoln Lincoln, Nebraska 68583-0908, USA





Seventy-five Angus x MARC II crossbred heifers were used to assess the effects of age at weaning on calf performance and blood constituents. Calves were weaned at 150, 210, or 270 days of age with comparisons made to their non-weaned counterparts. Changes in blood chemistry and hematology (cortisol, glucose, T3, total proteins, packed cell volume and the differential white blood cell count) were recorded on the day of weaning (day-0), and at 2, 7, 14, and 28 days post-weaning. Calves weaned at 210 days of age presented the highest plasma cortisol, glucose, and total protein concentrations during their early post-weaning period. Calves weaned at 150 days of age had significantly lower plasma T3 values. Packed cell volume and differential white blood cell counts showed no relationship to age at weaning. Calves weaned at 210 days of age presented the most rapid post-weaning growth performance, relative to the 150 or 270 days of age weaned calves.

Keywords: cattle, weaning, stress, blood constituents


Setenta e cinco bezerras cruzadas Angus x MARC II foram usadas para determinar os efeitos da idade à desmama sobre a performance e sobre os constituintes do sangue. As bezerras foram desmamadas com 150, 210 ou 270 dias de idade e foram sempre comparadas com um grupo de não desmamadas. Alterações plasmáticas nas características hematológicas e químicas (cortisol, glicose, T3, proteínas totais, contagem diferencial de células brancas) foram determinadas no dia da desmama (dia-0) e nos dias 2, 7, 14 e 28 após a desmama. As bezerras desmamadas aos 210 dias de idade apresentaram as maiores concentrações de cortisol, glicose e proteínas totais no início do período pós-desmama. Bezerras desmamadas aos 150 dias de idade apresentaram valores de T3 significativamente menores. As características hematológicas (PCV e contagem diferencial de células brancas) não apresentaram relação direta com a desmama. As bezerras desmamadas aos 210 dias de idade apresentaram resposta mais rápida de crescimento no período pós-desmama, comparadas com as desmamadas aos 150 e 270 dias de idade.

Palavras-chave: bezerra, desmama, estresse, constituintes do sangue




Amongst all living animals it is only within the mammalian populations that the young experience the process defined as "weaning" (i.e. the acute depravation of specialized nourishment from the maternal parent). Furthermore, it is only within that group defined as "domestic mammals" that the weaning process is an abrupt physical event. This acute, physical separation of dam and young as occurs within the livestock industries, in combination with an abrupt alteration in the source of nourishment and changes in environment and social structure, can be a very stressful event (Yousef, 1985; Fraser, Broom, 1990). Recognition of these events, selection of the most appropriate age, weight, and/or season to wean, and reduction of the level of stress incurred by the calf, is a challenge facing livestock producers (Veissier et al., 1989; Zavy et al., 1992; Lefcourt, Elsasser, 1995; Short et al., 1996).

Studies with cattle suggest that the physiological effects of weaning and the separation of the calf from its dam contribute to a decrease in the productive performance of the calves (Kubisch, Makarechian, 1987; Harvey, Burns, 1988; Grimes, Turner, 1991). These physiological alterations can be classified into three primary categories; 1) gross behavioral or clinical signs, 2) alterations in blood values, and 3) endocrine interposed changes (Asterita, 1985; Yousef, 1985). Cortisol functions to increase gluconeogenesis resulting in an increase in blood glucose, decrease glucose uptake by muscle cells, decrease cellular protein synthesis and increase the immuno-defense systems, including the white blood cell count (Jain, 1993). The thyroid gland, specifically triiodothyronine (T3), responds to stress by re-calibrating the metabolic state of the animal, which in turn alters nutrient metabolism and heat production, allowing the stressed animal to adapt to the environmental change (Khurana, Madan, 1986). Thus, both the rapid defense systems (i.e. cortisol) and long-term adaptation mechanism (i.e. T3) are affected by and respond to stress.

In this study, plasma cortisol, T3, glucose, total plasma proteins, hematocrit (PCV), differential white blood cell (WBC) count and body weight changes were recorded and analyzed to determine the effects of weaning calves at 150, 210 and 270 days of age.



A total of 75 Angus x MARC II (i.e. one-quarter each: Simmental, Gelbvieh, Hereford and Angus) crossbred heifer calves were used in the study. All animals were managed at the University of Nebraska, Dalbey-Halleck Ranch near Virginia, Nebraska. All calves were maintained with their dams on native grass pasture, and were vaccinated two weeks before the first weaning trial (150 days of age).

The experiment was conducted in three trials according to the age of calf at weaning (i.e. 150, 210 and 270 days of age). Each trial had duration of 28 days. At each weaning date all calves were gathered and processed through a squeeze chute where weight measurements and blood samples were collected. Weaned calves were then separated from their dams and taken to a post-weaning pen with free access to native grass hay and water. The non-weaned calves were returned to the pasture with their mothers. On subsequent days (i.e. 2, 7, 14, and 28 days post-weaning) weaned and non-weaned calves were brought back to the holding areas for blood collections and then returned to their respective treatment areas.

Blood samples were collected via jugular venipuncture using 18 gauge, 1 inch vacutainer needles and tubes. Blood samples were drawn into two 5ml vacutainer heparinized plasma tubes and a 10ml vacutainer tube containing potassium oxalate. An additional 5 ml blood sample was drawn and aliquoted into ethylene glycol (EGTA) tube for blood glucose analysis. All collection tubes were centrifuged at 2300xg for 20 minutes and the plasma used for determination of cortisol, T3, glucose, and plasma total proteins. Blood smears, for the differential WBC count, and hematocrit (PCV) were prepared immediately following collection.

Calves were grouped according to weaning date for statistical analysis. Further determination of the least square means for each blood parameter were conducted using general linear model procedures of Statistical… (1985). Correlation analyzes among all the blood parameters were determined using Proc Corr (Pearson correlation) procedure of Statistical… (1985).



Mean body weight of the calves, while similar across weaning age groups at the beginning of the trial, were statistically different for the October (P<0.07), December (P<0.05) and the February (P<0.06) weigh periods (Tab. 1). In October, the early weaned calves (150-d) weighed less than the non-weaned calves of the same age. For the December weigh period, calves weaned at 210 days of age (i.e. 60 days post-weaning) were heavier than calves weaned at 150 days of age (i.e. 120 days post-weaning) and also those calves weaned at 270 days of age. At the time of the February weigh period, the 210 day weaning group still maintained a weight advantage (P<0.06) over those weaned at the earliest date (i.e. 150 days), but not those weaned at 270 days of age. If we can assume that weight gain is a fair measure of the health of the calf, the data would suggest that weaning calves at 210 days of age produces the least amount of stress resulting in greater animal performance.



Plasma cortisol and glucose values as observed during each respective four-week post-weaning period, i.e. calves weaned at 150, 210 and 270 days of age, are presented in Tab. 2. Plasma cortisol values were noted to significantly decrease (P<0.05) for the age of weaning (day 0). By the second day post-weaning there were no measurable differences between ages of weaning groups. However, on subsequent days (i.e. day 7, 14, and 28), those calves weaned at 210 days of age achieved and maintained plasma cortisol values statistically higher (P<0.05) than those calves weaned at 150 or 270 days of age. When comparisons were made between calves weaned at 150 days of age and non-weaned calves (Tab. 3), and again for calves weaned at 210 days of age and non-weaned calves (Tab. 4), it was noted that weaning at 150 days of age failed to elicit a measurable difference in plasma cortisol values between weaned and non-weaned groups. However, when the calves were weaned at 210 days of age, weaning resulted in a significant (P<0.05) and continuous (i.e. day 2 through 28) increase in the plasma cortisol, relative to the non-weaned group. Note that all calves on experiment were weaned by 270 days of age, thus no comparison could be made for weaned versus non-weaned calves at this age group.







At the time of parturition normal birth calf’s cortisol values are high, decreasing abruptly within the first hours post-partum (Hoyer et al., 1990) and continue a slow, further reduction with advancing age. However severe dystocia may increase these values by 50 to 80 % (Massip, 1980; Cappel, 1996). Higher than normal levels of cortisol and corticosterone (i.e. 30% increase) appear to be closely related to visible signs of distress (Lefcourt, Elsasser, 1995; Lay et al., 1992).

Cortisol is considered one of the few hormones essential for life. Moderate increases in plasma cortisol have been shown to stimulate glycogenolysis, increase appetite, caloric intake, and lipogenesis (Ray et al., 1972). This may account for the observed relationship between plasma cortisol values and weight gain of calves weaned at 210-days of age.

Plasma glucose values were not different, across the three weaning groups, for the time of weaning (Tab. 2). However, statistical difference (P<0.05) was observed for plasma glucose concentrations on the four subsequent days post weaning. Generally, calves weaned at 210 days of age maintained higher plasma glucose values than those of the earlier (150 day) or later (270 day) weaning groups, as was observed for the plasma cortisol values. Plasma cortisol and plasma glucose concentrations exhibited a direct, positive relationship throughout the 7, 14, and 28 day sampling periods for all age of weaning groups. When comparing weaned and non-weaned calves (Tab. 3 and 4), weaning at 150 days of age resulted in no change in plasma glucose levels for the weaned versus the non-weaned group. However, when weaning occurred at 210 days of age those calves achieved significantly higher (P<0.05) plasma glucose levels than their non-weaned counterparts during the 7 and 28 day post-weaning periods. It is important to note that under stress, the release of cortisol initiates two primary defense mechanisms; 1) the immunodefense and 2) the initiation of gluconeogenesis in an effort to provide energy for the stress/recovery process (Dantzer, Mormede, 1983). A direct, and positive relationship was observed for the plasma cortisol and glucose concentrations of the calf weaning groups. The data suggests that calves weaned at 210 days of age are more capable of initiating favorable short-term stress response mechanisms.

The long-term adaptation mechanism to the stress of weaning, as associated with the age of weaning, was investigated via the study of circulating thyroid hormones (T3) and the plasma total protein values (Tab. 5). Plasma T3 values decreased significantly (P<0.05) with the age of weaning, which is in agreement with the observation that the metabolic activity, and thus T3, declines with age (Christopherson et al., 1979). However, for the 7, 14 and 28 day post-weaning periods, calves weaned at the earliest age had significantly lower (P<0.05) plasma T3 values compared to their 210 and 270-day weaned counter parts. Plasma T3 concentrations for the 210 and 270-day weaned calves were not statistically different throughout the experimental periods. Differences in plasma T3 concentrations were not observed for weaned and non-weaned calves at the time of weaning (Tab. 3 and 4, i.e. day-0). However, calves weaned at 150 days of age experienced a significant decrease (P<0.05) in plasma T3 concentrations during the post weaning periods, relative to the non-weaned calves (Tab. 3), while those weaned at 210 days of age were able to maintain plasma T3 concentrations similar to those of their non-weaned counterparts (Tab.4).



Stress in several forms alters the plasma total protein concentrations (Makarechian et al., 1988; Rombeau, Cadwelle, 1993). Jain (1993) indicates that plasma total proteins in cattle increase with age. Plasma total proteins (Tab. 5) were noted to increase with the age of weaning. However, throughout the remaining post-weaning periods (i.e. 2, 7, 14, and 28 days post-weaning) those calves weaned at 210 days of age maintained the highest levels of plasma proteins, and were greater (P<0.05) than the 150 and 270-day weaned calves for the 14 and 28 day post-weaned blood samples. When comparisons were made between weaned and non-weaned calves (Tab. 3 and 4), 2-day post-weaning initiated an increase (P<0.05) in plasma total proteins while subsequent samples (i.e. 14 and 28-day periods) indicated a decrease (P<0.05) in plasma total proteins, for both the 150 day and 210 day weaned calves.

The mean packed cell volume (PCV) and neutrophil to lymphocyte ratio (N:L) as observed in calves weaned at 150, 210 and 270 days of age is reported in Tab. 6. While differences in PCV were not detected for calves at the age of weaning, those calves weaned early (i.e. 150 days of age) achieved (day-2) and maintained (days 2 through 14) a higher (P<0.05) PCV than the 210 and 270-day weaning age groups. Differences were not observed between the 210 and 270-day weaning age groups.



While statistical differences were observed in the N:L ratio for calves at the time of weaning, and at various days post-weaning, no detectable patterns nor consistent relationships were noted (Tab. 6). However, because the main functions of glucocorticoids (primarily cortisol) are glyconeogeneses (i.e. increase plasma glucose concentrations) and immunodefenses (i.e. alterations in white blood cell numbers), and because plasma cortisol and plasma glucose concentrations observed in this study are positively related, while the N:L ratio shows no relationship to cortisol, it is safe to assume that the cortisol differences associated with the age of weaning relates to the energy demands of the animal and not to immuno-defense needs (Zavy et al., 1992).

Currently, livestock management enforces a relatively early separation of calves from dam (Fraser, Broom, 1990; Encarnação et al., 1995). The age of the calf and the method of weaning are two important variables to be accounted by producers. The effects of weaning can affect the development of the calves, performance of the cows and consequently livestock profitability (Kubisch, Makarechian, 1987; Harvey, Burns, 1988; Grimes, Turner, 1991; Short et al., 1996). The present data suggests the effect of early weaning, and perhaps the absence of a more favorable growth response to late weaning, demonstrate the need for a moderate degree of stress associated with weaning to facilitate favorable, yet limited physiological, hematological, and endocrine responses.



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Correspondence to
A.R. Bueno

Recebido para publicação em 27 de novembro de 2001
Recebido para publicação, após modificações, em 19 de julho de 2002

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