Grazing cattle are usually supplemented with additional feed to increase or maintain their BCS, mainly during the dry season, or when the quality of the pasture is insufficient to meet their nutrient requirements [5,6]. Feed supplementation of grazing cattle can lead to changes in ruminal fermentation. For example, the use of energy-protein supplements increases the degradation rate of insoluble fibrous compounds by improving the energy intake extracted from forage, which thus guarantees a rise in total DM intake due to a higher passage rate [25].
Data in the literature suggest that CP levels in diet of around 100 g/kg of DM improve fiber degradation [6], and CP quantity of around 145 g/kg DM is able to improve pasture voluntary intake [26]. In this experiment, total DM intake increased basically due to the supplement added to intake. However, supplementation did not increase pasture intake and apNDF digestibility. Basically, the quantity of CP herein used in the supplement and pasture (around 81 g/kg DM) was not enough to have a positive effect on forage voluntary intake and fiber degradation, which remained below the values suggested by the above-cited authors.
The supplemented animals displayed higher CP intake and OM and CP digestibly. This was expected because supplementation provides more dietary protein intake, apart from other high digestible nutrients. As the NS had lower protein intake and OM digestibility, nitrogen utilization efficiency was apparently enhanced, which led to higher Emic.
In the last 60 gestation days, cows display higher nutritional requirements [11] and, according to Silva et al. [7], supplementation administered in adequate amounts during this period can have beneficial effects on cow’s energy and protein metabolism, along with increased body reserves. Thus, it would seem to metabolically prepare cows for the postpartum period when supplements are no longer provided. Nevertheless, in the present study, and unlike Silva et al. [7] found, pre-partum supplementation had no effect on reducing the magnitude of BW lost during the post-partum as variation in ADG was noted in the SS animals during the experimental period, which was negative during the post-partum (Figure 1). These findings agree with Cardenas [8] and Sotelo et al. [9].
No difference in postpartum anestrus length was observed and supplementation had no effect on progesterone levels (Table 7). This indicates that supplementation was unable to contribute to greater reproductive efficiency. As expected, the above-cited results consequently reflect post-partum performance, with no difference found in pregnancy rate and days from calving to conception. These results might be explained by the fact that most of the cows presented appropriate BCS for reproduction at the beginning of the experiment (5 to 6.0 on a scale from 1 to 9) [28,29].
According to several works, BCS is a determining factor for cows to return to early estrus with improved conception rates [30, 3]. Furthermore, for cows with adequate BCS, there is evidence that body reserves can be used during late gestation without compromising the subsequent reproductive function [4]. This questions the need for supplementing cows with adequate BCS at the end of gestation. In other words, if grazing cows are appropriately conditioned (i.e. BCS 5 to 6) towards the end of gestation and do not mobilize body reserves extensively before calving; supplementation of extra feed will not lead to improvements of the reproductive performance during the subsequent reproductive cycle.
Although pasture had low quality (less than 7 g/kg DM of CP [25]) its availability to animals sufficed and allowed selective grazing, which led NS animals to maintain ADG throughout the experimental period (Figure 1). Therefore, no differences were found in either calves’ BW or milk production. It is well-known that adequate BCS in females throughout pregnancy does not influence the BW of the progeny at birth [30,31].
In fact, these affirmations are also supported by the results of hormones and metabolites, which mainly changed only during peripartum days.
The plasma glucose concentration declined regardless of the treatment at the end of gestation, which can be explained by the higher fetal demand during this period [11]. Upon calving, cows are stressed and, therefore, epinephrine acts by stimulating glycogen catabolism [32] to minimize stress during calving. Glucocorticoids act by promoting gluconeogenesis in the liver, and decrease glucose uptake and utilization in muscle and adipose tissue [33]. Hence glucose concentrations were higher during this period. The lower glucose concentrations after calving were probably caused by not only reduced DM intake, but also by higher energy demand for milk [34]. On days 30 and 45, serum glucose levels were restored and remained at basal levels.
Cholesterol levels progressively increased on all post-partum days regardless of the supplementation period. This agrees with Ruas et al. [35] and Godoy et al. [36] for postpartum blood cholesterol in lactating beef cows. Something similar occurs with HDL concentrations, which also increased after calving. In ruminants, lactogenesis increases plasma HDL concentrations, which is possibly due to an increase in either HDL synthesis or catabolism of VLDL by mammary tissue [37]. This would explain the low triglycerides and VLDL concentrations upon post-partum, and suggest its utilization as energy demand for lactation as they are important sources of fatty acids for milk fat synthesis [38]. Increased cholesterol during the postpartum could be related to precursors being needed for the synthesis of steroidal hormones [39]. While reproductive activity is re-established, avascularized granulosa cells are restricted to cholesterol uptake from HDL [40].
Data in the literature reveal that restricting pregnant cows’ intake during the late gestation period results in weight loss, BCS loss and high serum concentrations of NEFA and βHB, which lead to long periods of negative energy balance in both dairy [41,42] and beef cows [2].
In this study, NEFA concentrations were not affected by supplementation and these levels at calving indicate a higher adipose tissue rate of lipolysis [43,44]. The NEFA post-partum concentrations were lower compared to parturition, and remained at basal concentrations throughout the experimental period, which suggests the recovery of animals’ nutritional status. The same occurred with βHB levels upon post-partum, which differed between treatments for the pre-partum with higher concentrations for the NS animals.
Despite the difference between treatments in βHB concentrations on day -30, these levels do not indicate intense body reserve mobilization for the NS animals, and even ADG presented differences among treatments upon the pre-partum. It is important to emphasize that most studies into energy deficit in ruminants have been done with dairy cows, and despite lack of information on beef cows serum βHB levels, it may be understood that those levels herein do not suggest severe energy deficit, rather cows had differing nutrient balances during the pre-partum.
Nevertheless, βHB levels had not influence days from calving to conception, unlike Mulliniks et al. [2] who found that low βHB concentrations were associated with an earlier conception date in beef cows. The contrasting experimental results may be explained by the fact that the βHB values that impaired reproduction cited by the above authors were higher at -30-d (0.71 mmol/L) than those of our experiment (0.48 mmol/L). Under these conditions, the mobilization of body reserves does not lead to loss of performance and reproduction.
Albumin concentrations significantly lowered after calving, which could be related to amino acids demand for milk production [45]. Although the lowest value appeared on day 45, it still fell within the reference values (3.03-3.55 g/dL) [46]. On pre-calving day 30 and upon calving, globulin lowered in relation to the rest of the period, justified by the transfer of immunity to colostrum production [47], as reflected in the behavior of the total proteins concentrations.
Unlike the other protein status indicators, BUN levels were higher for the supplemented animals only during the pre-partum period. This is basically due to the effect of supplementation, which raised ammonia in the rumen. As urea is considered a short-time protein indicator, higher BUN levels were expected during the supplement period. Urea is synthesized in the liver in proportional amounts to the concentration of ammonia produced in the rumen, and its concentration is related directly to dietary protein levels [48].
The creatinine blood concentration is an index of muscle mass, insofar that creatinine excretion is proportional to lean body mass and is, therefore, proportional to an animal's BW [49]. As expected, creatinine concentrations lowered linearly throughout the peripartum due to weight loss, with the lowest values on post-partum days 30 and 45, but still within the reference values (1-2 mg/dL) [46].
Nutritional status also regulates the production of IGF-I and insulin, which are sensitive to nutritional plan [50] and are extremely helpful parameters. Feed-restricted animals display lower serum glucose and, consequently, less insulin, which reduces the somatotropin receptors in the liver, the main mediator of IGF-I production. Thus, animals in the catabolic state have lower plasma IGF-I concentrations [51-53]. Therefore, IGF-I and insulin are physiologically linked and both increase with enhanced BCS. However, the regulation of each hormone individually may vary according to metabolic status, and/or to the direction of changes in BW.
During the experiment, insulin and IGF-1 levels behaved similarly and were not influenced by supplementation. Under similar conditions, Silva et al. [7] reported no difference in insulin levels of cows supplemented during the pre-partum. The higher values for both hormones were observed on the calving day, possibly due to blood glucose increasing and then lowering during early lactation as part of the homeorhetic changes to support galactopiesis [42]. IGF-1 concentrations were restored after 30-d post-partum, as were insulin levels on day 45, which thus stimulated steroidogenesis [54] and led to higher progesterone levels by day 45.
Food-restricted ruminants adapt to lower maintenance requirements by means of a slowing down the basal metabolism rate [55] due to lowering circulating levels of thyroid hormones. During this experiment, total T3 and T4 reduced during the pre-partum for the NS animals, which could be explained by a lowering metabolic rate compared to SS. Several works have also reported that cows during the post-partum with a negative energy balance respond to lower total T3 and T4 concentrations due to both the energy deficiency state and the huge demand of these hormones by mammary glands [56]. Conversely from parturition to 45-d, total T3 and T4 levels behave distinctly. Coggins and Field [57] demonstrated that, compared to T3, T4 serum concentrations were a more sensitive indicator of energy balance in lactating beef cows. This supports our observation herein of lower T4 levels on post-partum day 15, unlike T3, which did not vary much during the post-partum.