Energy sources and supplementation levels for beef heifers raised during the dry season

This experiment aimed at evaluating the productive performance, intake and the apparent digestibility in the raising of beef heifers on Brachiaria decumbens Stapf. pastures under supplementation with starch or fibrous energy sources during the dry season. It was used 40 newly weaned heifers (20 Nellore and 20 crossbreed), at 8.5 months of age and 197.9 ± 3.74 kg initial weight. Two supplements were formulated: the first was corn mealand soybean meal-based diet (high in starch) and the second was based on wheat meal (high in fiber), which were given at the quantities of 0.5 or 1.0 kg/animal/day, compared to the offer of only mineral mixture (control). The animals were distributed according to a complete randomized block design in a 2 × 2 + 1 factorial arrangement with two sources of energy and two levels of supplementation plus the control treatment; the genetic group was used as a measure of local control. The average daily weight gain was higher for the animals under supplementation than those fed only mineral mixture (0.198 vs. 0.077 kg/animal/day). Moreover, performance of the animals under supplementation with starch energy source was higher than those fed wheat meal-based supplement (0.232 vs. 0.163 kg/animal/day). The intake of pasture dry matter was lower for the group given the highest supplementation level. The supplementation of beef heifers under pasture makes it possible to improve the productive performance. Compared to wheat meal, the use of the corn/soybean mixture meal as the basis of multiple mixture improves the productive performance of the heifers.


Introduction
Dry season is the most critical season in the year for bovine production cycle under grazing, not only because of a marked fall of forage grass growth rate but also for the increase on proportions of indigestible compounds in the material consumed by the animals.The use of multiple supplements during this time of the year is shown as an essential condition for maintenance of body weight of the animals under grazing, and even enabling weigh gain instead of losses.
Energy food with lower quantities of non-fibrous carbohydrates, containing higher concentrations of fiber of rapid ruminal degradation could contribute for higher digestibility of dietary fiber, on the contrary, food high in non-fibrous carbohydrates reduce digestibility of the dietary fibrous portion especially in diets with low protein contends (Souza et al., 2010).
According to Paulino et al. (2008), supply of small quantities of energy-mineral-protein supplements is indicated for the raising phase, and levels from 0.1 to 0.4% of the body weight are suggested for this phase or during the entire life of the animal; so, higher levels of supplementation in the diet are usually associated to better performances.
Therefore, it was aimed in this work to evaluate the productive performance, intake and apparent digestibility in raising of beef heifers on Brachiaria decumbens Stapf pasture, under starch energy or fibrous source supplementations during the dry season.

Material and Methods
This experiment was carried out in Setor de Bovinocultura de Corte in Departamento de Zootecnia at Universidade Federal de Viçosa during the dry season from July to September 2007, with a duration of 89 days divided in three experimental periods.
It was used 40 heifers, 20 Nellore breed and 20 crossbred, with predominance of Zebu breed, newly weaned at 8.5 months of age and initial average weight 198.0 ± 3.74 kg.The animals were grouped in five treatments, using a complete random block design in a 2×2+1 factorial scheme (two energy sources and two levels of supplementation plus a control group, only fed mineral mixture), and the genetic group was adopted as measurement of local control.
The experimental area for the animals was composed of five 2.5-ha paddocks formed with Brachiaria decumbens Stapf., provided with covered water drinkers and troughs.
It was evaluated two supplements with approximately 30% crude protein (Table 1), one containing corn (starch source) and the other containing wheat meal (fibrous source) as energy source, given at 0.5 or 1.0 kg/animal/day ratio in comparison to mineral mix supply.Soybean meal was added to corn-based supplement in order to keep urea level even and content of crude protein in the multiple mix constant.
The supplements were given every day at 10 a.m. in a grouped feeder with two meters in length and access on both sides in order to allow simultaneous access by the animals.Mineral mix was freely given for all the animals.
At the beginning of the experiment, all animals were submitted to control for ecto and endoparasites and also for ticks, cloth and horn flies which was carried out during the experimental period, when it was necessary.
Animals were weighted without fast at the beginning of the experiment and at every 28 days, always in the morning, aiming at reducing possible interferences which could alter the intake or harm the performance and the correct data measurement.
Every 14 days, animals were rotated among paddocks in order to eliminate possible interferences on the results due to differences among paddocks (grazing availability, localization of water drinkers and trough, relief, shading, etc).For follow up and evaluation of daily average weight gain in each experimental period, weighing of the animals was carried out every 28 days.Total weight gain (TWG) was quantified by the difference between final and initial weight, in which the average daily weight gain is the ratio between total weight gain and number of experimental days.
Forage samples were collected on the first and on the last day of each experimental period in different paddocks through cut at five centimeters from the soil of five area delimitated by a 0.25-m 2 metal square, randomly chosen in each experimental paddocks following recommendations by McMeniman (1997).
After collection, each sample was weighted and homogenized and from them, it was taken two compound samples: one for evaluation of total dry matter (DM) availability and potentially digestible dry matter (pdDM) and another for analyses of availability of fresh leaf, dry 10 grams per animal per day immediately before supply of the supplement, so measurement of individual intake of the supplement could be done.In addition to TiO 2 , animals were given as an external marker, on the last three days of adaptation, LIPE ® (Rodriguez et al., 2006), which was used to calculate fecal excretion and it was applied via esophageal probe always around 12 p.m. On the eighth, ninth and tenth days, fecal collection was carried out at 4 p.m., 12 p.m., and at 8 a.m., respectively, aiming at obtaining representative fecal sampling of each animal during the experimental period.Feces were collected immediately in the rectum or right after defecation of the animals, in quantities around 200 g.These samples were per day and per animal identified and dried in forced ventilation oven at 55ºC for around 72 hours being ground in a Wiley mill with 1.00-mm porosity sieve.Following to separation of an aliquot for estimation of LIPE® concentration, samples were stored as compound samples, per animal, in properly identified and sealed plastic pots.
Individual intake of the supplement was calculated using titanium dioxide as external marker.The estimate of the supplement individual intake was obtained using the following equation: SupI = FE * iF / iSup in which SupI = supplement individual intake (kg/day); FE = fecal excretion (kg/day); iF = concentration of feces marker (g/kg); iSup = concentration of marker in the supplement (g/kg).
Estimations of iNDF in the feces, in the pasture samples obtained by manual grazing simulation and in the supplements were obtained after in situ incubation for 240 h as suggested by Casali et al. (2008).
It was calculated the replacement coefficient (RC) of the consumed forage by consumed supplement, so positive values of RC indicates that supplement was ingested by substituting forage; null value of this coefficient indicate that supplement ingestion was not performed to replace grazing and negative values demonstrate additive effect of supplement intake over pasture dry matter intake.Replacement coefficient was obtained through the equation: RC = (PDMIc -PDMIt) /SDMI leaf, fresh stem and dry stem.After being separated, samples were immediately taken to air forced circulation oven at 55ºC for 72 hours for posterior determination of pasture dry matter total availability.Potentially digestible dry matter (pdDM) was calculated by Paulino et al. (2008) using the following equation: pdDM = 0.98 (100 -NDF) + (NDF -iNDF) in which: NDF = neutral detergent fiber and iNDF = indigestible neutral detergent fiber.
The average temperature within the trial period was 18.6 o C in which there was accumulation of 35.8 mm of rain (Figure 1).
Samples for qualitative evaluation of pasture consumed by the animals were obtained through grazing manual simulation every 14 days, to which, together with samples with concentrated food, were evaluated for content of dry matter (DM), ether extract (EE), acid detergent fiber (ADF) and lignine (H 2 SO 4 72% w/w), following techniques by Silva & Queiroz (2002).
Contents of neutral detergent fiber (NDF) were calculated following recommendations by Mertens (2002).Correction regarded to ash and protein contained on neutral detergent fiber were carried out following recommendations by Mertens (2002) and Licitra et al. (1996), respectively.
For evaluation of intake and digestibility of the ingested diet, from the 10 th day in the second experimental period, a 10-day trial was carried out, in which, by following methodology by Titgemeyer et al. (2001), seven days were for adaptation of the animals to titanium dioxide (TiO 2 ), which was mixed to the supplementation at the quantity of in which: PDMIc = average intake of pasture dry matter by the control group animals; PDMIt = average intake of pasture dry matter by the animals in the question group and SDMI = average intake of supplement dry matter by the animals in the question group.
On the last day of digestibility trial, urine spot samples (10 mL) were collected in spontaneous urination of the animals and blood samples collection was done approximately four hours after supplement supply.After collection, samples of urine were diluted in 40 mL H 2 SO 4 0.036 N (Valadares et al., 1999) and frozen at -20ºC for later determination of creatine, urea and purine derivatives.Blood samples were collected at the end of urine collection period, using vacuum collection tubes and the samples were immediately centrifuged and the serum frozen at -20ºC.
Samples of urine were analyzed for creatine contend using a modified Jaffé method (Bioclin K016-1) for the urea contends and uric acid by colorimetric-enzymatic method (Human do Brasil) and blood samples were analyzed for urea contend using the same method utilized in urine analyses.Commercial kits were used for these analyses.
Calculations for daily urinary volume were done by using the relationship among creatinine daily excretion (CE) and its concentration in the spot samples adopting as a reference the equation proposed by Chizzotti et al. (2004): CE (mg/kgBW) = 32.27-0.01093 × BW in which: BW = body weight.
In this way, daily urinary excretion of nitrogenous compounds was the product between its concentration in the spot samples and the estimated value of the urinary volume.
Analyses for alantoine contend in the urine was performed using colorimetric method following Fujihara et al. (1987), quoted by Chen & Gomes (1992).
Total excretion of purine derivatives was calculated by the sum of alantoine and uric acid quantities excreted in the urine, expressed in mmol/day.
Statistical analyses referring to performance of the animals were carried out following a complete random block design in 2×2+1 factorial scheme and comparisons among observed means were performed through decomposition of the sum of squares for treatments in orthogonal contrasts related to supplementation and no supplementation, corn and wheat meal and 0.5 kg/animal/day and 1.0 kg/animal/day and interaction among the factors.Statistical procedures were carried out using SAS (Statistical Analysis System) computer program, adopting 0.10 as critical probability level for type I error.

Results and Discussion
Wheat meal or corn-based supplements showed contents of crude protein, neutral detergent fiber and non-   2).Average availability of forage in the paddocks was 5,013 kg DM/ha (Figure 2).At the beginning of the experiment, when pasture was deferred, dry matter availability of fresh leaf was 908 kg/ha and at the end of the experimental period this availability was reduced for 178 kg/ha, which means an 80% reduction.It is also observed an outstanding replacement of fresh stems by dry stems.A 50% reduction of fresh stem availability is observed while availability of dry stem raised approximately 23% confirming the outstanding drop in the nutritional quality of tropical forages during the year dry season.
Tropical forage species present great production potential, however, accumulation of dry matter is followed by reduction in the leaf:stem ratio and as dry season advances there is an outstanding reduction in the dry matter fresh:dry ratio, denoting the need for corrections in balance of the nutrients in grazing animal diets.
Forage collected trough manual grazing simulation showed an average content of 8.31% (Table 2), which is higher than the 6.13% considered by Sampaio (2007) as a lower limit for the minimal activity of ruminal microorganisms.Nevertheless, this value is lower than 10.83% crude protein showed by Lazzarini et al. (2009) as the level of crude protein that would maximize dry matter intake.Therefore, supplementation of protein in the diet explains the higher consumption of dry matter (P<0.10) by the animals under supplementation than those that were not given supplement (Table 3).
The highest dry matter intake is commonly associated to better performances, which can be confirmed in the present work when supplemented animals showed (P<0.01)CV = coefficient of variation; NDFacp = neutral detergent fiber correct for ash and protein; WB = body weight.
1 Described probability levels (P value) for Type I error for contrasts.
Table 3 average intake of pasture and supplement nutrients and digestibility of DM and NDFacp weight gain (17.6 kg) higher than the non-supplemented animals (6.9 kg) (Table 4).According to Lazzarini (2007), dietary levels of crude protein above 8.55% provide sufficient microbial growth that do not harm the digestibility of potentially degradable neutral detergent fiber.As the close value of crude protein showed by B. decumbens (8.31%) in this work, there was no significant differences (P>0.10) in NDFacp digestibility for supplemented animals in comparison to non-supplemented animals (Table 3).
Supplementation promoted improvements on the performance of the animals by increasing digestibility (P<0.10) and consequently the dry matter intake (P<0.10)(Table 3) and average daily weight gain (P<0.10)(Table 4).Performance in total weight gain of the animals was on average 20.6; 14.5 and 6.9 kg for animals supplemented on corn; wheat meal and non-supplemented animals, respectively (Table 4).
Despite of being low, daily average weight gain of the animals are compatible to those found by Silveira et al. (2007), who also worked during the dry season of the year with tropical grass, in which animals were given the quantity equal to 0.72 kg of the supplement/animal/day and based on corn, soybean meal and urea containing 25.9% of crude protein.The authors obtained average weight gain of 0.005 and 0.183 kg/animal/day for animals given only mineral mixture and supplemented animals, respectively.
The basic principle of pasture supplementation is to give catalytic doses of nutrients that maximize the use of basal substrate, in this case, the forage.Therefore, the effects of replacement of the pasture by supplement with no improvement on the performance are not desirable.It was observed that daily levels of supplementation of 0.5 kg/ animal resulted in a similar performance as compared to levels of 1.0 kg per day (Table 4).There was a higher pasture dry matter consumption (P<0.05) by the animals fed 0.5 kg of supplement without making difference on total dry matter intake (P>0.05) when it was compared to animals supplemented with 1.0 kg (Table 3), so the higher supplement intake by the animals than those which consumed 1.0 kg of the supplement was because of the replacement to pasture intake.So, replacement coefficient was -0.72 for animals supplemented with 0.5 kg of supplement/animal/day versus 0.48 for the animals supplemented with 1.0 kg of supplement/animal/day.
Use of supplements high in non-fibrous carbohydrates provides the occurrence of "carbohydrate effect" (Arroquy et al., 2005), which is the reduction on fiber digestibility by the fall in the activity of fibrolytics bacteria.Animals supplemented with corn-based ration showed non-fibrous Probability described levels (P value) for Type I error for contrasts carbohydrate intake higher than those fed wheat mealbased supplement (P<0.01), in counterpart, the expected fall regarded to NDFacp digestibility did not happen in a significant way.The occurrence of carbohydrate effect occurs basically because of a competitive advantage that a non-fibrous carbohydrate source provides to amylolytic bacteria in response to fibrolytic using nitrogen present in the medium.However, as it can be seen, levels of crude protein used in the supplement at about 32.0% outlined the effects of deficiency of this nutrient, which is very common in diets for bovines under tropical grass grazing.This fact can be supported even by the higher levels of ureic nitrogen observed in the blood of supplemented animals (Table 5).
The highest value of serum ureic nitrogen presented by animals fed wheat meal suggests higher utilization of nitrogen by ruminal microorganism when animals are supplemented with energy source high in starch, since they showed the lowest nitrogen absorption in the rumen, indicating lower ammonia concentrations.This value suggests even lower N requirements in the rumen when starch energetic source is replaced by a rapid degradation fibrous source.This fact is important since that protein is the most expensive nutrient in diets for animals.
The best performance of animals fed corn proves that, despite of being the highest in these animals, ruminal nitrogen requirement was completely met by the supplements.

Conclusions
Supplementation for grazing beef steers during the dry season enable an increase on productive performance.The use of corn/soybean meal mixture as base of multiple mixture improves the productive performance of the steers in comparison to wheat meal.

Figure 1 -
Figure 1 -Precipitation (in millimeters) and average temperature (°C) during the experimental period.
Probability described levels of contrasts for Type I error contrasts and means of initial body weight, final body weight, body weight gain, and average daily weight gain for the different treatments 1

Table 1 -
Composition of the supplements based on natural matter

Table 2 -
Chemical composition of supplements and of Brachiaria decumbens pasture

Table 5 -
Microbial nitrogenous compounds, serum ureic nitrogen, ureic urinary nitrogen and microbial efficiency in beef heifers on palisade grass pasture given supplement with two energy sources during dry season 1 Probability described levels (P value) for Type I error for contrasts.