Digestibility and microbial efficiency in steers fed diets based on corn silage hybrids and concentrate levels

A trial involving a 2x2 factorial design was conducted to evaluate the effect of corn silage hybrids and concentrate levels (25 and 50%) on intake and digestibility of nutrients, ruminal characteristics and microbial efficiency in steers. Four ruminal and abomasal cannulated steers (512±25kg of birth weight), were used in a 4×4 Latin square design. Treatments consisted of 75% silage A + 25% concentrate; 50% silage A + 50% concentrate; 75% silage B + 25% concentrate; and 50% silage B + 50% concentrate on dry matter (DM) basis. There were no differences in the intakes of DM, organic matter (OM), crude protein, and ether extract. The intake of non fiber carbohydrates and total digestible nutrients were positively affected by concentrate levels. The digestibility of DM and OM were also positively affected by concentrate levels. There were no effects of treatments on ruminal pH values, ruminal ammonia-N, and microbial efficiency.


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The feeding is the most expensive component of feedlot systems, mainly the concentrate ration cost.Thus, interactions and impacts of the use of different forage and concentrate ratios are extremely important to analyze the optimal relationship between cost and animal performance.In addition, the quality of silage used is also fundamental to get good economic results, because when using good quality silage, the utilization of concentrate can be decreased and consequently the cost of the diet will be lower.
Whole-plant corn silage is a popular forage source for ruminants due to its high yielding properties, energy content, relatively high palatability, and easy incorporation into total mixed ration.Furthermore, the corn plants have high water soluble carbohydrate content, adequate lactic acid production and, consequently, good results in quality silage.
Arq. Bras.Med. Vet. Zootec., v.64, n.3, p.649-657, 2012 However, hybrid, maturity, and moisture content are some of the factors that can alter the nutritive value of corn silage (Johnson et al., 2002).Several studies have shown differences between hybrids in nutrient composition of whole plant corn and yield of dry matter (DM; Hunt et al., 1993;Xu et al., 1995;Melo et al., 1999;Oliveira et al., 2003;Qiu et al., 2003).Commercial corn hybrids indicated for silage have been selected on the basis of agronomic traits such as grain yield and disease resistance (Clark et al., 2002) and differences in the nutritive value of whole plant corn silages related to corn genetics have been ignored.Therefore, the choice of hybrid for silage should include agronomic traits and evaluation of nutritive value, as well as the intake and performance of animals fed those silages.
The commercial corn hybrids AGN35-A42 (Agromen) and A3663 (Bayer) are indicated for both corn grain and whole-plant corn silage although feeding trials to evaluate ruminal characteristics and microbial efficiency involving these corn silage hybrids at the same environment condition are limited.The objective of this study was to determine the effects of two corn silage hybrids and two concentrate levels on intake, digestibility, ruminal characteristics, and microbial efficiency in crossbred steers.

MATERIAL AND METHODS
Two corn hybrids, (Agromen, AGN35-A42, and Bayer, A3663) were used in this trial.Agromen (AGN35-A42) is a double cross hybrid and has an ultra-short-season life cycle while Bayer (A 3663) is three-way cross hybrid with a shortseason life cycle.Both corn hybrids are indicated for the production of grains and whole plant silage.
Four Holstein x Zebu crossbred steers, averaging 512kg±25kg of birth weight (BW), and fitted with abomasal and ruminal cannulas, were used in a 4×4 Latin Square design to evaluate intake and apparent total tract and partial digestibility of nutrients, ruminal pH and ammonia-N, and microbial efficiency.Each experimental period had 18 d: 10 d for diet adaptation, 6 d to collect fecal and abomasal samples, 1 d for ruminal pH measurements and collection of ruminal fluid samples, and 1 d to collect ruminal contents to isolate bacteria.The experiment was conducted for 72 d (4 periods of 18 d).Steers were surgically fitted with ruminal and abomasal cannulaes in agreement with techniques described by Leão and Coelho da Silva (1980).
Steers were randomly assigned to four dietary treatment sequences and fed individually ad libitum twice daily (07:00 and 15:00h).Diets were fed as total mixed ration, being corn silage and concentrate (previously mixed) weighed and mixed at feeding time.Orts were collected and weighed once daily and the feed offered was adjusted daily to yield orts of about 5 to 10% of total offered.Animals had free access to water at all times.Feed ingredients and orts were sampled daily and composed by weight for each steer within each period.
Feces and abomasal digesta samples (approx. 200g and 500mL, respectively) were collected between the d 11 and 16 of each period with intervals of 26h between the samplings.Indigestible acid detergent fiber (iADF) was used as an internal marker to estimate apparent nutrient digestibility and fecal and abomasal output.After drying at 60°C for 72 h, feed, orts, and fecal and abomasal samples were ground to pass a 1-mm screen (Willey mill) and period composites per steer were prepared.
Ruminal contents (100mL) were obtained at 0, 1, 2, 4, 6 and 8 h after the morning feeding on d 17 of each period and subsequently strained through 2 layers of cheesecloth.pH was measured immediately.The ruminal fluid was preserved through the addition of 1mL of 9 M H 2 SO 4 , and stored at -20 o C for analyses of NH 3 -N concentration.
On d 18, the rumen contents were obtained 4h post-feeding and squeezed through two layers of cheesecloth to yield about 1,500mL of strained fluid.Particles retained on the cheesecloth were mixed with 500mL of 9g of NaCl/L, blended for 1min, refiltered through cheesecloth, and added to the 1.5L ruminal fluid sample.Bacteria were isolated by differential centrifugation (500g and 27,000g) according to procedures by Cecava et al. (1990).The resulting bacterial pellets were dried at 60 o C for 48h and ground in a ball mill.The dried bacterial samples were ground and analyzed for DM, ash, and total N, and total purines were determined as proposed by Ushida et al. (1985).
The determinations of the contents of NH3-N of ruminal and abomasal fluid samples were done according to the original procedures by of Fenner (1965) and adaptations by Vieira (1980).To quantify microbial protein and subsequently determine microbial efficiency, approximately 400mg of dry abomasal digesta samples were used, which were also analyzed for purines according to Ushida et al. (1985).
Data of intake, digestibility, and microbial efficiency were analyzed with the GLM procedure of SAS assuming a 4x4 Latin square design with a 2x2 factorial arrangement of treatments.The ruminal characteristics data collected over time were analyzed as repeated measures (Kuehl, 2000) using the MIXED procedure by SAS (Statistical..., 1990).When treatment interacted (P<0.05) with sampling time, variables were analyzed within time periods.The variance-covariance structure AR(1) was used for estimating covariances.Differences were considered to be significant when P≤0.05.

RESULTS
The chemical composition of silages is presented in Table 1.Both corn silage hybrids had similar nutrient values, except for DM and NFC, which were numerically lower in corn silage hybrid A than those observed in corn silage hybrid B. In addition, corn silage A had a pH value of 3.6 and N-NH 3 /Total N of 8.4% while corn silage B had a pH of 3.5 and N-NH 3 /Total N of 6.3%.

DISCUSSION
Overall, both silages had similar nutrient contents, except for a % of DM, NDF and NFC (Table 1).Freitas et al. (2003) also observed no effect of silages of five corn genotypes on DM intake and digestibility.According to Muck and Pitt (1993), both silages had good quality due to their adequate percentage of N-NH 3 /Total N and pH values.The diets with 50% concentrate in association with either corn silage hybrids provided higher amounts of DM, NFC and TDN and lower NDF than 25% concentrate diets, certainly due to a higher percentage of corn ground in those diets.
Although some authors have found linear (Dias et al., 2000;Souza et al. 2002;Pereira et al., 2006) or quadratic (Verás et al., 2000;Ítavo et al., 2002;Silva et al., 2005;Costa et al., 2005) increases in DM intake with an increase of concentrate levels in beef cattle diets, no effects of treatments were observed in this study.Moraes et al. (2002) also found no increase in DM intake with the concentrate levels.These variations among studies probably occurred due to other variables related to the kind of animal, age of animal, climatic conditions, days of feeding, etc, which can affect DM intake.
The higher intake of NFC and TDN and lower intake of NDF in steers fed diets with 50% concentrate likely occurred as a result of higher amounts of ground corn in these diets than those with 25%.Similar results were found by Costa et al. (2005)  Ruminal pH values were not affected either by silage or by concentrate levels, which means that the ruminal environment was adequate for microbial growth.Overall, the mean ruminal pH value observed was 6.02, which is greater than the 5.0 to 5.5 range that was suggested by Hoover (1986) in which ruminal digestibility of fiber is negatively affected.

Table 1 .
Chemical composition of corn silagesThe nutrient composition of the diets is shown in Table2.As expected, diets with 50% concentrate provided higher amounts of DM, NFC and TDN than diets with 25% concentrate.The intake of nutrients is shown in Table3.There were no treatment effects (P>0.05) on the intakes of DM, OM, CP, and EE.The intakes of NDF, NFC and TDN were influenced (P<0.05) by concentrate levels.

Table 3 .
Nutrient intake according to corn silage hybrids and concentrate levels

Table 4 .
Apparent total tract digestibility of nutrients and % of total digestible nutrients according silage hybrids and concentrate levels a Calculated as % of digested.DM= dry matter; OM= organic matter; CP= crude protein; EE= ether extract; NDF= neutral detergent fiber; NFC= non-fiber carbohydrates; S = silage source effect; C= concentrate level effect; S×C = silage and concentrate interaction effect.
. VALADARES FILHO, S.C. et al.Intake, digestibility, and performance of steers fed diets based on two corn silage hybrids and two concentrate levels.Arq.