Performance trial and residual feed intake estimative
The experiment was conducted from April to August 2013 at the Federal University of Goias, Goiania, Goias, Brazil (16º 40 ' S, 49º 15' W), where the climate is classified as Aw according to Köppen-Geiger - tropical wet and dry climate.. The performance trial lasted 84 days after a 14-day adaptation period. One-hundred-and-twenty Nellore bulls (20 months of age and initial body weight of 394.33 ± 37.42 kg) from 13 farms, participants in Nelore Qualitas® genetic improvement programme, were used. They were dewormed and randomly allocated to individual pens (12.5 m2). Bulls were weighed every 28 days after a 16-h water and feed withdraw. Metabolic weight was calculated as BW0.75
The diet was formulated to meet a predicted gain of 1.6 kg/d (National Research Council, 2000) and an estimated intake of 2.5% BW. Feed was offered once a day at 13.00 and feed offered and refused was recorded daily. The diet was adjusted to allow approximately 10% of orts. Feed and orts were collected weekly, and samples were composed over a 28-day period. Chemical and nutrient compositions of diets are presented in Table 1.
Table 1: Diet composition and nutritional characteristics
Ingredients
|
g kg-1 DMa
|
Corn silageb
|
180
|
Sugarcane bagasse
|
50
|
Sorghum meal
|
468
|
Soybean hulls
|
243
|
Soybean meal
|
35
|
Urea
|
8
|
Mineral premix
|
16
|
Chemicalcomposition (g kg-1 DM)
|
Dry matter (g kg-1 as fed)
|
618
|
Crude protein
|
147
|
Neutral detergent fiber
|
360
|
Acid detergent fiber
|
240
|
Ether extract
|
25
|
Mineral salts
|
45
|
Total carbohydrates
|
780
|
Non fibrous carbohydrates
|
420
|
Total digestible nutrients
|
669
|
a dry matter; b corn silage without cob
Daily dry matter intake (DMI) of each animal was calculated as the difference between feed offered and orts. The following linear regression model was adjusted to estimate DMI (eDMI) according to Archer et al. (1997):
eDMI = β0 +( β1 × ADG) + (β2 × BW0.75) + εi,
where β0 and β1 are the partial regression coefficients of DMI on ADG and metabolic weight, and ε is the random error (which represents RFI). Residual feed intake was calculated as the difference between observed DMI and expected DMI (eDMI).
After 84 days in the feedlot, animals were classified into three groups, according to the methodology of Basarab et al. (2003), as follow: low residual feed intake (RFI), medium RFI and high RFI. Data from ten bulls from each group were used to estimate digestibility and to evaluate ingestive behaviour.
Digestibility trial
Indigestible NDF (iNDF) was used as an internal marker to estimate faecal output. Feacal collection from all 120 animals occurred during three consecutive days, at 0800h, 12h00 and 16h00, respectively, according to Ferreira et al. (2009), during the experiment. Faeces were collected after spontaneous defecation or manual collection direct from rectum. Composite samples were elaborated per animal. Analyses of faeces were done in 30 selected animals.
Samples of feed ingredients, orts and faeces were dried in a ventilated oven at 55°C for 72 h, ground in a Wiley mill (sieve of 1 mm). Dry matter (DM, method 934.01), ash (method 942.05), ether extract (EE, method 920.39) and crude protein (CP, method 945.18) were determined according to AOAC, (1990). Neutral detergent fiber (NDF), NDF exclusive of protein and residual ash (NDFomp) and acid detergent fiber (ADF) were determined according to Detmann et al. (2012). Total carbohydrates (TC) were calculated according to NRC (2000) as follows: TC= 100- (%CP + %EE+ %Ash). Non fiber carbohydrates (NFC) were calculated according to Hall (2000): NFC= 100 – [% Ash+ %EE+ %aNDF + (%CP-%CPu + U)], in which CPu is the CP from urea and U is the urea content. Total digestible nutrients (TDN) were calculated according to National Research Council (2001): TDN= % digestible CP + 2.25x % digestible EE + %digestible NDFap + % digestible NFC.
For iNDF quantification, composite samples (feed, orts and feces) were put in non-woven textile bags (100g/m2) at a ratio of 20 mg DM/cm2 of surface (Nocek, 1988). The bags were kept for 264 h (Casali et al., 2008) in the rumen of a cannulated bull. After that, the bags were cleaned with tap water, sequentially oven dried. The bags were then submitted to extraction with neutral detergent.
Digestibility was calculated as follows: DMD (%) = 100-[100 × (Mfd / Mfc)], where Mfd is the marker concentration in the feed and Mfc is the marker concentration in faeces.
Ingestive behaviour trial
Ingestive behaviour was evaluated for 24 h at intervals of five minutes for the selected animals at d 85. The following behavioural categories were evaluated: feeding, ruminating, and idling (ingestion of water, interaction with other animals, rest). Feeding efficiency (FE) and rumination efficiency (RE) were calculated as FE (kg) = DMI (kg)/ feeding time (h); RE = DMI (kg)/ ruminating time (h).
Statistical Analysis
Data was analyzed as a completely randomized design with three treatments and ten replicates, using “easyanova” procedure of R (Arnhold, 2013). The statistical model was as follows:
Yij= μ+Ti +eij,
where Yij is the dependent variable, μ is the overall mean for each parameter, Ti is the effect of treatment and eij is the residual error.
The Tukey test was used to compare treatment means, and significant differences were declared at P≤0.05 and tendency at P≤0.10. Pearson’s linear correlation analysis was performed between the behaviour variables, apparent digestibility, performance data and RFI.