Introduction
Early weaning for calves reared in a feedlot production system may be defined as separating calves from their dams before 180 days of age (Rasby, Reference Rasby2007). This management technique has been used to improve body condition score of the cows after calving (Houghton et al., Reference Houghton, Lemenager, Horstman, Hendrix and Moss1990; Arthington and Kalmbacher, Reference Arthington and Kalmbacher2003; Arthington and Minton, Reference Arthington and Minton2004), since they have greater nutritional demands during the lactation phase (Arthington et al., Reference Arthington, Spears and Miller2005). In this context, the majority of primiparous cows are in a condition that needs a greater amount of nutrients to support lactation and continued growth (Cooke et al., Reference Cooke, Daigle, Moriel, Smith, Tedeschi and Vendramini2020).
Although early weaning of calves has been used to preserve primiparous dams, the allocation of their calves in a feedlot system might be a viable strategy for producers. Some studies (Myers et al., Reference Myers, Faulkner, Ireland, Berger and Parrett1999; Arthington et al., Reference Arthington, Spears and Miller2005; Arthington and Vendramini, Reference Arthington and Vendramini2016) have reported that early-weaned calves have exceptional gain : feed and an improvement in their carcase quality when fed a total mixed ration (TMR) in the feedlot compared to normal weaned animals. Nevertheless, it is worth mentioning that early-weaned beef calves are in the growth phase, and therefore, have different nutritional requirements compared to beef cattle in the finishing phase. Thus, starch-rich feedlot diets could provide a greater early fat deposition and negatively influence the performance of these animals, being important to provide diets that meet their nutritional requirements.
However, the nutrient requirements and performance of early-weaned Bos indicus beef calves allocated in a feedlot system during the growing and finishing phases are not well documented in the literature. Energy and protein requirements for calves have been established based on diets containing milk proteins, which have a high digestibility and biological value (NRC, 2001; Valadares Filho et al., Reference Valadares Filho, Costa e Silva, Gionbelli, Rotta, Marcondes, Chizzotti and Prados2016) compared to other ingredients. According to Davis and Drackley (Reference Davis and Drackley1998), calves may not use non-milk proteins at such high efficiencies as the protein from milk, and some adjustments may need to ensure an adequate supply of amino acids for growth when such protein sources are used. Moreover, protein requirements could be lower for beef calves in feedlots compared with those in pasture systems.
According to Valadares Filho et al. (Reference Valadares Filho, Costa e Silva, Gionbelli, Rotta, Marcondes, Chizzotti and Prados2016), suckling beef calves [140 days of age, 150 kg and 0.6 kg/day average daily gain (ADG)] in pasture feeding systems have been shown to have crude protein (CP) requirements of approximately 145 g CP/kg dry matter (DM). Amaral et al. (Reference Amaral, Mariz, Zanetti, Prados, Marcondes, Santos, Detmann, Faciola and Valadares Filho2018) showed that male beef calves in traditional weaning (240 days of age) allocated in a feedlot should receive diets with a CP content of approximately 140 g CP/kg DM to achieve an ADG of 1.2 kg during the initial growing phase (112 days). However, these authors suggest that the CP content could be reduced to 120 g CP/kg DM during the finishing stage for Nellore and to 100 g CP/kg DM for Crossbred. Similarly, Menezes et al. (Reference Menezes, Valadares Filho, Pacheco, Pucetti, Silva, Zanetti, Paulino, Silva, Neville and Caton2019) suggested that the protein requirements for finishing young Nellore bulls can be met through dietary CP concentrations of 105–125 g/kg DM. Therefore, these data suggest that CP requirements decrease with age and as cattle reach final body weight (BW) (Todd et al., Reference Todd, Cole, Clark, Flesch, Harper and Baek2008; Amaral et al., Reference Amaral, Mariz, Zanetti, Prados, Marcondes, Santos, Detmann, Faciola and Valadares Filho2018), it being possible to adjust diets according to the phase of the production system.
Fine grinding is the primary maize processing method adopted by most Brazilian feedlots (Millen et al., Reference Millen, Pacheco, Arrigoni, Galyean and Vasconcelos2009; Oliveira and Millen, Reference Oliveira and Millen2014; Pinto and Millen, Reference Pinto and Millen2018). However, feedlots have shown interest in using processing methods based on ensiling flint maize with high moisture (Pinto and Millen, Reference Pinto and Millen2018; Bernardes and Castro, Reference Bernardes and Castro2019) to maximize starch–protein matrix breakdown (Mahanna, Reference Mahanna2008; Hoffman et al., Reference Hoffman, Esser, Shaver, Coblentz, Scott, Bodnar, Schmidt and Charley2011; Silva et al., Reference Silva, Pacheco, Godoi, Silva, Zanetti, Menezes, Pucetti, Santos, Paulino and Valadares Filho2020a, Reference Silva, Pacheco, Godoi, Alhadas, Pereira, Rennó, Detmann, Paulino, Schoonmaker and Valadares Filho2020b), and consequently, increase starch digestibility and nutrients’ utilization (Owens et al., Reference Owens, Zinn and Kim1986). In that context, snaplage is a type of maize silage that usually contains maize grain, cobs and husks, which are harvested when the maize grain contains approximately 40% moisture (Mahanna, Reference Mahanna2008). The use of a forage harvester fitted with a maize snapper header and an on-board grain processor allows simultaneous harvesting and chopping of snaplage components (Lardy and Anderson, Reference Lardy and Anderson2016; Ferraretto et al., Reference Ferraretto, Shaver and Luck2018). Also, snaplage is considered a high-energy feed product (Lardy and Anderson, Reference Lardy and Anderson2016) that could be used as an exclusive fibre and energy source in feedlot diets or in combination with other feeds (Godoi et al., Reference Godoi, Silva, Menezes, Silva, Alhadas, Trópia, Silva, Andrade, Schoonmaker and Valadares Filho2021b). Therefore, diet formulation using snaplage will vary according to the harvest methods and desired performance. Furthermore, the use of snaplage in feedlots may improve nutrients’ digestibility and facilitate daily operations.
Based on the protein requirements suggested by Valadares Filho et al. (Reference Valadares Filho, Costa e Silva, Gionbelli, Rotta, Marcondes, Chizzotti and Prados2016) and previously mentioned, we hypothesize that: (1) it is possible to feed early-weaned beef calves with lower feedlot dietary CP content without adversely affecting animal growth and performance and (2) lower dietary CP concentration would reduce nitrogen (N) excretion through urine and faeces to the environment. Therefore, this study was conducted aiming to verify the effects of dietary CP contents of early-weaned calves.
Regarding maize processing methods, we hypothesize that snaplage can be used as an exclusive fibre and energy source in feedlot cattle. Thus, this study was conducted aiming to verify the influence of flint maize processing methods on intake, total tract nutrient digestibilities and performance of Nellore heifers.
Materials and methods
The experiment was conducted at the Experimental Feedlot of the Animal Science Department at the Universidade Federal de Viçosa (UFV), Viçosa, Minas Gerais, Brazil. The total experiment lasted 196 days. In phase 1, three CP contents were evaluated in early-weaned Nellore female calves for 112 days. The CP contents were: (1) low (130 g CP/kg DM), (2) medium (145 g CP/kg DM) and (3) high (160 g CP/kg DM). In phase 2, maize processing methods were evaluated in the growth and finishing of Nellore females and lasted 84 days. The results of both phases, phases 1 and 2, were combined in the current study to provide information regarding the performance of early-weaned calves allocated in a feedlot system until the finishing phase.
Animal handling, experimental designs and diets
Fifteen early-weaned Nellore heifer calves (initial BW of 108 ± 13.1 kg) were used in the study. The animals were weaned with approximately 4 months of age in the beef cattle sector, and they were immediately transported to the Experimental Feedlot of the Animal Science Department at the Universidade Federal de Viçosa. Nellore female calves were born from primiparous cows. During 40 days before the weaning, calves received ad libitum creep-feeding supplementation as a previous acclimation period to the feedlot diet. The supplement was formulated with ground maize, wheat bran, soybean meal, urea, ammonium sulphate, salt and mineral mix. Moreover, each calf was fitted with an ear tag (left ear) containing a unique radio frequency transponder (FDX-ISO 11784/11785; Allflex, Joinville, Santa Catarina, Brazil), and treated for the elimination of internal and external parasites by the administration of injectable ivermectin (Ivomec; Merial, Paulinia, São Paulo, Brazil).
Phase 1
The remaining 15 early-weaned Nellore female calves were randomly assigned to receive one of the three ad libitum dietary treatments (n = 5 female calves per treatment) with different CP concentrations, as follows: (1) low (130 g CP/kg DM), (2) medium (145 g CP/kg DM) and (3) high (160 g CP/kg DM). Urea + ammonium sulphate (9 : 1) and soybean meal were used as a strategy to increase the CP content of the diets. It is worth mentioning that urea + ammonium sulphate and soybean meal ratio was similar (approximately 1 :7 .5) to avoid a large variation in the proportion of rumen degradable protein (RDP) and rumen undegradable protein (RUP) among the experimental diets. Feedstuffs and chemical composition of experimental diets during phase 1 are detailed in Table 1. Diets were formulated according to the BR-CORTE system (Valadares Filho et al., Reference Valadares Filho, Costa e Silva, Gionbelli, Rotta, Marcondes, Chizzotti and Prados2016) to achieve an ADG of 0.6 kg/day. The RDP was calculated according to the Brazilian Tables of Chemical Composition of Feeds described by Valadares Filho et al. (Reference Valadares Filho, Costa e Silva, Gionbelli, Rotta, Marcondes, Chizzotti and Prados2016), and the RUP was estimated by difference.
Table 1. Feedstuffs and chemical composition of experimental diets during phase 1
a Urea + ammonium sulphate in a 9 : 1 ratio.
b Mineral mix = 7.83 g S/kg; 5950 mg Co/kg; 10 790 mg Cu/kg; 1000 mg Mn/kg; 1940 mg Se/kg; 1767.4 mg Zn/kg.
c Calculated according to Brazilian Tables of Chemical Composition of Feeds described by Valadares Filho et al. (Reference Valadares Filho, Costa e Silva, Gionbelli, Rotta, Marcondes, Chizzotti and Prados2016).
d Rumen undegradable protein (g/kg CP) = 1000 − rumen degradable protein (g/kg CP).
e Corrected for residual ash and nitrogen compounds.
f NFC = 100 − [(CP − CP from urea + urea) + NDF corrected for residual ash and residual nitrogenous compounds + EE + ash].
Phase 2
At the end of phase 1, the animals were randomly redistributed into two groups (n = 6 female calves per treatment). Each group received two animals from each previous treatment. So, the remaining three animals were assigned to another study. The animals were fed ad libitum, receiving the following treatments: (1) 0.60 dry ground maize grain, 0.30 whole-plant maize silage plus 0.10 mineral-protein supplement (DM basis; DMG), or (2) 0.90 snaplage plus 0.10 mineral-protein supplement (DM basis; SNAP-90). The proportions of snaplage used in this experiment were approximately 0.78 grains, 0.13 cob and 0.9 husk. Diets were formulated according to BR-CORTE recommendations (Valadares Filho et al., Reference Valadares Filho, Costa e Silva, Gionbelli, Rotta, Marcondes, Chizzotti and Prados2016) to provide 125 g CP/kg DM and support an ADG of 1.0 kg/day (Table 2).
Table 2. Feedstuffs and chemical composition of experimental diets during phase 2
a Premix guarantees (per kg of DM): 200–220 g of Ca, 10 mg of Co (min), 500 mg of Cu (min), 22 g of S (min), 333 mg of Fe (min), 178.41 mg of F (max), 10 g of P (min), 25 mg of I (min), 17 g of Mg (min), 1500 mg of Mn (min), 1100 mg of monensin, 100 × 109 CFU of Saccharomyces cerevisiae (min), 6.6 mg of Se (min), 50 g of Na (min), 100 000 IU of vitamin A (min), 13 000 IU of vitamin D3 (min), 150 IU of vitamin E (min) and 2000 mg of Zn (min).
b Urea + ammonium sulphate in a 9 : 1 ratio.
c Total digestible nutrients (TDN) was estimated using concentrations of CP, EE, ash, NDF, lignin, acid and neutral insoluble CP as described by the BR-CORTE system (Valadares Filho et al., Reference Valadares Filho, Costa e Silva, Gionbelli, Rotta, Marcondes, Chizzotti and Prados2016) and then metabolizable energy (ME) was calculated using the following equations: DE = TDN × 4.4; and ME = 0.9455 × ED − 0.3032 (Valadares Filho et al., Reference Valadares Filho, Costa e Silva, Gionbelli, Rotta, Marcondes, Chizzotti and Prados2016) multiplied by the conversion factor from Mcal/kg to MJ/kg of 4.184.
d Corrected for residual ash and nitrogen compounds.
Facilities and experimental procedures
In both phases, the animals were housed in collective pens (48.0 m2) with electronic feeder (model AF-1000 Master; Intergado Ltd., Contagem, Minas Gerais, Brazil) and waterer (model WD-1000 Master; Intergado Ltd.). Animals were weighed at the beginning and the end of each phase after undergoing a 16-h fasting period to measure the initial and final BW. Also, animals were weighed every 28 days to evaluate and monitor ADG and BW.
The TMRs were provided twice a day at 07.00 h and 15.00 h. Feed delivery was adjusted daily to maintain minimum orts the next day and ad libitum intake. The appropriate feed delivery for each group was based on orts weight each morning. Electronic feeders were evaluated at 06.00 h daily to quantify orts and adjust daily feed delivery to a maximum of 2.5% orts. According to the amount of orts, the TMR was reduced (more than 2.5% orts at morning evaluation) or increased (less than 2.5% orts at morning evaluation) to reach ad libitum intake. Each treatment was delivered to the electronic feeder and consequently provided unique access to individual animals. Using the electronic identification tags, individual daily feed intake was recorded and measured using electronic equipment (model AF-1000 Master; all Ltda., Contagem, Minas Gerais, Brazil; Chizzotti et al., Reference Chizzotti, Machado, Valente, Pereira, Campos, Tomich, Coelho and Ribas2015).
All feeds were sampled daily. Feed samples were oven-dried (55°C), ground in a knife mill (R-TE-650/1, Tecnal, Piracicaba, São Paulo, Brazil) using 1-mm screen sequentially, packed in plastic bags, and stored at room temperature (20°C) for further laboratory analyses.
Sample collections, digestibility assays and slaughter procedures
To evaluate apparent total tract digestibility and to estimate nutrient excretion, two digestibility assays were performed in each phase. The spot sampling of faeces was collected through rectal palpation over a 3-day period. In phase 1, spot faecal samples were collected on day 50 to 52 and day 106 to 108. Moreover, in phase 2, spot faecal samples were collected on day 40 to 42 and day 80 to 82. The collections were conducted at 06.00 h on day 1, at 12.00 h on day 2 and at 18.00 h on day 3 of each digestibility assay (Prados et al., Reference Prados, Sathler, Silva, Zanetti, Valadares Filho, Alhadas, Detmann, Santos, Mariz and Chizzotti2017; Amaral et al., Reference Amaral, Mariz, Zanetti, Prados, Marcondes, Santos, Detmann, Faciola and Valadares Filho2018). Faeces were packed in aluminium trays, partially dried in a forced ventilation oven at 55°C, and then ground in a knife mill as described above. These collection times were used to obtain proportional and representative samples for each treatment. A composite sample from each animal was created per period. Indigestible neutral detergent fibre (iNDF) was used as a marker to estimate faecal DM excretion.
Spot urine samples were collected by spontaneous urination simultaneously to faeces collection at 12.00 h on day 2 of each phase (Valadares et al., Reference Valadares, Broderick, Valadares Filho and Clayton1999; Prados et al., Reference Prados, Sathler, Silva, Zanetti, Valadares Filho, Alhadas, Detmann, Santos, Mariz and Chizzotti2017; Zanetti et al., Reference Zanetti, Godoi, Estrada, Engle, Silva, Alhadas, Chizzotti, Prados, Rennó and Valadares Filho2017; Makizadeh et al., Reference Makizadeh, Kazemi-Bonchenari, Mansoori-Yarahmadi, Fakhraei, Khanaki, Drackley and Ghaffari2020). After each collection day, 10 ml of urine was diluted in 40 ml of 0.036 N sulphuric acid solution to avoid bacterial destruction of purine derivative (Chen and Gomes, Reference Chen and Gomes1992; Menezes et al., Reference Menezes, Valadares Filho, Costa e Silva, Pacheco, Pereira, Rotta, Zanetti, Detmann, Silva, Godoi and Rennó2016). These samples were stored at −20°C for further laboratory analyses.
The heifers were slaughtered after 196 days at an average shrunk body weight (SBW) of 285 kg, close to the average for young heifers in the Brazilian system (Costa e Silva et al., Reference Costa e Silva, Engle, Valadares Filho, Rotta, Valadares, Silva and Pacheco2015; Valadares Filho et al., Reference Valadares Filho, Costa e Silva, Gionbelli, Rotta, Marcondes, Chizzotti and Prados2016). Before slaughter, heifers were solid fasted for 16-h to estimate SBW. Heifers were slaughtered via captive bolt stunning followed by exsanguination. After slaughter, the carcase of each heifer was separated into two halves, weighed to quantify hot carcase weight and dressing percentage, and then chilled at 4°C for 18 h. The next day, half-carcases were removed from the cold chamber, weighed again and cold carcase yields were calculated. Subcutaneous fat thickness was then measured using a digital caliper in the region between the 11th and 12th rib cut.
The left half-carcase of each animal was totally dissected into muscle, fat and bone, and each portion was weighed separately. After that, the muscle and fat of each bull were homogenized and ground to obtain a composite sample. The bones were sliced with a band saw (Skymsen, model SFL-315HD, Santa Catarina, Brazil) in subsections of 1.5-cm length to obtain a representative sample of the bones. The composite sample of muscle and fat and the sample of bones were lyophilized and then were ground in a knife mill (Fortinox, Piracicaba, São Paulo, Brazil) with a 1-mm screen sieve to evaluate the DM, organic matter (OM), N and ether extract (EE) contents.
Laboratory analyses and calculations
Individual feed ingredients and faeces were quantified in terms of DM, OM, N and EE according to the AOAC (2012, method numbers 934.01, 930.05 and 981.10; 2006, method number 945.16, respectively). NDF was analysed according to the technique described by Mertens et al. (Reference Mertens, Allen, Carmany, Clegg, Davidowicz, Drouches, Frank, Gambin, Garkie, Gildemeister, Jeffress, Jeon, Jones, Kaplan, Kim, Kobata, Main, Moua, Paul, Robertson, Taysom, Thiex, Williams and Wolf2002) without the addition of sodium sulphite, but with the addition of thermostable alpha-amylase to the detergent (Ankom Tech. Corp., Fairport, NY). The analyses of NDF were performed by using a fibre analyser (Ankom Technology, Macedon, NY, USA). The NDF content was corrected for residual ash and protein (apNDF). Estimations of neutral detergent insoluble nitrogen followed the technique described by Licitra et al. (Reference Licitra, Hernandez and Van Soest1996).
The faecal DM excretion was obtained by dividing iNDF intake by the faecal iNDF concentration. To quantify iNDF, the faecal samples, concentrate, orts, maize silage and snaplage were placed in filter bags (model F57, Ankom) and incubated in a rumen-cannulated animal for 288 h (Valente et al., Reference Valente, Detmann, Valadares Filho, Cunha, Queiroz and Sampaio2011). Non-fibre carbohydrates (NFC) were calculated following the recommendations of Detmann and Valadares Filho (Reference Detmann and Valadares Filho2010) regarding the use of urea (or urea : ammonium sulphate mixture) in diets. The faecal content of NFC and its digestibility were calculated based on the equation for feed NFC (NRC, 2001; Makizadeh et al., Reference Makizadeh, Kazemi-Bonchenari, Mansoori-Yarahmadi, Fakhraei, Khanaki, Drackley and Ghaffari2020).
The starch analysis was based on an enzymatic method following the recommendations of Silva et al. (Reference Silva, Godoi, Valadares Filho, Zanetti, Benedeti and Detmann2019). Briefly, sequential incubations in a water bath with solutions of thermostable α-amylase and amyloglucosidase were performed in standards of d-glucose anhydrous (0, 50, 100, 150, 200 and 250 mg) and samples (i.e. feed and faeces) previously weighed in screw-cap test tubes. This procedure is used to establish a process of starch hydrolysis into glucose. Then, an o-toluidine solution is used to allow glucose quantification. The standard absorbance obtained through reading a spectrophotometer (630 nm) is used to adjust the simple linear regression equation without an intercept according to the Lambert–Beer law (Skoog et al., Reference Skoog, Holler and Crouch2017). Thus, the sample content of glucose released from starch is converted into starch content (% of DM).
Urine samples were analysed for N concentration (method 981.10; AOAC, 2012). Uric acid, creatinine and urea in urine were analysed with kits Bioclin® (K0139, K067 and K056, Belo Horizonte, Brazil) and determined by an automated biochemical analyser (Mindray, BS200E, Shenzhen, China). Allantoin was analysed according to the descriptions in Chen and Gomes (Reference Chen and Gomes1992). Microbial efficiency was estimated as described by Barbosa et al. (Reference Barbosa, Valadares, Valadares Filho, Pina, Detmann and Leão2011) in accordance with daily purine derivative excretion and measured by sum of allantoin and uric acid excretion via urine. Microbial efficiency was expressed in grams of microbial protein synthesized by kilograms of TDN intake (g MCP/kg NDT) and by kilogram of digestible OM intake (g MCP/kg DOM). N balance was calculated based on the difference of N intake, faecal N and urine N.
Statistical analyses
Data from both phases were analysed under a completely randomized design by PROC MIXED procedures in SAS (version 9.4, SAS Institute Inc., Cary, NC, USA), where the animals were considered as the experimental units and treatments were fixed effects in the model.
In phase 1, the orthogonal contrast was used to identify linear and quadratic effects of CP concentration. Analysis of variance assumptions were verified for all variables. The regression equations were performed using PROC REG procedures in SAS (version 9.4, SAS Institute Inc., Cary, NC, USA). In phase 2, the least-squares means were compared using the Fisher's least significant difference test. Results were deemed significant when P ≤ 0.05 and tendency was considered when 0.05 < P ≤ 0.10.
Results
Phase 1
Voluntary intake and digestibility of DM, OM, NDF and starch were not different (P > 0.10) among treatments (Table 3). However, there was a linear tendency (P = 0.07) for N intake with increasing dietary CP content (Table 4). The apparent digestibility coefficient of N increased linearly (P < 0.05) as dietary N concentration increased (0.75, 0.77 and 0.79 for low, medium and high based diets, respectively). The linear regression to estimate the apparent digestibility coefficient of N was as follows: apparent digestibility coefficient of N = 0.588 + 0.0013 × CPc (R 2 = 0.40), where CPc is the crude protein content of the diet (g/kg).
Table 3. Voluntary intake and apparent digestibility coefficient by calves fed three CP contents during phase 1
a Low = 130 g CP/kg DM; medium = 145 g CP/kg DM; high = 160 g CP/kg DM.
Table 4. Nitrogen (N) balance of calves fed three CP contents during phase 1
a Low = 130 g CP/kg DM; medium = 145 g CP/kg DM; high = 160 g CP/kg DM.
b Urine volume (litres/day) was estimated based on daily urinary creatinine excretion [UCE (mg/day) = 37.88 × SBW0.9316; where SBW is the animal shrunk body weight (kg)] and creatinine concentration in urine spot sample as described by the BR-CORTE system (Valadares Filho et al., Reference Valadares Filho, Costa e Silva, Gionbelli, Rotta, Marcondes, Chizzotti and Prados2016).
c Microbial CP synthesis.
There was no significant difference (P > 0.10) on faecal N when dietary CP increased. On the other hand, urinary N (g/day and g/kg of excreted N) increased linearly (P < 0.05) as dietary CP increased. The linear regression to estimate the urinary N was as follows: urinary N (g/day) = 0.302 × CPc (R 2 = 0.98). No difference (P > 0.10) was observed on N retained (g/day and g/kg of N intake) between the diets. A significant linear effect (P < 0.05) was observed for urinary urea N (g/day) with increasing dietary CP content. The linear regression to estimate the urinary urea N was as follows: urinary urea N (g/day) = −42.47 + 0.543 × CPc (R 2 = 0.62).
No difference (P > 0.10) was observed on microbial N production (g/day) or microbial efficiency (g MCP/kg NDT and g MCP/kg DOM) with increasing dietary CP content.
Animal performance was not affected (P > 0.10) by dietary CP content (Table 5).
Table 5. Performance of calves fed three CP contents during phase 1
a SBW, shrunk body weight; ADG, average daily gain; G : F, gain-to-feed ratio.
b Low = 130 g CP/kg DM; medium = 145 g CP/kg DM; high = 160 g CP/kg DM.
Phase 2
The different flint maize processing methods on the finishing phase of heifers did not alter (P > 0.10) the voluntary intake and digestibility of DM, OM, NDF, starch and N (Tables 6 and 7).
Table 6. Voluntary intake and apparent digestibility coefficient by heifers fed different flint maize processing methods during phase 2
a DMG = 0.60 dry ground maize grain, 0.30 whole-plant maize silage and 0.10 mineral–protein supplement; SNAP-90 = 0.90 snaplage and 0.10 mineral–protein supplement.
Table 7. Nitrogen (N) balance and microbial efficiency of heifers fed different flint maize processing methods during phase 2
a DMG = 0.60 dry ground maize grain, 0.30 whole-plant maize silage and 0.10 mineral–protein supplement; SNAP-90 = 0.90 snaplage and 0.10 mineral–protein supplement.
b Urine volume (litres/day) was estimated based on daily urinary creatinine excretion [UCE (mg/day) = 37.88 × SBW0.9316; where SBW is the animal shrunk body weight (kg)] and creatinine concentration in urine spot sample as described by the BR-CORTE system (Valadares Filho et al., Reference Valadares Filho, Costa e Silva, Gionbelli, Rotta, Marcondes, Chizzotti and Prados2016).
c Microbial CP synthesis.
There was no significant difference (P > 0.10) in faecal N, urinary N (g/day and g/kg of excreted N), and N retained (g/day and g/kg of N intake) among the diets. Also, no difference (P > 0.10) was observed on animal performance, carcase characteristics, and carcase composition of heifers fed different flint maize processing methods (Table 8).
Table 8. Performance, carcase characteristics and carcase composition of heifers fed with different flint maize processing methods during phase 2
a DMG = 0.60 dry ground maize grain, 0.30 whole-plant maize silage and 0.10 mineral–protein supplement; SNAP-90 = 0.90 snaplage and 0.10 mineral–protein supplement.
b SBW, shrunk body weight; ADG, average daily gain; G : F, gain-to-feed ratio.
Discussion
Phase 1
Dietary protein can be divided into RDP and RUP (NRC, 1985). Ruminal microorganisms could degrade the digestible proteins and non-protein compounds of diets into peptides, amino acids and eventually to ammonia (NH3; Hristov and Jouany, Reference Hristov, Jouany, Pfeffer and Hristov2005), which is the primary source of protein for ruminal microbial growth (Cotta and Russell, Reference Cotta and Russell1982).
Rumen microbial growth, and consequently, the microbial CP synthesis can be compromised when ruminants are fed with inadequate amounts of RDP. Also, the inadequate flow of MCP and RUP to the small intestine can compromise animal performance (Hristov et al., Reference Hristov, Hanigan, Cole, Todd, Mcallister, Ndegwa and Rotz2011) since the metabolizable protein requirements of ruminants are met by MCP, RUP and endogenous protein secretion (NRC, 2001). From a nutritional point of view, non-protein nitrogen sources, such as urea, have been included in diets to increase the RDP (Rufino et al., Reference Rufino, Detmann, Gomes, Reis, Batista, Valadares Filho and Paulino2016), and consequently, the amount of NH3. Also, the use of non-protein nitrogen sources could reduce costs compared with the use of RUP (Rufino et al., Reference Rufino, Detmann, Gomes, Reis, Batista, Valadares Filho and Paulino2016), being economically viable to the production system.
Regarding the requirements of weaned calves, the ruminant phase of calves starts from 8 weeks onwards of age (Gupta et al., Reference Gupta, Khan, Rastogi, ul Haq and Varun2016). Therefore, calves begin to derive their nutrients from solid feeds, mainly through microbial fermentation in the reticulo-rumen (NRC, 2001), resembling the rumen characteristics of adult animals.
The increase in dietary CP content did not affect the intake and digestibility of DM, OM, NDF and starch, suggesting that intake variable are not restricted or stimulated by dietary CP content (Menezes et al., Reference Menezes, Valadares Filho, Pacheco, Pucetti, Silva, Zanetti, Paulino, Silva, Neville and Caton2019). However, as would be expected, the planned difference in dietary N content resulted in a tendency of the linear increase in N intake as dietary CP content increased. Also, there was a subsequent increase in apparent N compounds’ digestibility. Also, according to Rufino et al. (Reference Rufino, Detmann, Gomes, Reis, Batista, Valadares Filho and Paulino2016), CP apparent digestibility coefficient is proportional to CP intake. A similar pattern was observed by Menezes et al. (Reference Menezes, Valadares Filho, Pacheco, Pucetti, Silva, Zanetti, Paulino, Silva, Neville and Caton2019) where young Nellore bulls fed high CP diet (145 g CP/kg DM) had greater apparent CP digestibility than bulls fed diets based on medium (125 g CP/kg DM) or low CP content (105 g CP/kg DM).
Microbial protein synthesis was estimated using purine derivative excretion. Microbial protein synthesis and microbial efficiency was not affected by the dietary CP content. According to Clark et al. (Reference Clark, Klusmeyer and Cameron1992), the availability of energy and nitrogen are the main determinants of microbial growth. Therefore, it suggests that both diets fulfilled minimum requirements for microbial growth and feed degradation in the rumen (Kidane et al., Reference Kidane, Øverland, Mydland and Prestløkken2018).
Regarding N excretion, the increasing content of CP in diets did not influence the excretion of faeces N. Studies (Menezes et al., Reference Menezes, Valadares Filho, Costa e Silva, Pacheco, Pereira, Rotta, Zanetti, Detmann, Silva, Godoi and Rennó2016; Jennings et al., Reference Jennings, Meyer, Guiroy and Andy Cole2018) have reported the lack of knowledge of dietary effects on faecal N excretion for finishing bulls. On the other hand, the increase of dietary CP content resulted in a linear pattern on urinary N (g/day and g/kg of excreted N) and urinary urea N (g/day). When the protein contents of diets are greater than animal requirements, the urinary N excretion will increase linearly with protein intake (Menezes et al., Reference Menezes, Valadares Filho, Costa e Silva, Pacheco, Pereira, Rotta, Zanetti, Detmann, Silva, Godoi and Rennó2016).
Ruminal microorganisms usually degrade the digestible proteins and non-protein compounds of diets into ammonia-N to be used in microbial protein synthesis (Hristov and Jouany, Reference Hristov, Jouany, Pfeffer and Hristov2005). However, the inefficient use of N by ruminal microorganisms may promote an excess of NH3 in the rumen. Thus, this excess is absorbed by the rumen wall and transferred through portal blood to the liver, where it is converted to urea and subsequently released into the blood (Hristov et al., Reference Hristov, Hanigan, Cole, Todd, Mcallister, Ndegwa and Rotz2011). The blood urea is partially filtered in the kidney and excreted in urine or recycled back to the gastrointestinal tract (Lapierre and Lobley, Reference Lapierre and Lobley2001). Therefore, it may explain why N losses are much more variable in urine than in faeces (Dijkstra et al., Reference Dijkstra, Oenema, van Groenigen, Spek, van Vuuren and Bannink2013) since faecal N is made up of undigested dietary proteins, indigestible microbial CP and endogenous N (Prados et al., Reference Prados, Chizzotti, Valadares Filho, Chizzotti, Rotta, Costa e Silva, Valadares Filho, Costa e Silva, Gionbelli, Rotta, Marcondes, Chizzotti and Prados2016).
The N retained (g/day or g/kg of N intake), ADG (kg/day) and final SBW (kg) were not affected by dietary CP content, suggesting that the increase in CP in diets does not bring any evident benefit to the performance of early-weaned calves. Commercial dairy calves are usually weaned under 90 days of age since transitioning calves from milk to solid feed is a way to decrease feed costs in commercial dairy production systems (Kertz et al., Reference Kertz, Hill, Quigley III, Heinrichs, Linn and Drackley2017). Thus, NRC (2001) recommended 180 g CP/kg DM for conventionally reared dairy calves, which generally weigh less than 100 kg. On the other hand, according to Valadares Filho et al. (Reference Valadares Filho, Costa e Silva, Gionbelli, Rotta, Marcondes, Chizzotti and Prados2016), suckling beef calves (140 days of age, 150 kg and 0.6 kg/day ADG) in pasture systems have been shown to have CP requirements of approximately 145 g CP/kg DM. Also, Amaral et al. (Reference Amaral, Mariz, Zanetti, Prados, Marcondes, Santos, Detmann, Faciola and Valadares Filho2018) showed that beef calves in traditional weaning (240 days of age) allocated in a feedlot should receive diets with a CP content of approximately 140 g CP/kg DM to achieve an ADG of 1.2 kg/day during the initial growing phase (112 days), being reduced to 120 g CP/kg DM in the finishing phase. Therefore, these data suggest that it is possible to adjust diets according to each phase and targeted animal ADG.
The results suggest that dietary CP concentrations of 130 g/kg DM can be indicated for early-weaned Nellore calves since it meets their nutritional requirements. Moreover, there is the potential to reduce the environmental impacts of beef cattle production and increase economic returns to producers by the reduction of N losses. Nevertheless, it is also important to highlight that the small sample size was one of the limitations of this study.
Phase 2
Greater amounts of grain in beef cattle diets associated with the use of feeds rich in readily available carbohydrates may increase the propensity for acidosis, which is a common ruminal digestive disorder in feedlots (Elam, Reference Elam1976; Owens et al., Reference Owens, Secrist, Hill and Gill1998; Nagaraja and Titgemeyer, Reference Nagaraja and Titgemeyer2007). Animals with subacute acidosis decrease (Elam, Reference Elam1976; Allen, Reference Allen1997) or oscillate (Devant et al., Reference Devant, Quintana, Aris and Bach2015) their DM intake in an attempt to stabilize the rumen environment. Also, ruminal pH below 6.0 may decrease fibre digestibility as well as the microbial yield (Hoover, Reference Hoover1986). Therefore, acidosis can lead to marked reductions in cattle performance (Owens et al., Reference Owens, Secrist, Hill and Gill1998; Nagaraja and Titgemeyer, Reference Nagaraja and Titgemeyer2007). According to Owens et al. (Reference Owens, Secrist, Hill and Gill1998), acidosis can also be diagnostic when animals show symptoms such as anorexia, diarrhoea and lethargy. None of these factors was observed during the feedlot period, suggesting the non-occurrence of acidosis. Animals fed diets based on SNAP-90 or DMG showed similar intake and digestibility of DM, OM, CP, NDF and starch.
Some studies (Akins and Shaver, Reference Akins and Shaver2014; Ferraretto et al., Reference Ferraretto, Shaver and Luck2018; Bernardes and Castro, Reference Bernardes and Castro2019; Godoi et al., Reference Godoi, Silva, Silva, Pucetti, Pacheco, Souza, Lage, Rennó, Schoonmaker and Valadares Filho2021a, Reference Godoi, Silva, Menezes, Silva, Alhadas, Trópia, Silva, Andrade, Schoonmaker and Valadares Filho2021b) have reported an improvement in starch digestibility when maize is harvested, stored and fed as snaplage. However, many factors may interfere with the digestibility of starch in diets based on grain silages, such as the moisture content at harvest time and ensiling (Owens et al., Reference Owens, Secrist, Hill and Gill1997; Lardy and Anderson, Reference Lardy and Anderson2016), particle size (Rémond et al., Reference Rémond, Cabrera-Estrada, Champion, Chauveau, Coudure and Poncet2004; Lardy and Anderson, Reference Lardy and Anderson2016) as well as the length of storage time (Hoffman et al., Reference Hoffman, Esser, Shaver, Coblentz, Scott, Bodnar, Schmidt and Charley2011).
Microbial protein synthesis and microbial efficiency were not affected by flint maize processing methods. Similarly, animal performance, carcase characteristics and carcase composition did not differ when heifers were fed diets based on SNAP-90 or DMG. Hence, corroborating with our hypothesis, snaplage can be used as an exclusive fibre and energy source in feedlot cattle. However, it is worthy to note that a small sample size was one of the limitations of this study.
Although no differences were observed for animals fed diets based on SNAP-90 or DMG, harvest maize as snaplage can increase productivity per hectare compared to the harvest of dry grains (Mahanna, Reference Mahanna2008). Also, the ensiling process of grains with high moisture may promote an improvement in starch availability, and consequently, in feed efficiency (Hoffman et al., Reference Hoffman, Esser, Shaver, Coblentz, Scott, Bodnar, Schmidt and Charley2011; Junges et al., Reference Junges, Morais, Spoto, Santos, Adesogan, Nussio and Daniel2017; Silva et al., Reference Silva, Pacheco, Godoi, Silva, Zanetti, Menezes, Pucetti, Santos, Paulino and Valadares Filho2020a, Reference Silva, Pacheco, Godoi, Alhadas, Pereira, Rennó, Detmann, Paulino, Schoonmaker and Valadares Filho2020b).
In conclusion, based on the current experimental condition, the dietary CP concentrations of 130 g/kg DM can be adequate for early-weaned Nellore calves since this content can cover the N requirement of animals at this age. However, more studies are recommended to validate this result and to evaluate content below 130 g CP/kg DM in early-weaned Nellore calves. Moreover, this diet reduces the environmental footprint related to N excretion. Regarding flint maize processing methods, snaplage could be used as an exclusive fibre and energy source for cattle finished in feedlot.
Financial support
This study was supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq); Instituto Nacional de Ciência e Tecnologia – Ciência Animal (INCT – CA); Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES); and Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG). The funding agency had no role in the study design, data collection and analyses, decision to publish or preparation of the manuscript.
Conflict of interest
The authors declare that they have no competing interests.
Ethical standards
The experiment was conducted following the approval of the animal handling and procedures described herein by the Ethics Committee for Animal Use (protocol CEUAP/DZO/UFV 05/2020).
