Collection and processing of wet BSG
The wet BSG of barley origin was procured at regular intervals from a local brewery (M/s United Breweries Limited, Kalyani, West Bengal, India). The collected wet BSG was dried by spreading over a tarpaulin sheet for 3–4 days under sunlight with at least three manual rotations a day. The initial moisture content of the wet BSG was ~ 75%. Following sun drying, the moisture level was brought down to approximately 10% and the same was stored in plastic bags for subsequent use during feeding trial.
For chemical characterisation (described in the later sections), the sun-dried BSG samples of different batches were pooled and oven-dried for 24 h, ground and stored in airtight plastic containers awaiting analyses.
Animals, diets, housing and general management
Ten apparently healthy male Jersey crossbred growing cattle aged 4–9 months were selected from the herd maintained at the Eastern Regional Station of ICAR—National Dairy research Institute, Kalyani, India. All the animals were vaccinated, dewormed and treated for ectoparasites. The initial BW of each animal was noted before the start of the experiment. Following this, animals were randomly stratified into two experimental groups of control (T1) and treatment (T2) of five animals each in such a way that both had an average comparable live BW of 59.3 ± 5.9 kg. Animals in group T1 were fed with a diet containing approximately 15% chopped green oats (Avena sativa; harvested at 9 weeks) and 45% paddy (Oryza sativa) straw along with 40% commercial concentrate mixture. The diets for animals in group T2 were similar to that of T1, except 20% (w/w) of the diet (i.e., 50% of concentrate mixture) was substituted by equal proportion of sun-dried BSG. Thus, both the experimental diets had similar forage-to-concentrate ratio of 60:40, offered in a typical component-fed system. Animals were fed individually in a specially built mangers, whereby they had no access to feed/forage allocated to the neighbouring animal. All animals had ad libitum provision to drinking water throughout the experiment spanning 105 days. Feeding management was done to fulfil the nutrient requirements of growing cattle as recommended by the NRC (2001) feeding standard.
All animals were housed in an experimental shed having amenities for individual care and management. The cement-floored house was well ventilated and received sufficient sunshine. Periodically, the floor was disinfected with antiseptic solution to observe high hygienic standards. On a weekly basis, all animals were given bath with plain water. Feeding and all other protocols relating to handling and management of experimental animals were performed in compliance with the Institutional Animal Ethics Committee rules laid out by CPCSEA, New Delhi, Government of India.
The amount of offered feedstuffs and the residues from individual animal was weighed daily using spring balance and sampled at weekly intervals for determining dry matter (DM), to calculate average DM intake (DMI) as below:
$${\text{DMI (kg/d)}} = {\text{DM offered (kg/d)}}{-}{\text{DM in the residue (kg/d)}}$$
Before commencing morning feeding and watering, BW of each animal was recorded on an electronic platform scale, at weekly interval to estimate BW changes, and the diets were adjusted accordingly to commensurate with change in BW. Using the data of DMI and average daily gain (ADG), FCR was computed. Similarly, ratio of intakes of energy and protein to that of ADG yielded the respective conversion ratios. Based on feed cost per unit growth in group T2, relative to that of T1, termed as relative feed cost was calculated as exemplified by Mahesh (2022).
Digestion trial, sampling and processing
A digestibility trial of 6 days was conducted towards the end of feeding trial in a total faecal collection method. During this trial, samples of offered feeds (green oats, paddy straw, concentrate mixture and BSG), corresponding residues and faeces were collected the next morning separately for each animal. These samples were collected daily and immediately dried in hot-air oven at 60 °C until achieving a constant weight to estimate DM and then ground in a laboratory hammer mill to pass through 1-mm sieve. Samples were pooled for 6 days and then stored in airtight plastic containers for further chemical analysis. On individual animal basis, the suitable aliquots of fresh faeces was sampled daily, preserved in plastic bottles previously containing 25% (v/v) sulphuric acid, and pooled over for 6 days. About 5 g of this wet faeces was properly mixed, and the pooled sample was subjected for nitrogen (N) estimation by Kjeldahl method.
Chemical analysis
Representative samples of offered feeds, residue left and faeces were subjected to wet chemistry analysis. The DM, ash, CP (as Kjeldahl N × 6.25), ether extract (EE) and crude fibre (CF) were estimated as per the standard methods of AOAC (2005). Fibre fractions such as neutral detergent fibre (NDF), acid detergent fibre (ADF) and acid detergent lignin (estimated with concentrated sulphuric acid, 720 g/kg) were assayed by the procedures outlined by Van Soest et al. (1991), without using heat-stable amylase and sodium sulphite in the former. Gravimetric procedures were followed to determine calcium (Ca) and phosphorus (P) content in BSG.
Calculation of digestibility and energy value
Based on the intake and faecal excretion of a particular nutrient, apparent digestibility co-efficient was computed, as below:
$${\text{Apparent digestibility (g/kg)}} = [{\text{Intake (kg)}}{-}{\text{faecal excretion (kg)}}] \times 1000 \div {\text{intake (kg)}}$$
Using the digestibility coefficients of individual nutrients, apparent total digestible nutrients (TDNa) of diet was calculated according to Owens et al. (2010), as follows:
$${\text{TDNa (g/kg)}} = [d{\text{CP intake (kg)}} + d{\text{CF intake (kg)}} + d{\text{NFE intake (kg)}} + d{\text{EE intake (kg)}} \times {2.25}] \times 1000 \div {\text{DMI (kg)}}$$
where d = co-efficient of apparent digestibility (g/kg) and NFE = nitrogen-free extract.
Metabolisable energy (ME) value of diets was calculated from TDNa by multiplying the latter with 0.15 (1 kg TDNa = 15.13 MJ of ME; NRC 2001).
In vitro gas measure
Hohenheim in vitro gas production method of Menke and Steingass (1988) was used to measure net gas production (GP) after 24 h of incubation of 0.2 g BSG, as below:
$$\begin{aligned} & {\text{GP}}\;({\text{mL/g DM}}) = [({\text{Syringe reading at 24 h}}{-}{\text{initial syringe reading}}) \\ & \quad - ({\text{blank reading at 24 h}}{-}{\text{initial blank reading}}) \times 0.2 \times 5] \\ & \quad \div \;{\text{actual dry weight of sample incubated}} \\ \end{aligned}$$
Statistical analysis
The data were expressed as mean ± standard error for all parameters. Statistical analysis was performed by applying student’s T test using Statistical Package for the Social Sciences for Windows (SPSS, Chicago, IL, USA) with the below model:
$$Y_{i} = \mu + D_{i} + e_{i} ,$$
where Yi = independent response variable (e.g. DMI, digestibility, FCR, etc.); µ = overall mean; Di = effect of ith treatment (i = T1 and T2), and ei = residual error component of ith observation.
The significance among means of two groups was established at 5% level of probability when the P value for null hypothesis was ≤ 0.05. The data on DMI and ADG were analysed on every weekly basis, and since there was no significant interaction found with period, only the main effect of treatment was considered.