Seven metabolism experiments were conducted to examine the effect of level and type of supplement on the degree of substitution (decline in forage intake per unit of supplement) in steers given forage based diets with the aim of developing strategies to minimise the degree of substitution.
All experiments used a similar design (two incomplete 5 x 5 Latin Square) with two supplement types in each experiment. Each supplement type was consumed at five levels which was replicated over three runs (each of three weeks) with 10 steers used per experiment. Intake, digestibility, microbial protein production and various ramen parameters were measured in all experiments.
The first three experiments examined the effect
of high lipid supplements on the degree of substitution of steers receiving low quality forage. In Experiments 1 and 2, five levels (0, 0.25, 0.50, 0.75 and 1.0% body weight per day (W/d)) of palm kernel meal (PKM) or copra meal (CM), were supplemented to steers fed Rhodes grass hay (Experiment 1), or Green panic hay (Experiment 2). A maximum intake level of 0.72 and 0.79% W/d of supplement was achieved in Experiment 1 due to a lack of supplement acceptance. In Experiment 1, increased PKM intake linearly decreased (P<0.01) hay intake with a substitution rate of 1.47 (kg DM hay intake decrease per kg supplement DM intake). Increased CM intake resulted in a quadratic response (P = 0.07) in forage intake and substitution rates of 0.08, 0.23, 0.51 at the intake levels of 0.25, 0.50, and 0.79% W/d, respectively. Hay intake responded quadratically (P<0.01) with increasing PKM or CM intakes in Experiment 2. Substitution rates at supplement intakes of 0.5, 0.75 and 1.0% W/d,
were 0.02, 0.16, 0.38 and 0.02, 0.13, 0.25 for PKM and CM, respectively.
In Experiment 3, whole cottonseed (WCS) or a urea-ammonium sulphate mix (US) were supplemented at the rates of 0, 0.15, 0.30, 0.45 and 0.60%W/d and 0, 1, 2, 3 and 4% hay respectively. There was a curvilinear response of hay intake (P<0.01) to WCS or US supplementation. The maximum hay intakes were obtained at 0.30% W/d of WCS or 2.33% US inclusion. The substitution rate was very low (0.02) at the highest WCS intake.
Experiments 4 and 5 examined the effect of the extent of starch digestion of a supplement in the rumen of steers consuming low or medium quality forage on substitution. Dry-rolled sorghum (DRS) and expanded sorghum (ES) represented low and high ramen starch
digestion, respectively, which were fed at 0, 0.5, 1.0, 1.5 and 2.0% W/d. Increased DRS and ES intakes were associated with quadratic and linear decreases (P<0.05) in low and medium quality hay intake respectively. For the medium quality hay, the rate of substitution was 0.72 and 0.77 for DRS and ES respectively. Supplementation with DRS and ES at the rate of 0.5, 1.0, 1.5 and 2.0% W/d on low quality forage resulted m a substitution rate of 0.03, 0.11, 0.22 0.38, and 0, 0.06, 0.20, 0.40, respectively. There was no significant difference (P>0.05) between extent of starch digestion in the rumen on substitution rate.
Experiments 6 and 7, examined the hypothesis that increasing the protein to energy (P/E) ratio of nutrient supply could reduce the rate of substitution. Barley gram (B) and barley plus protein (BP), which represent low and high P/E ratio
of absorbed nutrients respectively, were supplemented to steers fed low or medium quality hay. Supplement was fed a 0, 0.5, 1.0, 1.5 and 2.0% W/d. There was no difference (P>0.05) between B and BP intake on the rate of substitution of steers fed the low quality hay and both resulted in a quadratic effect (P<0.01). The rates of substitution were 0.07,0.21,0.34, 0.59 and 0.04,0.15,0.30,0.52 at supplement intakes of 0.5, 1.0, 1.5 and 2.0% W/d of B and BP, respectively. However, on medium quality hay, substitution was linear for both B and BP (P<0.05) but BP had a lower substitution rate than B (0.42 vs 0.85). All supplements increased MCP production and eMCP primarily in response to RDP supply but at high levels of RDP supply other factors contributed to significant variation in eMCP and hence MCP supply and P/E ratio.
It may be
concluded that substitution with a high lipid supplement was primarily governed by two factors: RDP availability, and lipid intake. Furthermore, with grain supplementation, substitution is primarily a function of ME intake, rather than extent of rumen starch digestion or P/E ratio of nutrient supply once RDP requirements in the rumen have been met. A high P/E ratio of the supplement can reduce but not eliminate substitution. In short, the main determinant of forage substitution by grain was ME intake from supplement and the level of substitution was therefore related to the ME concentration of the supplement.