Degradabilities of dry matter and crude protein from perennial herbage and supplements used in dairy production systems in Victoria

Abstract

Summary The nylon bag technique was used to estimate the degradability of the dry matter (DM) and crude protein (CP) of herbage samples and supplementary feeds from recent grazing experiments with dairy cows conducted in Victoria. The cumulative disappearance data for CP were used to calculate the effective rumen degradability of CP (ERDP, g/kg DM) for these feeds at rumen outflow rates of 0.02, 0.05 or 0.08/h. Crude protein was more extensively degraded than DM in all herbage samples collected before grazing, or in those samples taken to represent the herbage consumed by grazing cows.

The ERDP of spring pastures (excluding the dead material and postgrazed samples) was generally in the range 103–197 g/kg DM, at an outflow rate of 0.02/h and in the range 74–142 g/kg DM, at an outflow rate of 0.08/h. For summer pastures, the ERDP was generally in the range 80–128 g/kg DM, at an outflow rate of 0.02/h and in the range 58–108 g/kg DM, at an outflow rate of 0.08/h. For autumn pastures, ERDP ranged from 108 to 170 g/kg DM, at an outflow rate of 0.02/h and from 84 to 140 g/kg DM, at an outflow rate of 0.08/h. There were only 2 herbage samples collected during the winter, which had ERDP of 237 and 249 g/kg DM, at an outflow rate of 0.02/h and 213 and 222 g/kg DM, at an outflow rate of 0.08/h. Values for ERDP tended to be higher for these winter samples compared with spring samples. In contrast, autumn samples tended to have lower ERDP than spring samples.

It is shown that metabolisable protein is unlikely to limit milk production of cows consuming 17 kg herbage DM/cow. day and producing up to 30 L milk/day, when milk production is determined from metabolisable energy supply. The excesses in metabolisable protein ranged from 0.48 to 1.21 kg/cow. day from a subset of spring herbage samples cut to represent that selected by cows and from 0.14 to 0.23 kg/cow. day from summer herbage samples. The degradability characteristics of the herbage samples were similar to some estimates for temperate herbages in the United Kingdom and from limited data from Australia and should form a useful basis on which to assess whether additional metabolisable protein is required as supplements for dairy cows grazing pastures in Victoria.

There was a positive (P<0.01) relationship between CP content and ERDP (R² = 0.93), across herbage samples indicating it could be possible to estimate ERDP for herbage samples that have been assessed for CP content. There was also a significant (P<0.01) positive relationship between the effective degradability of DM of herbage and ERDP (R² = 0.77). The relationships between in vitro dry matter digestibility or neutral detergent fibre and ERDP were also significant (P<0.01), but accounted for even less of the variability.

The ERDP in cereal grain-based supplements ranged from 87 to 111 g/kg DM, at a outflow rate from the rumen of 0.02/h, compared with 70–92 g/kg DM, at an outflow rate of 0.08/h. The compounded supplements containing varying levels of cereal grain, lupins and cottonseed meal, had effective degradabilities of CP that generally declined as the content of lupins decreased and the content of cottonseed meal increased. However, ERDP increased to 152 g/kg DM because of the increasing CP content with increasing cottonseed meal. These results are compared with previously published estimates of effective degradability of CP for supplementary feeds.