The nutritive characteristics of perennial ryegrass and implications for diet formulation for grazing dairy cows

Abstract

The dairy industry in south-eastern Australia is dominated by pasture-based feeding systems, where perennial ryegrass (Lolium perenne L.) is the most common pasture species grown. Pasture alone cannot provide optimal nutrition for high milk production; therefore, the majority of farmers feed concentrates such as cereal grain, protein supplements and dairy pellets to supplement the diet of grazing dairy cows. The challenge is to formulate an optimal supplementary ration that will complement the nutrients from pasture; thus, an understanding of the seasonal variation in nutritive characteristics is important, but there has been limited research investigating a range of perennial ryegrass cultivars. Diet formulation using nutrition models in overseas dairy industries that feed total mixed rations has been previously documented, but there has been limited research in pasture-based systems. This thesis aimed to investigate the nutritive and degradation characteristics of perennial ryegrass cultivars to subsequently inform the formulation of supplementary rations for grazing dairy cows. The first experiment (Chapter 3) investigated the nutritive characteristics of perennial ryegrass cultivars, to provide data for use in future chapters of this thesis. Up to 18 cultivars of perennial ryegrass from Gippsland, northern Victoria and south-west Victoria were harvested during early spring, late spring, summer, autumn and winter and analysed for their nutritive characteristics. There were minimal differences between individual cultivars, however tetraploid cultivars had greater metabolisable energy (ME), while diploid cultivars had greater neutral detergent fibre (NDF) concentrations. Early- and mid-flowering cultivars had greater lignin concentrations than late-flowering cultivars. Seasonal variation included greater ME and concentrations of crude protein (CP) when the pasture was in the vegetative growth stage, and greater NDF concentrations during the flowering and reproductive growth stages. Season accounted for the most variation for ME, CP, NDF, and other nutrients and minerals. Regional differences were also evident, influenced by the specific climatic conditions in each region. The results of this experiment demonstrated that an understanding of the seasonal variation in nutritive characteristics was important to consider when formulating diets for grazing dairy cows, without the need to specify the individual cultivar but considering the geographic location of the farm. The next three experiments (Chapters 4 and 5) investigated the ruminal degradation of nutrients in three perennial ryegrass cultivars between different seasons and regions, as degradation data can improve the ration to be formulated. The three cultivars investigated were Bealey NEA2 (late-flowering, tetraploid), Trojan NEA2 (late-flowering, diploid) and Victorian SE (early-flowering, diploid). Degradation parameters were determined by in situ incubation in the rumens of non-lactating, rumen-fistulated Holstein-Friesian cows using a nylon bag technique. Bealey NEA2 had greater effective degradability of CP and NDF harvested during early spring, late spring and summer but there were no differences between cultivars harvested during autumn. Cultivars harvested during early spring and autumn had greater effective degradability of CP and NDF than cultivars harvested during late spring and summer. These results enabled us to quantify the reduction in ruminal degradation of perennial ryegrass as the pasture matures, with greater degradation when the pasture is vegetative. We have concluded that incorporating degradation data to formulate rations for grazing dairy cows is best done with average seasonal data, also without needing to specify the individual cultivar. The fifth experiment (Chapter 6) used the information of the nutritive characteristics of perennial ryegrass reported in Chapter 3 to formulate supplementary grain rations for grazing dairy cows during late spring in Gippsland. Ninety-six spring calving, Holstein-Friesian cows grazed pasture at an allowance of 33 kg dry matter (DM)/cow per day (to ground level) and were fed 7.5 to 8.0 kgDM/cow per day of one of four grain rations, for 28 days. Two of the rations were formulated using a nutrition model to complement the nutrients the cows were expected to consume from pasture during late spring, called designer grain mixes, and both included lower amounts of protein supplements due to the high CP concentration of the pasture during late spring. There were no differences in pasture or total dry matter intake (DMI). Cows fed either of the two designer grain mixes had the same milk yield as cows fed a ‘standard’ ration that contained a greater amount of protein supplement. Re-running the model after the experiment, using the nutritive characteristics of the pasture offered during the experiment, showed the importance of using the nutritive characteristics of perennial ryegrass to formulate rations that more accurately predicted milk yield, as well as a correct estimate of pasture intake. The research reported and integrated in this thesis is the first to investigate and quantify the nutritive and degradation characteristics of individual cultivars of perennial ryegrass in different regions and seasons. The thesis highlighted the importance of using near-real time data of the nutritive characteristics of perennial ryegrass during a season to formulate supplementary grain rations that will complement the nutrients from pasture. The combination of experimental data with modelling simulation can be used by Australian dairy farmers to achieve practical milk production benefits in grazing systems. Further research is warranted to investigate the best grain rations to feed grazing dairy cows throughout other seasons.