Grazing season length and stocking rate affect milk production and supplementary feed requirements of spring-calving dairy cows on marginal soils.

In pasture systems, milk production is aimed to be maximised from grazed pasture with merely minimum supplementation to meet dairy cow requirements when necessary. The economic impacts of increased pasture production and utilization on farm profitability is well known, but the influence of maximising utilisation in different circumstances is not always known, one being soil type. Therefore, the objective of the study of the authors cited was to investigate the effect of increasing stocking rate (SR) and extending the grazing season (GS) on pasture and cow productivity on a marginal, poorly draining soil.

The study extended over years, and evaluated whole farm systems in a 2 x 2 factorial design, where the systems comprised two GS treatments: average (AGS – 205 days) and extended (EGS – 270 days); and two stocking rates: medium (2.5 cows/ha) and high (2.9 cows/ha). These treatments created four grazing intensities, respectively 500, 600, 700 and 800 cow grazing days per hectare per year. In Year 1, cows were randomly allocated to one of the four whole farm systems pre-calving and they remained on the same treatments for the three years thereafter.

The results showed that there were no significant differences in total average annual pasture production (14.1 ± 0.5 ton DM/ha) or sward chemical composition between GS and SR treatments over the four-year period, with the exception of average crude protein content. Crude protein for EGS was lower (211 g/kg DM) compared with AGS (218 g/kg DM). The grazed pasture production was significantly increased by 0.76 ton DM/ha in EGS treatments compared with AGS (9.9 ton DM/ha), whereas conserved silage DM production was greater for AGS by 0.72 ton DM/ha, compared with EGS (3.6 ton DM/ha). Neither GS or SR had a significant effect on daily or total lactation milk yield or fat and protein production per cow. However, increased SR resulted in increased milk fat plus protein yield per hectare based on increased grazed pasture utilization.

These results assist in understanding the important additive contributions of both extended grazing and SR intensification in achieving high levels of grazed pasture utilization, also on these soils, and milk production per hectare, and thereby allowing reduced supplementary feed allocation.