Effects of autumn and spring pasture availability and cow production efficiency of intensive pasture-based dairy systems

Maximising both the utilisation of grazed pasture and cow production efficiency on-farm is highly beneficial in order to build a more sustainable and resilient dairy system. The objectives of this thesis were to identify pasture management strategies in autumn and spring to increase the proportion of utilised pasture while also evaluating various efficiency metrics and identifying animal characteristics associated with the efficient conversion of grazed pasture into high value milk output. Over 3 consecutive years, 144 Holstein-Friesian (HF) and Holstein-Friesian × Jersey crossbred (JFX) dairy cows were randomly assigned to 1 of 3 pasture availability (PA) treatments with a target average pasture cover (APC) of 800, 600 and 400 kg DM/ ha for high (HPA), medium (MPA), and low (LPA) on December 1. Grazing rotation length during the 11-wk measurement period in autumn was extended by +13 and +7 days for HPA and MPA, respectively, beyond that required by LPA (37 days) while all animals were housed at similar dates at the end of the grazing season. Besides achieving significant differences in APC and therefore pre-grazing herbage mass, PA had no significant impact on dry matter intake (DMI), milk production, or body condition score (BCS) in late lactation. During the first grazing rotation in spring, daily herbage allowance and total pasture utilised were greatest for HPA (11.7 kg DM/cow and 4,099 kg DM/ ha), intermediate for MPA (10.2 kg DM/cow and 3,319 kg DM/ ha) and lowest for LPA (8. kg DM/cow and 2,794 kg DM/ ha), with the deficit covered by additional grass silage supplementation so that total feed allowance did not differ between PA. There was no effect on daily or cumulative milk yield or milk composition in early lactation. Furthermore, an evaluation of a cost-effective efficiency parameter calculated as the annual fat and protein (milk solids; MS) production per unit of mid-lactation BW demonstrated that highly efficient grazing animals were characterised by a greater and more consistent daily MS yield, an increased DMI earlier in lactation and a greater intake per kg BW. Moreover, the most efficient animals also showed a greater MS yield both per kg of BW and per kg of DMI. An analysis evaluating the production efficiency of HF and JFX on commercial dairy farms identified Jersey crossbred animals to be more efficient in terms of MS yield per unit of mid-lactation BW and total energy requirement per kg of MS produced. However, large variation existed between farms (0.73 to 1.14 kg MS/ kg BW) and between animals within herds (0.42 to 1.47 kg MS/ kg BW). The results from this thesis corroborate the importance of achieving adequate pasture supply at the commencement of calving so that grazing days per ha and the utilisation of grazed pasture can be maximised, irrespective of between year weather and pasture growth variations. Equally important is the identification and breeding of elite animal genetics capable of harvesting additional quantities of fresh pasture as the cheapest feed source particularly in spring and efficiently converting it into a high value milk output.