Strategies to increase spring herbage production and animal dry matter intake to maximize animal performance during early lactation in pasture-based dairy production systems

Spring is a time of low grass growth rates and rapidly increasing herd demand as cows begin lactation. Improved spring grassland management has the potential to increase farm profitability and sustainability through increased grass utilisation, animal performance, reduced feed costs and improved nitrogen use efficiency (NUE) with the strategic application of nitrogen (N). The objective of this thesis was to identify grassland management strategies to increase spring herbage production and animal dry matter intake (DMI) to maximize animal performance in early lactation. A plot experiment investigating three spring N rates (30, 60 or 90 kg N/ha), three N application strategies (0:100, 50:50 or 33:66) split across February and March and two phosphorus (P) application rates (0 or 13 kg P/ha with the first N application) was carried out (Chapter 3). The results of this study illustrate that early spring N application is necessary as withholding N application until March reduced spring herbage production (-185 kg DM/ha). The 60 kg N/ha treatment increased spring herbage production compared to the 30 kg N/ha treatment (+ 445 kg DM/ha) and increased N response compared to the 90 kg N/ha treatment (+ 2.9 kg DM/kg N applied). In scenarios where feed deficits do occur, silage supplementation may be necessary to ensure the energy requirements of cows are fulfilled. In Chapter 4, supplementing grass silage during the first 6 weeks of lactation had no effect on DMI or milk production, however, the inclusion of silage from week 7 – 12 of lactation significantly reduced milk yield and protein content (- 0.9 kg/cow and - 1.1 g/kg milk, respectively) with similar total DMI (TDMI). As TDMI is more difficult to measure in grazing animals, there is little knowledge available on TDMI during early lactation. Individual TDMI was measured using the n-alkane technique from week 2 (13.2 kg DM/cow/day) until week 12 (17.7 kg DM/cow/day) of lactation (Chapter 5). The greatest increase in TDMI occurred from week 2 to 6 of lactation (+ 0.8 kg DM/cow/week) and this decreased to 0.3 kg DM/cow/week from week 7 to 12 of lactation. This measured intake data was also used in Chapter 6 to evaluate an existing intake prediction model based on milk yield potential, bodyweight (BW), body condition score (BCS), parity, stage of lactation and stage of gestation. Increasing the accuracy of models to predict intakes will allow for more informed grassland management and feeding decisions. The strategic application of spring N fertiliser can increase spring herbage production (Chapter 3) to meet early lactation intake requirements (Chapter 5 and 6), reduce supplementation and improve overall animal performance (Chapter 4).