Data from two multi-year experiments undertaken at the DairyNZ research farm, No 2. Dairy were collated and analysed. The effects of:
1) Breed (Jersey or Holstein-Friesian; JER, HF) at optimum or high comparative stocking rates (CSR; 80 or 100 kg body weight (BW)/t dry matter (DM) of feed available; CSR80 and CSR100, respectively); and
2) Changing the mean calving date (January, April, July, or October; JAN, APR, JUL, OCT)
on biophysical measurements (i.e., milk production, pasture growth) and farm profitability were determined.
Changing these strategic management variables affected the amount of pasture grown and consumed. For example, there was an interaction between breed and CSR in many of the measured pasture and milk production variables in experiment one; whilst, in experiment two, changes in the month of calving affected pasture dry matter intake (DMI) in early-mid and mid-late lactation, and annual milk production. Annual pasture production was greatly reduced at CSR100 on the HF farmlet, but not on the JER farmlet. Month of calving affected pasture DMI during early-mid lactation and mid-late lactation.
A breed x CSR interaction reduced milk production per cow at CSR100, an effect that was greater in the HF breed than in the JER. Month of calving affected milk production per cow with the JUL herd producing the highest yield, compared with the JAN, APR and OCT herds.
Other breed x CSR interactions were also detected: JER cows had the lowest mean days in milk (DIM) to first heat at both CSR80 and 100; furthermore, there were negative effects of an increase in CSR from 80 to 100 kg body weight/t DM feed in the HF breed on DIM to first heat. Total metabolisable energy (ME) requirements per cow was affected by a breed x CSR interaction. At CSR80, the HF used more ME per cow than the JER, whilst at CSR100, both breeds used less total ME per cow than at CSR80, but the HF again used more than the JER.
From a profitability perspective, HF cows had a greater operating profit per hectare than JER at CSR80; however, JER cows were more profitable at CSR100. The JUL herd had the most profitable farm system, in both the base economic model and stochastic model. Results of the stochastic modelling with no premium included in the milk payment (NZ$/kg fat and protein) variable revealed the operating profit per hectare was greatest for the JUL herd, compared with the JAN, APR, and OCT herds. Inclusion of a premium for milk supplied during 16 May to 15 July in the milk payment (NZ$/ kg fat and protein) variable as well as a downward adjustment in milk price for JUL calving cows because of the high milk production peak in spring did not overcome this difference in profitability. Therefore, the JUL calving scenario was most profitable. The APR seasonal calving strategy resulted in a 10% reduction in operating profit per hectare, compared with the JUL herd, while the JAN and OCT strategy had the lowest operating profit per hectare.
