Long term impact of the plane of nutrition on milk protein production

The plane of nutrition (PON) influences the production potential of the dairy cow. One of the major limitations in grazing systems is the climatic effect where pasture productivity varies considerably throughout the year (Smit et al 2005) resulting in restricted feed intake often compromising milk protein synthesis (Walker et al 2004). The objectives of this study were to determine whether a short (3 weeks) and long term (9 weeks) reduction in the PON affects milk yield and protein production and if the length of nutritional restriction impairs the capacity of the udder to respond to a subsequent increase in PON. Thirty-six Holstein Friesian cows in mid-lactation (87 ± 13 days in milk on day one) were fed a diet comprising of ryegrass pasture (~7kg DM/day), canola meal/mineral mix (1.3kg DM/day), barley/sorghum grain mix (2.7 kg DM/day) and equal amounts of maize and forage sorghum silage (~5kg DM/day). The 20-week experiment included three treatments; 1) a high PON (C), 2) a low PON for 9 weeks (L9) and; 3) a low PON for 3 weeks (L3). Four periods were applied including a covariate period: high PON for all treatments (week 1-5); period 2A: low PON for the L9 cows (week 6-11); period 2B: low PON for the L9 and L3 cows (week 12-14) and period 3: high PON for all treatments (week 15-20). The low PON treatments were imposed by removing the silage from the diet to elicit a 4-5 litre reduction in milk yield/day. Pasture dry matter intake was measured and allocated using a calibrated rising plate meter. All other feeds were fed out individually. Milk yield was recorded twice daily with individual milk samples collected twice weekly and analysed for milk protein, fat, and lactose concentration. Liveweights were recorded daily and body condition score (BCS) was assessed weekly. There was no significant difference in milk yield during period 2A for the L9 cows. A more severe feed restriction imposed during period 2B caused a more rapid decline in milk yield for the L3 cows. There was no significant difference in milk protein, fat and lactose concentration between all treatment groups and across all periods. In comparison to the covariate period, milk protein yield during period 2B was 34% lower for the L9 cows and 30% lower for the L3 cows. There was no significant difference in milk protein yield during the recovery period. There was no change in BCS or liveweight between treatments throughout the experiment.