A systems and component experimental approach was taken to investigate the factors affecting milk protein concentration and composition in the southeast Queensland region of northern Australia, with the aim of developing management strategies to increase summer milk protein concentrations.
The research approach included two systems-based monitoring programs (on-farm and farmlet monitoring programs) and a component experiment (3 x 3 Latin Square with 4 replicates). An extensive range of nutrition, climate, management and milk composition parameters were collected within the two monitoring programs. Milk yield, body condition score (BCS), liveweight and detailed milk composition, including true protein, fat, lactose, casein, whey protein and urea concentration and the proportion of αS1-, αS2-, β- and κ-casein, were recorded in all studies. Various rumen function parameters were measured for the farmlet monitoring program and the component experiment. Dietary parameters were calculated using a nutrition-based model (NRC 2001) in all studies.
The on-farm monitoring program (Chapters 3 and 4) examined the seasonal variation and factors affecting milk protein concentration and composition over 13-month period on twelve commercial dairy farms in the southeast Queensland region from September 2000 to September 2001. Approximately 1400 cows were monitored in total. Farms were categorised as high (HMP) or low (LMP) milk protein farms. Mean annual true protein concentration was significantly higher (P<0.05) on HMP farms. There was a significant (P<0.01) seasonal variation in milk true protein (decline in summer), whey protein and urea concentration and the proportion of αS1-, αS2- and κ-casein. Regression tree analysis identified dietary non-fibre carbohydrate (NFC) content as the main factor affecting milk protein and casein and the proportion of β-casein within casein respectively. Dietary ADF content, THI, radiation and maximum temperature were the main factors affecting whey protein and urea concentration and the proportions of αS- (αS1- and αS2-casein) and κ-casein fractions of all farms respectively (HMP and LMP). There was a positive relationship between milk protein concentration and average herd BCS (R2= 29.8%; P<0.001) and average herd ABV for milk protein concentration (R2= 53.6%; P=0.01). Increasing BCS at calving increased milk protein concentration throughout lactation. The shape of the protein concentration curve throughout lactation was influenced by time of calving, and the shape was dependant on month and season.
The farmlet monitoring program (Chapter 5) investigated the effect of feeding system on the seasonal variation and factors affecting milk protein concentration and composition. Five farmlets (feeding systems) containing 20 cows/farmlet were evaluated over a 13-month period from March 2002 to March 2003. There was a significant (P<0.05) effect of feeding system (farmlet) on true protein concentration and yield. There was also significant (P<0.05) seasonal variation in true protein, casein, whey protein and urea concentration and the proportion of the αS1-, αS2-casein and κ-casein fractions. The amount of concentrate fed and DIM were the main factors affecting milk protein concentration within feeding systems.
The component experiment examined the effect of level and type of ME intake on milk protein concentration and composition. Three dietary treatments were imposed on 12 lactating dairy over three separate runs (each 21 day period). Treatments were either a low quality forage plus low level of concentrate diet (LFLC), a high quality forage plus low level of concentrate diet (HFLC) or a low quality forage plus high level of concentrate diet (LFHC). All cows were κ-casein AA phenotype with an equal number (6) of cows being β-Lactoglobulin AB and BB phenotype. True protein and casein concentration were significantly increased (P<0.05) by the fibre (HFLC; 0.14 and 0.09% units respectively) and starch-based (LFHC; 0.22 and 0.17% units respectively) diets above the control (LFLC) diet, with starch increasing true protein concentration by 0.08% units/kg starch fed. Whey protein and urea concentration were significantly (P<0.05) increased and decreased respectively, by the starch-based (LFHC) diet only. There was no significant (P>0.05) effect of ME level or type on the proportion of the casein fractions. β-Lactoglobulin BB phenotype significantly (P<0.05) increased true protein and casein concentration.
It was concluded that dietary non-fibre carbohydrate content over the range of 35 to 40%, is the main factor affecting milk protein concentration. Increasing ME intake of dairy cows with starch-based concentrates will increase milk protein and casein concentration. The type of feeding system influenced the level of milk protein and the amount of seasonal variation seen, with feeding systems that provide a higher plane of nutrition and a more even supply of nutrients having higher true protein concentrations and less seasonal variation respectively. Increased BCS at calving had a positive impact on milk protein concentration throughout lactation, particularly in early to mid lactation. Time of calving also influenced milk protein production curves throughout lactation, with autumn and winter calving cows having higher levels at peak lactation. The main factors that affected milk protein composition were nutrition and climate-related, with a number of different variables within these factors contributing to the seasonal variation seen in milk protein
components. There was a significant seasonal effect seen in the proportions of the casein fractions and feeding system had no effect on milk protein composition.