THE EFFECT OF AN EXTENDED INTER-CALVING PERIOD ON MILK QUALITY AND OTHER MANAGEMENT FACTORS

Sometimes we need to think outside the box and test the unconventional. Below is an example based on what are the consequences if one alters the conventional lactation period: In some parts of the world, extended lactation is being used as a strategy in which rebreeding is deliberately delayed to maximize lactation persistency rather than peak production. In conventional systems, dairy cows calve approximately once a year (a 12- month calving interval). The question is what would be the advantages and disadvantages if the lactation period is extended beyond the 12-month calving interval. If the calving interval is extended, the number of calves per year is reduced and, accordingly, so is the susceptibility to calving risks and post-calving metabolic diseases. Research has also shown that fewer calves per year results in a reduction of the number of dry days per year, replacement heifers and feed use, thereby potentially lowering greenhouse gas emissions per kilogram of milk produced. Furthermore, some economic benefits may be associated with extended lactation, such as reduced costs per cow as a result of mating, calving, health and replacement. However, there may be disadvantages associated with milk quality and udder health if the lactation period is extended. Therefore, the aim of the study by Dr G.M. Maciel and co-workers was to compare the quality of milk produced during an extended lactation period, especially by evaluating milk indicators related to udder integrity and cheese-making properties when cows were fed low- or high-energy diets in the early lactation mobilization period. The results were published in the Journal of Dairy Science, Volume 99 of 2016, pages 8524 to 8536, with the title: Good sensory quality and cheese-making properties in milk from Holstein cows managed for an 18-month calving interval. 

Forty-seven Danish Holstein cows (15 first-calf heifers and 32 older cows) were fed one of two diets in early lactation: 22 cows were fed a high-energy diet for approximately 42 days in milk (DIM) followed by a low-energy diet and 25 cows were fed a low-energy diet throughout the whole experiment. Milk quality was investigated in three lactation periods: 140 to 175 DIM (P1), 280 to 315 DIM (P2), and 385 to 420 DIM (P3); the reason for the unusual periods was to have sufficient variation to detect statistical differences. 

Lactation period was found to be the main factor affecting milk yield, quality and cheese-making properties. The first-calf heifers kept the same daily milk yield throughout the study periods, whereas the older cows produced on average 10.2 kg per day less in P3 compared with P1. Fat, protein and casein concentrations increased, respectively, by 18, 16, and 16%, from P1 to P2 and P3. Cheese-making properties, such as curd-firming rate, gel strength and wet and dry curd yield, improved from P1 to P3 and were strongly correlated with the concentrations of protein and casein in the milk. The udder integrity indicators, SCC, degree of proteolysis and milk pH, remained unchanged throughout the studied lactation periods.                                                                                                                             

The results in general show that feeding cows either high- or low-energy diets during the early lactation mobilization period did not exert any relevant carryover effect on milk composition and thus had no effect on the cheese-making properties in extended lactation. The trained sensory panel assessed the quality of the milk from P3 as reflecting the sensory descriptors related to the increased levels of fat and protein over lactation, and, importantly, the milk produced in P3 did not present sensory demerits when compared with milk produced in P1. Thus, high-yielding Holstein cows in an 18-month calving interval cycle produced high-quality milk from mid to extended lactation. In terms of practical on-farm decisions one needs to weigh up the loss in milk in an extended lactation period against the possible benefits associated with calf rearing, metabolic diseases, dry period and cow costs related to mating, calving, health etc. These will differ from farm to farm.