Agri Kultuur December / Desember 2018 - Page 25

cost which increases daily. The mortality cost of lost heifers should be included as part of the rearing cost of the surviving heifers. Milk yield per cow The average milk yield of a dairy herd is a common point of discussion among dairy farmers. It is often regarded as an indication of the standard of a dairy herd, i.e. high milk yields being a “top” herd while low milk yields are regarded as a poor performing herd. However, the emphasis should rather be the amount of milk collected by the processor rather than the farmer’s herd estimations. The reason for this is that not all milk produced is sold. Some milk is used in calf rearing and for household purposes. Milk is also discarded being contaminated when cows are treated for mastitis and other infections. Each treatment has a specific milk withdrawal period. This is required to prevent milk becoming contaminated by antibiotics. The amount of milk cows produce is also reduced by the level of subclinical mastitis as indicated by herd (or bulk tank) somatic cell counts. This also has a negative effect on the milk price because of poor milk quality. Milk income – feed cost This indicator is based on the difference between the milk income and feed cost and is estimated as the gross margin per cow per day. This figure changes every time milk or feed prices change. Similarly, the breakeven point of production is also affected by feed and milk prices. This refers to the minimum milk yield per cow per day to cover the dairy herd’s production cost of milk. Milk yield must be higher when the feed price is high or when the milk price is low. AgriKultuur |AgriCulture The cost of feeding and management of the non-producing animals in the herd should also be included when estimating the production cost of milk. An easy way to do this is compile a list of all the different groups of animals in the herd, i.e. lactating and dry cows, heifers at different ages and young calves, the diet (forages and concentrates) and the feed intake per animal per day that each group receives. Dairy farmers should, on a monthly basis, compile all the costs required to produce milk as well as the herd milk income. Using only the concentrate feeding cost can be misleading as this may be less than 30% of the total cost of production per litre of milk. Days in milk The average number of DIM is based on the interval between the present date and the calving down dates for all cows currently in milk. This figure increases when a greater proportion of lactating cows in the herd is in late lactation. Cows calving down regularly keeps the average number of DIM at a lower level. The number of DIM usually increase because of reproductive problems in getting cows pregnant. This results in extending the number of days from calving to conception or days open. The lactation period then extends past the normal 300-day lactation period as cows are mostly milked until 50- 60 days before the expected calving date. Because of the lactation curve, milk yield is lower at this lactation stage and with more cows in the herd, the average milk yield of the herd is reduced. Milk yield (and milk income) decreases when the interval days open (or calving interval) increases. Modelling lactation curves for different calving intervals of 12, 13, 25