Lewis C.D.1,4, Ho C.K.M.2, Malcolm B.3, Williams S.R.O.1, and Marett L.C.1

1Agriculture Victoria Research, Department of Economic Development, Jobs, Transport and Resources, Ellinbank, Vic, Australia.

2Agriculture Victoria Research, Department of Economic Development, Jobs, Transport and Resources, Agribio, 5 Ring Road, Bundoora, Vic, Australia.

3Faculty of Veterinary and Agricultural Sciences, The University of Melbourne, Parkville, Vic, Australia.

4Corresponding author: claire.lewis@ecodev.vic.gov.au


Heat stress reduces the milk yield of dairy cows. Digestion and metabolism of feed generates heat which contributes to the heat load on cows. An experiment was designed to compare the effects of different concentrates on dry matter intake and milk production of cows during hot weather. Prior to commencement of the experiment, an economic threshold analysis was conducted to determine the saving in lost milk production during hot weather required from the concentrate supplements to break-even with a control diet. Diets were formulated as a mixed ration with only the concentrate component changing. Concentrate treatments were wheat (control), barley, maize or a mix of canola meal plus wheat. All diets contained 8 kg DM/cow.day of concentrate and 13 kg DM/cow.day of conserved forage. In the economic threshold analysis, the reduction in the heat-related loss of milk solids (MS) for each treatment diet that would give the equivalent contribution to farm profit as the control diet was estimated. It was assumed the diets were fed every day from 1st November to 28th February (120 days). Reductions in heat-related losses of milk production from the treatment diets were assumed to be realised over this period. Market prices for concentrates delivered to Gippsland, Victoria, Australia were used and an 8% p.a. opportunity cost of variable capital was included in the cost of each diet.  A four-year Victorian average milk price of $6.18/kg MS was used. Feeding barley resulted in a saving of $0.21/cow.day compared to the control diet. This meant the MS from cows fed barley could decline 4.4 kg (60 L/cow @ 4.0% fat and 3.3% protein) more than the yield decline of cows fed the control over the 120 days, before profit would be penalised by feeding barley. Replacing wheat with higher cost maize increased the cost of the diet by $0.60/cow.day, resulting in the maize diet needing to reduce the loss in MS from hot weather by 12.7 kg MS/cow (174 L/cow) to break-even with feeding wheat. Feeding 2 kg DM/cow.day of canola meal plus 6 kg DM/cow.day of wheat had to prevent a reduction in MS from heat stress of 5.7 kg MS/cow (77 L/cow), to be equally as profitable as feeding wheat. Pre-experimental economic threshold analysis provides insight into the biological requirements for changes to diets to be economically sound substitutes for the control diet. All diets were analysed here on an as offered basis. Marginal changes in total DM intake with each diet (including pasture and conserved fodder), to achieve the required changes in milk production, were assumed to come from feed which would otherwise have been grown or fed but not utilised. This assumption will be revised as experimental results become available.

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