2.1. Pre-Experimental Analysis

The extra costs associated with feeding each experimental diet compared to feeding the control diet alone was used to establish the milk production thresholds. The diets proposed in the animal experiment were formulated as a mixed ration with only the supplement component changing. The three experimental diets were (1) BET—control diet plus 16 g betaine (trimethylglycine as a powder; Feedworks, Romsey, Victoria, Australia) per cow per day, (2) FAT—control diet plus 0.7 kg canola (Brassica napus L.) oil per cow per day, or (3) BET + FAT—16 g betaine (trimethylglycine) and control diet plus 0.7 kg canola oil per cow per day. The daily control diet offered to each cow was a total mixed ration (TMR) comprised of 7 kg dry matter (DM) lucerne hay (Medicago sativa L.), 6 kg DM pasture silage (predominantly perennial ryegrass, Lolium perenne L.), 5.0 kg DM grain mix (500 g/kg wheat grain (Triticum aestivum L.), 500 g/kg barley grain (Hordeum vulgare L.), 1.5 kg DM solvent extracted canola meal (Brassica napus L.), 0.2 kg DM of minerals and vitamins (Ca 134 g/kg, Mg 110 g/kg, P 60 g/kg, Zn 6.4 g/kg, Mn 2.4 g/kg, Cu 1.2 g/kg, I 80 mg/kg, Co 100 mg/kg, Se 24 mg/kg, Vitamin A 165 IU/g, Vitamin D3 24 IU/g, Vitamin E 800 mg/kg), 0.1 kg DM salt, and 42 mL of Bloat Drench (271 g/L alcohols, C12-15 ethoxylated; VicChem, Coolaroo, Victoria, Australia). The chemical compositions of the main dietary components are given in Table 1.

Composition of the main dietary ingredients used in the animal experiment (as published by Williams et al. 2021).

¹ Grain mix consisted of 500 g/kg wheat grain and 500 g/kg barley grain.

The amount of canola oil fed was restricted to 0.7 kg/cow per day so that the dietary fat concentration did not exceed 70 g/kg DM to avoid a reduction in ruminal fermentation or voluntary DM intake [19]. The betaine dose rate (16 g/cow per day) was the average of dose rates used in experiments from published literature where there had been a positive production response or reduction in heat stress from feeding betaine [10,11,20]. The dose rate also aligned with that recommended by the manufacturer of 15 to 20 g/cow per day [21].

For our pre-experimental economic analysis, all diets were evaluated on a DM offered basis as intake was yet to be measured. Marginal changes in the total DM offered with each diet, such as in conserved fodder or pasture that may be needed to achieve the required changes in milk production, were assumed to come from feed which would otherwise have been grown or fed, but not utilized. Therefore, there were no other costs in addition to the costs of the dietary supplements. It was assumed that the diets were fed every day for 212 days from 1 September, prior to the peak period for hot weather, to 31 March, the end of the hot weather period in northern Victoria. The benefits to milk production from feeding the experimental diets were assumed to be realized within the 212-day feeding period and that no carryover benefits occurred for the remainder of lactation. For each diet, the additional cost of the dietary supplement was calculated assuming an average price of AUD 8.50/kg for betaine and AUD 1.40/kg for vegetable oil, including delivery to northern Victoria, Australia [21,22]. The price of vegetable oil was used instead of canola oil, as this is a more commonly used component of livestock diets, and the two oils were deemed interchangeable as fat supplements. Compared to the control diet, this equated to an extra cost of AUD 0.14/cow per day for the BET diet, AUD 0.98/cow per day for the FAT diet, and AUD 1.12/cow per day for the BET + FAT diet. An 8% p.a. real opportunity cost of variable capital, equating to 4.65% for the 212-day period, was used to calculate the economic threshold (Equation (1)) using an average Victorian real milk (fat + protein) price of AUD 6.28/kg for the period 2013-14 to 2018-19 [23,24]. A litre of milk was assumed to contain 72 g of milk fat plus protein, referred to as milk solids in this analysis. Results were tested using a price sensitivity analysis where the average supplement and milk prices were adjusted by ±20%.

where MS_e equals the extra milk solids required over the 212 days from feeding the experimental diet for it to make an equal contribution to farm profit as the control diet, $_c and $_e are the costs of the control and the experimental diets, respectively for 212 days, r is the real opportunity cost of variable capital for the 212 days, and $_MS is the price per kg of milk solids.

While producers may implement dietary strategies for reasons in addition to immediate milk production responses, such as animal welfare, these factors were not accounted for in the economic analysis presented here.