The response of dairy cows of high genetic merit to increasing proportion of concentrate in the diet with a high and medium feed value silage

Forty-eight high genetic merit Holstein/Friesian dairy cows with Predicted Transmitting Abilities for milk fat plus protein yield, calculated using 19 95 as the base year (PTA(95) fat plus protein), of 40.0 kg, were used in an eight treatment, two period (period length = 8 weeks) change-over design e xperiment Diets were offered ad libitum in the form of a complete diet. Tre atments comprised a high and medium feed value silage, with concentrates be ing incorporated into the diet at proportional inclusion rates of 0.10, 0.3 0, 0.50 and 0.70 of total dry matter with the high feed value silage, and a t 0.32, 0.48, 0.64 and 0.80 of total dry matter with the medium feed value silage. The high feed value silage was produced from a primary regrowth her bage (regrowth interval of 33 days) which was wilted prior to ensiling, whi le the medium feed value silage was produced from a primary growth herbage harvested on 9 June 1995. The high and medium feed value silages had estima ted metabolizable energy concentrations of 12.4 and 11.3 MJ/kg DM respectiv ely. Animals commenced treatments after a mean post calving interval of 26 days. Irrespective of silage feed value, total dry matter intake and milk yield t ended to increase with increasing proportion of concentrate in the diet. Mi lk protein concentrations increased to concentrate inputs of 0.50 and 0.64 of total DM with the high and medium feed value silages respectively, but d ecreased at the highest concentrate inclusion rates. Milk fat concentration s tended to decrease at higher concentrate inclusion rates. The milk yield responses to increasing proportion of concentrate in the diet have been des cribed by a pair of significantly different equations: Y = 34.2 - 11.9 x 0. 0101(x) and Y = 34.2-28.5 x 0.0101(x) for the high and medium feed value si lages respectively (where Y = milk yield (kg/day) and x = concentrate propo rtion in the diet on a dry matter basis). Similarly, milk fat and milk prot ein yields for the two silage types have been described by pairs of signifi cantly different equations. Using these milk yield response equations, it c an be calculated that at a concentrate proportion in the diet of 0.32 total dry matter, an additional 3.7 kg of milk per day would be produced by offe ring the high, rather than the medium feed value silage, while at a concent rate proportion of 0.70 total dry matter, the milk yield benefit associated with the high feed value silage was reduced to 0.6 kg per day. Similarly, total dry matter intakes for animals offered the high feed value silage wer e 7.4 and 0.8 kg per day greater than for animals offered the medium feed v alue silage, at concentrate proportions in the diet of 0.32 and 0.70 total dry matter, respectively. These responses indicate that at low concentrate inclusion rates in the diet, silage feed value has a major influence on ani mal performance, while at higher concentrate inclusion rates, the importanc e of silage feed value is reduced.