Milk composition and production efficiency within feed-to-yield systems on commercial dairy farms in Northern Ireland.

The introduction of concentrates in the diet has contributed markedly to increasing milk production per cow and limiting the effect of the negative energy balance during the period when dry matter intake (DMI) is comparatively low. A wide range of concentrate feeding approaches are used including total mixed rations (TMR), or presenting the concentrate portion separately from the forage via in-parlour or out-of-parlour feeding systems, or a combination of these approaches. In TMR feeding is ad lib, but with separate feeding systems concentrates are often offered on a feed-to-yield (FTY) basis, which should be more accurate in terms of meeting nutrient requirements and economics. However, the literature is unclear as to the outcomes, one reason being that cow performance associated with FTY approaches has been only investigated on a limited scale and not on a scale involving commercial farms. The objectives therefore of the study cited were therefore to investigate how FTY systems operate in practice on commercial farms and to examine the relationship between milk yield and DMI, milk composition and production efficiency. Specifically, the study sought to identify if a reduction in milk fat concentration was observed in cows offered high levels of concentrates, and if so, how much of the reduction could be attributed to cow genetics.

The study was conducted on 26 dairy farms in Northern Ireland involving a total of 3471 cows. The cows were fully housed and were given concentrates on the FTY basis. The forage which was fed ad lib consisted mostly of grass silage. Individual cow genetic information was obtained for 18 herds. Concentrate intakes of individual cows were either obtained from the farms or calculated, while milk yield and composition data were obtained from test-day milk recording. Mean test-day milk yields during months two to five post-calving were calculated for each cow, and cows within each of lactation 1,2,3 and 4+ were placed into one of six equal-sized milk yield groups in kg per cow per day, which depending on lactation number ranged between 20 and 50L+. Total DMI was calculated from the equation:

DMI (kg/day) = 11.032 + (0.554 x lactation number) + (0.343 x ECM kg) + (-3.194 x Fat:Protein) + (0.107 x week-in-milk)

where, ECM is energy corrected milk.

Dietary effects and performance responses to milk yield groups were tested for linear and quadratic effects.

Concentrate DMI showed a quadratic response with increasing milk yield in first lactation cows and a linear increase in multi-lactation cows, whereas total DMI showed a positive linear increase with increasing milk yield within all lactations. Although significant in particular groups, the effect of milk yield on forage intake was small (< 5%). Milk fat and protein percentage showed a linear decline with increasing milk yield group in all lactation numbers, whereas fat plus protein yield and ECM yield increased with increasing milk yield group. Nitrogen use efficiency increased quadratically with increasing milk yield, so did ECM/DMI. For genetic evaluation, Profitable Lifetime Index and Predicted Transmitting Ability for milk yield, showed trends to decrease with increasing lactation number but did increase with increasing milk yield group.

The results suggested that when concentrates are offered on FTY basis, the response may be different than when fed the traditional way: Total DMI continued to increase with increasing milk yield, with little evidence of forage substitution by concentrates. The lack of substitution probably indicates that a basal diet will sustain a common milk yield across a range of performance levels. The decrease in milk fat and protein concentration with increasing milk yield, is probably partly due to cow genetics, diet and dilution effects. Differences in cow genotype between the two extreme milk yield groups were responsible for up to 21% of the variation in milk fat concentration and up to 13% of the variation in milk protein concentration. Cows with greater milk yields had improved N and energy use efficiency due to the dilution of maintenance as milk yield increased. However, concentrates offered per kg of ECM increased with increasing milk yield, which although sustaining higher milk yields, the reduction in milk composition and the subsequent impact on the value of each kg of milk value produced, may impact the economics of providing concentrates according to the FTY approach. Thus, there may be an optimum which is probably not at the maximum concentrate level fed.