USEFUL EQUATIONS – IMPLICATIONS TO EFFICIENCY PROGRESS IN THE DAIRY HERD

Background:

Feed efficiency traits such as dry matter intake (DMI) and gross feed efficiency (GFE) [kg milk/ kg DMI] are imperative in the improvement of herd performance, profitability and sustainability. However, these traits are normally not included in selection indices, because of the difficulty to measure DMI. Consequently, efficiency of milk production is usually evaluated only in terms of milk yield, on the assumption that because maintenance requirements become a smaller proportion of total at higher yields, the cow with a high milk yield is considered more efficient, which in general is true. However, a cow can also produce more milk because of an exceptional high feed intake and maintenance requirement, and as a result is then not that efficient. To identify those cows that are really efficient the relationship between the energy requirements (which can be converted to DMI) to produce particular amounts of milk has been compiled, and the data of cows evaluated against this theoretical relationship. The difference between the actual DMI of the cow and the theoretical value has become known as residual feed intake (RFI). If the difference is more than the theoretical, that cow is not efficient, if it is less, that cow is more efficient and the one to be chosen in selection. However, to identify the efficient ones individual DMI and RFI measurements are still required and worldwide there has been a comprehensive effort to come up with indirect easily measured and proxy methods to predict individual DMI and RFI. Genomic and physiological investigations to understand efficiency are also done. Although promising, the accuracy as yet is not sufficient for reliable use in selection indices.

Useful equations:

Despite the difficulties discussed, prediction equations for DMI and GFE are useful to the dairy farmer to estimate formulated feed requirements and efficiencies at the herd level, and for pasture-based systems the amounts of herbage required in relation to herbage biomass to plan rotational frequency.

International sufficiently comprehensive and accurate prediction equations using either animal-based factors such as milk components, body weight, milk production, and stage of lactation, or based only on feed fibre (NDF, ADF) components of the diet are:

DMI (kg/day) = (0.372FCM) + (0.0968LW^0.75) x (1 – e^{(-0.192 x (WOL + 3.67))})

where: FCM = 4% fat corrected milk (kg/day) [calculated as milk (kg/day) x (0.4 + 15 x fat%)]; LW = live weight; WOL = week of lactation.

The fibre-based DMI model which predicts satisfactory and is useful for pasture systems is:

DMI (kg/d) = 12.0 − 0.107FNDF + 8.17ADF/NDF + 0.0253FNDFD – 0.328(ADF/NDF – 0.602) × (FNDFD − 48.3) + 0.225MY + 0.00390(FNDFD − 48.3) × (MY – 33.1)

where: FNDF = forage NDF content of diet (% of DM); ADF/NDF = ADF as a fraction of NDF in the ration; FNDFD = digestibility of forage NDF measured in vitro or in situ (% of FNDF), and MY = mean milk yield (kg/day).

Both models were developed for cows of 500 kg + live weight.

For the animal-based factor model, farmers should record cow body weight, milk production, and milk components of individual cows daily, and by using the DM intake prediction equation, can calculate DMI and GFE, whereas for the fibre-based model laboratory analyses or records of NDF, ADF and FNDFD are required.

South African useful equations for cows weighing more than 500 kg fed TMR, and of application in the first 15 weeks of lactation are:

• DMI (kg/day) = 8.81 + 0.212Milk(kg/day) + 0.010LW(kg), R² = 0.90                                        where: DMI = dry matter intake and LW = live weight of the cow.                                         Gross feed efficiency (GFE) [milk (kg)/DMI (kg)] is then simply the cow’s milk yield divided by the calculated DMI
• Using milk composition data, GFE can be calculated as: GFE (kg/kg) = 1.881 + 1.344BFY −0.003LW, R² = 0.91.                                                                                                                            

where: BFY = butterfat yield (milk fat % x milk yield)

The relationship of GFE with butterfat is explained by better energy mobilization in more efficient cows. Utilizing this equation in selection will enhance both milk fat and milk yield which is beneficial. However, unabated selection for milk fat % could be negative to protein % and should be controlled. Of further significance in the second equation, and also implied in the first equation when calculating GFE, is that LW is negatively associated with GFE, which means that GFE declines with LW. Farmers should therefore put a ceiling on cow LW to maintain GFE and limit maintenance needs of the cow.

These implications to GFE are supported by results of a large international study based on genomic data, which strongly suggest that selecting smaller sized cows with lower DMI, but higher milk, fat, and protein production, tends to increase GFE and profitability. Apart from an individual farmer perspective, from a breed point of view, national indices should therefore prioritize reducing body size of cows for improved feed efficiency and profitability. This should also benefit the quest to reduce greenhouse gas emissions as the amount of methane produced in the rumen in dairy cow diets is largely determined by the amount of feed consumed.