Milk Fat Depression at Grass

23 February 2018

Claire Beckett - Milk Fat Depression at Grass

Milk fat depression (MFD) during grazing continues to be a problem for a subpopulation of Irish herds, typically during April, May and June. Bulk tank milk fat of 3% and below have been reported from some Irish producers.

By Claire Beckett, Dairy Technical Support Co-ordinator, Trouw Nutrition Ireland

Milk recording data in Ireland from 2004 to 2014 showed that between 10 and 20% of herds experience MFD, where MFD was defined as bulk tank milk fat < 3.3% (Carty et al., 2017). The highest prevalence of MFD in autumn calved cows occurred at 181-210 days in milk (DIM). In spring calving cows, the highest prevalence occurred at 61-90 DIM (Carty et al., 2017).

MFD is not an individual cow problem, rather a herd level problem, with milk fat levels dropping to well below what is typically expected, even taking account of the expected milk fat decline associated with the lactation curve. MFD typically occurs on the second rotation in full time grazing herds and it is very difficult to recover from for the remainder of the year once it occurs. A grass-only system of young, fresh, lush, immature grass provides ‘the perfect storm’ for MFD.  

Irish milk recording data highlighted additional associations to MFD; prevalence of MFD was shown to be highest in cows with the highest genetic values for milk yield, whilst lowest prevalence of MFD was shown in cows with the highest breeding values for milk fat percent (Carty et al., 2017).

Without an industry standard on what constitutes an acceptable butterfat level, we need first to agree on an acceptable target for butterfat percentage during grazing for Irish dairy farms. Research highlighted in 2007 at the American Association of Bovine Practitioners (AABP), by G Oetzel outlined that milk fat percentages of 3.4 - 4.0% constitutes normal composition (with some seasonal variation) for Holstein herds, and for Ayrshire and Milking Shorthorn herds alike. For Jersey herds, normal milk fat corresponds to 4.2 – 5.0%. G Oetzel suggested that milk fat should never drop below 3.2% in Holstein herds.

As base price butterfat is generally set at 3.8% by many Irish milk processors, Trouw Nutrition Grass Watch uses milk fat below 3.8% as a potential indicator of MFD. During 2017 Trouw Nutrition Ireland’s (TNI) Grass Watch Trials seen mean weekly butterfat levels drop below 3.8% from 24th April to the end of May (Graph 1).  

Graph 1 – TNI GrassWatch Mean Weekly Milk BF% 2017

The ‘mean’ butterfat drop looks reasonably acceptable however, given that MFD only typically occurs on 10-20% of Irish herds, the mean will therefore mask what is typically occurring on an individual farm basis. On an individual farm basis, butterfat drops to 3% were recorded. John is one of our Grass Watch farmers whose herd experienced this. On approximately the 24th April, butterfat dropped from 3.81% to 3.48% and on 15th May, butterfat percentage reached a low point of 3%. Butterfat % in John’s herd failed to raise significantly again for the remainder of the grazing season, despite John introducing straw and 20% beet pulp into the diet. This re-emphasises the difficulty of increasing herd milk-fat once the depression has occurred.

Milk fat production

Approximately 50% of milk fat is synthesized de novo in the mammary epithelial cells, using precursors such as beta-hydroxybutyrate and acetate, which are absorbed from the blood. The remaining 50% of milk fat originates from mammary uptake of preformed fatty acids from the gastro-intestinal tract, which reflects dietary and microbial fatty acids. Unless cows are in a negative energy balance, mobilisation of body fat reserves will constitute a relatively small proportion of milk fat.

How does milk fat depression occur?

There are 3 theories to explain MFD. Two are connected, as they both focus on the balance of volatile fatty acids (VFAs) in the rumen. It is proposed that decreased rumen actetate and butyrate, associated with the lower effective fibre (peNDF) in grass diets, will limit milk fat synthesis, as acetate is one of the building blocks of milk fat. Alternatively, starch rich diets, (from concentrates fed during grazing) increase rumen propionate, which consequently increase blood glucose and insulin. Insulin will divert nutrients away from the mammary gland and toward body fat reserves.

The third, and the most physiologically plausible accepted theory is the ‘biohydrogenation theory’. As unsaturated fatty acids (FAs) are toxic to rumen bacteria, biohydrogenation in the rumen is a necessary process to convert unsaturated FAs to saturated FAs. These saturated FAs are then absorbed and contribute to milk fat. Under certain dietary influences, rumen conditions are changed, altering this biohydrogenation process to produce FA intermediates (e.g. conjugated linoleic acid isomer trans-10, cis-12 CLA). These intermediates, escape the rumen, travel in the blood to the mammary gland where they signal a decreased expression of mammary lipogenic enzymes, thus inhibiting milk fat synthesis.

Dietary risk factors for MFD in a pasture-fed system include high polyunsaturated FA (PUFA) intake and a reduction in ruminal pH; both which favour the production of alternative biohydrogenation pathways (resulting in trans-10, cis-12 CLA intermediates).

Fresh grass can have a FA content up to 6%, and can be particularly high in rumen available PUFA, which can precipitate MFD. Grazing at the optimal agronomic 3 leaf stage, will limit FA intake. Premature entry at low leaf emergence, coupled with nitrogen fertilizer use, increases the FA content of plant tissue. When this is coupled with a lack of peNDF at these lower leaf emergence points, it could exacerbate the risk of MFD further.

Reductions in ruminal pH as causal of MFD is most likely associated with the imposed changes in bacterial populations towards those that favour alternative biohydrogenation pathways. Factors that can result in marked reductions in ruminal pH during grazing, include:-  

  • Alterations to the carbohydrate profile and rate of degradation of carbohydrate fractions
  • High concentrate feeding in-parlour. In some cases, 5kg of highly fermentable concentrates per milking  altered rumen fermentation / reduced rumen pH
  • Reduced peNDF in grass increasing rumen throughput
  • Reduced production of salivary buffers (to buffer rumen), as a function of reduced peNDF supply

Graph 2 - Relationship between ruminal pH and milk fat (Allen, 1997)

Over half of Irish dairy cows are affected by either acidosis or subacute ruminal acidosis (SARA) during grazing (Graph 3). Clinical acidosis and SARA are not prerequisites for MFD; only subtle pH drops are required (e.g. from pH 6.2 to 5.8). Therefore, in line with Graph 3, a considerable proportion of Irish cows are vulnerable to MFD.

Graph 3 - Percentage of Irish dairy cows affected by SARA during grazing (O’Grady et al., 2008)

  •  11% of cows classified as affected by SARA pH ≤ 5.5
  • 42% of cows marginally affected by SARA pH 5.6-5.8
  • 47% of cows normal pH >5.8 

Trouw Nutrition GrassWatch MFD Alert

No single dietary factor is responsible for MFD, rather an interaction between various dietary components (Adam Lock, Michigan State University). The TNI Grass Watch farm trial 2017 findings re-emphasised this statement. Table 1 shows the average grass nutrient parameters associated with MFD for all 2017 GrassWatch farms. All the grass parameters change sequentially, therefore by remembering the value of one/two parameters, this will act as a potential future butterfat drop indicator.

Table 1 - TNI GrassWatch 2017 Mean Grass Parameter Values Associated with the Period of MFD

Grass parameters associated with MFD

Low NDF and fibre index will result in reduced production of the milk fat precursor acetate, as well as a lower rumen pH due to reduced rumination and saliva production. High sugars, acid load and RFC will lower rumen pH, favouring microbial populations with alternative biohydrogenation pathways. Low NDF and fibre index, coupled with high RFC reduces rumen retention times and increase the likelihood of partially-hydrogenated FA intermediates escaping the rumen. High oil and RUFAL will increase the concentration of lipolysed unsaturated C18 FAs in the rumen. Elevated FA concentration will cause changes in the ruminal fermentation characteristics and microbial population distribution, due to the antimicrobial nature of unsaturated FAs. As certain bacterial species are more susceptible than others, the result is a microbial shift in the rumen. This bacterial shift results in altered fermentation of carbohydrates, fibre digestion and redirection of the biohydrogen pathway towards CLA intermediates.

Table 1 also shows John’s grass values for the period of MFD. Those grass parameters displayed in red are in the risk category. As the majority of grass values are displayed in red, and associated with MFD, this possibly explains why John suffered such a severe MFD. In addition, the sub optimal low grass covers (<1000kgDM/ha) grazed by John’s herd at the point of MFD may have been a contributory factor, as structurally inefficient, high acid loading grass composition was recorded on John’s farm at the point of MFD. Furthermore, John’s herd was 60-90DIM at the point of MFD, the end of the risk time for negative energy balance with associated low blood NEFA concentrations, meaning that the contribution of NEFA to milk fat is less.

In summary, the composition of grass (most of the parameters in the risk category) on a pasture-only system, in one of the highest yeilding herds in the GrassWatch study, (yielding >34litres and feeding 6-7kg of concentrate at the point of MFD), and reaching the end of the risk period for negative enery balance, may have provided the ‘perfect storm’ for MFD in John’s herd.

Nutriopt GrassWatch Butterfat Risk Alert

Table 2 illustrates the Nutriopt Grass Watch Butterfat Risk Alert parameter values for the ‘complete diet’ for low, moderate and high risk of MFD. Balancing the Nutriopt nutrients RFC, acid load, fibre index and RUFAL, using the Nutriopt Dairy model allows the diet to be fine tuned to maintain as optimal rumen conditions as possible, through dietary nutrient manipulation.

Table 2 - Nutriopt GrassWatch Butterfat Risk Alert

Controlling MFD during grazing

It is very difficult to control poly unsaturated FA intake on full time grazing diets. Nevertheless diets should be formulated to ensure RUFAL remains at an acceptable level. Bypass fats (C16) have been succesfully used to artifically improve milk fat % – but economics must be considered. To control rumen pH, reduce diet fermentability and increase peNDF, buffer feed chopped straw or silage or include rumen buffers and yeast (for oxygen scavagenging and lactate utilisation).

Note: a balance needs to be struck between reducing carbohydrate fermentability of the diet so as to avoid shifts in biohydrogenation pathway, and maintaining sufficient carbohydrate fermentability of the diet to avoid reducing nitrogen utilisation efficiency at grass. As there is not a single cause to herd MFD, the solution should be a multi-structured approach.