21 March 2013

8 minute read

Introduction

Maintaining healthy animals is a key component of animal welfare. Ensuring good rumen health in dairy cattle is key for the maintenance of efficiency and productivity, and thus herd profitability. In dairy cattle, the rumen environment is designed to function optimally within a pH range of 6.2–7.2.

To maintain healthy rumen function, dairy cows require diets that contain adequate amounts of physically effective fibre (peNDF). Ensuring adequate intake of peNDF can be difficult because most commercial dairy rations, designed to maximize milk production, contain high levels of concentrate and high-quality forages, often limiting in peNDF (Beauchemin and Yang, 2005).

When ruminants consume excessive amounts of rapidly fermentable (non-fibre) carbohydrates, combined with low intake of peNDF, they are not able to maximize their rumination time and salivary buffer flow to the rumen, and thus ruminal pH drops below normal physiological levels.

Sub-optimal ruminal pH (e.g. pH 5.2–5.8) is often referred to as subacute ruminal acidosis (SARA) (Owens et al., 1998).

SARA is a major concern in terms of both productivity and animal welfare. Rumen pHet al., 2008).

Despite our vast knowledge of the etiology of SARA and its consequences, the prevalence of this digestive condition, which is estimated to range from 19 to 29 percent in dairy cows in early- and mid-lactation, remains very high because we try to maximize milk production through encouragement of maximum intake of diets containing high proportions of highly-fermentable carbohydrates (Krause and Oetzel, 2006).

This case study focuses on how dietary selection (sorting) of dairy rations may lead to depressed rumen pH, thus increasing the risk of SARA. Through a series of research studies, we have established evidence indicating that excessive sorting of certain ration components by dairy cattle can lead to SARA and also promote variability in nutrient intake.

We have also investigated the impact of this resultant health condition on cow productivity and, thus, also profitability. Finally, a discussion of means to reduce this behaviour is outlined.

Feed Sorting and Intake Consistency

Dairy cattle are commonly fed their feed components in the form of a total mixed ration (TMR). Total mixed rations are designed as a homogenous mixture with the goals of helping to minimize the selective consumption of individual feed components by dairy cattle, promoting a steady-state condition conducive to continuous rumen function and ingesta flow, and ensuring adequate intakes of fibre (Coppock et al., 1981).

It is not surprising that providing feed as a TMR is standard on most commercial dairies, particularly for lactating animals. Unfortunately, even when providing feed as a TMR, cows have been shown to preferentially select (sort) for the grain component of the TMR and discriminate against the longer forage components (Leonardi and Armentano, 2003).

To date, our collective research has demonstrated that, on average, when dairy cows are fed TMR they selectively consume approximately five to ten percent more of the smallest ration particles and ten to 20 percent less of the longest ration particles than that which we would predict they would consume based on the original ration formulation.

Impact of Feed sorting on Rumen Health and Productivity

The result of sorting of TMR by dairy cows can, therefore, be that the ration actually consumed by cows is higher in fermentable carbohydrates than intended and lower in effective fibre. Such nutrient consumption patterns lead to excessive acid production and reduced buffering capacity in the rumen.

Not surprisingly, therefore, we have demonstrated that such consumption patterns are related to depressions in rumen pH (Figure 1; DeVries et al., 2008) and, thereby, increase the risk of SARA. In fact, we have been able to explain between 45 and 97 percent of the variability in measures of rumen pH (minimum, maximum, mean and range) based on the feed sorting patterns of lactating dairy cows (DeVries et al., 2008).

Given the relationship between sorting and rumen pH, and the fact that depressed rumen pH may lead to milk fat depression, it is not surprising that we have recently demonstrated a clear association between sorting against long ration particle and producing milk of lower fat percentage. In two recent separate studies, we observed that milk fat decreased by 0.15 percent for every 10 percent refusal of long forage particles in the ration (DeVries et al., 2011; Fish et al., 2012).

Sorting of a TMR can also reduce the nutritive value of the TMR remaining in the feed bunk, particularly in the later hours past the time of feed delivery (DeVries et al., 2005) after the greatest amount of sorting has already occurred (Hosseinkhani et al., 2008).

Figure 2 illustrates how feed sorting deteriorates nutritional value, as indicated by the quadratic increase in fibre content in the feed remaining in the feed bunk throughout the course of the day. For group-fed cows, this may be detrimental for those cows that do not have access to feed, at the time when it is delivered, for example when there is high competition at the feed bunk or when lameness incidence is high.

In such cases, cows that are forced to return to the feed bunk at later time points during the day may end up consuming a ration very different from that originally formulated for them and may, thus, not be able to maintain adequate nutrient intake to maintain high levels of milk production (Krause and Oetzel, 2006); whereas those accessing the feed bunk for a greater period earlier in the day are more likely to experience SARA because of selective intake of highlyfermentable feed particles.

Economic Cost of Feed Sorting and Subacute Ruminal Acidosis

Reduced profitability associated with feed sorting and any resultant SARA is related to both direct and indirect costs. As noted above, our research has indicated that milk fat decreases by 0.15 percent for every ten percent refusal of long forage particles in the ration (DeVries et al., 2011; Fish and DeVries, 2012).

The financial impact of such a drop in milk fat is substantial, particularly in areas where milk value is related to component prices. For example, a ten percent refusal of long ration particles in a 500-cow Canadian dairy herd may result in approximately 90 000 Canadian dollars in annual lost revenue due to production of less milk fat. Indirect costs of sorting, including greater within-herd variability of nutrient intakes, would add to this decrease in profitability.

The direct financial impact of SARA was demonstrated by a field study on a large dairy farm in New York State where it was found that SARA reduced milk yield by 2.7 kg/day, milk fat production by 0.3 percent and milk protein production by 0.1 percent (Stone, 1999).

These production losses associated with SARA alone were estimated to be US$1.12/ day/cow (Stone, 1999). To place that cost in further context, if a 1 000-cow dairy herd had a 25 percent prevalence rate of SARA, the production losses associated with SARA would be approximately US$102 200 annually.

These costs exclude the indirect financial impact of associated disorders and risks (such as lameness, reduced reproductive efficiency and increased culling) and veterinary treatment, which would be in addition to the cost of lost milk production.

Reducing the Degree of Sorting of Total Mixed Rations

As described above, the high-energy diets, which are low in neutral detergent fibre (NDF) and high in starch, that are typically fed to lactating dairy cows can put the cows at risk of SARA.

Interestingly, our research indicates that lactating dairy cows demonstrate higher degrees of sorting against longer forage particles and for smaller grain concentrate particles when fed lower forage diets (DeVries et al., 2007; 2008).

Thus, in situations where rations are being heavily sorted, it is recommended that a greater proportion of forage be included in the ration. Besides the quantity of forage, the type and particle size of forages will also affect feed sorting (Leonardi and Armentano, 2003).

A greater proportion of dry forage (hay) in the ration will increase the amount of sorting against longer fibrous particles. Greater particle size of forages, including alfalfa hay and corn silage, also results in increased sorting against long forage particles in the TMR. Thus, sorting can be reduced by chopping forages into smaller lengths; however, care must be taken in those situations not to reduce particle size to the extent that the forage loses its physical effectiveness for stimulating rumination and rumen buffering.

Beyond changing forage characteristics or content, sorting can be influenced by other dietary measures. It is commonly believed that adding water to a dry TMR will help bind particles together and make it harder for dairy cattle to sort out smaller particles.

Research has shown that the amount of feed sorting (against long particles) can be reduced when water is added to a dry TMR (>60 percent dry matter), particularly those rations containing a high proportion of dry forage (Leonardi et al., 2005; Fish and DeVries, 2012).

Recent research has also indicated that for wetter rations, other liquids (for example, a molassesbased liquid feed) may be more effective at reducing feed sorting (DeVries and Gill, 2012). It is recommended that in situations where sorting of mixed rations is evident, producers try adding water or liquid feed to their TMR, and carefully monitor the effects that these may have.

There are also feeding management practices which influence the degree of feed sorting. Increasing the frequency of TMR delivery from once per day to twice (or more often) per day has been shown to reduce feed sorting (DeVries et al., 2005; Endres and Espejo, 2010).

Given this, and the fact that more frequent feeding also promotes more equal access to freshly delivered feed and a more even distribution of feeding time over the course of the day, feeding cows more often has the potential to reduce the variation in composition of ration consumed.

Acknowledgements

For their financial support of much of the research described in this case study, the author wishes to thank the National Science and Engineering Research Council (NSERC) of Canada, Dairy Farmers of Canada, Agriculture and Agri-Food Canada, the Canadian Dairy Commission, Westgen, the Investment Agriculture Foundation of British Columbia, the Canadian Foundation for Innovation, the Ontario Research Fund, the Ontario Ministry of Agriculture, Food, and Rural Affairs, the University of Guelph and the University of British Columbia Animal Welfare Program.

Further Reading You can view the full report by clicking here.

March 2013