Mortality and Early-Lactation Cull Rates in PA Dairy Herds

Posted: April 20, 2009

Evidence has been accumulating that lactating cow mortality rates have increased more than 2 fold since 1980.

Evidence has been accumulating that lactating cow mortality rates have increased more than 2 fold since 1980. Legislative changes that limited the sale of downer cows caused a jump in mortality rates in 2003 - 2004, but levels were on the rise long before then. There are obviously severe economic consequences to high mortality rates. Producers lose 100% of a cow’s salvage value, and rather than growing internally some herds have to buy cows just to maintain their current herd size. While the economic costs of high cow mortality rates are serious, the impact on cow welfare and public perception of dairy production are more troubling.

Several studies have been initiated to investigate mortality rates, including an analysis of DHI data from Pennsylvania. Mortality rates in Pennsylvania herds have not been firmly established, but they are lower than other states in most studies. Our objectives were to estimate mortality in Pennsylvania using data from 2,574 herds, and to identify production differences between high and low survival herds. In addition to mortality, we considered the proportion of a herd that is culled or died between 21 days before a cow is due through 60 days in milk (which we will call the transition cull rate). We analyzed transition cull rate because healthy cows rarely exit a dairy herd during this time period due to the high milk production potential for the remainder of lactation and because not all herds report cow mortality for various reasons. We estimated the overall lactating cow mortality rate in Pennsylvania to be somewhere between 5% and 5.5%, and the transition cull rate to be 7.6%. These percentages are below the national average, with mortality rate estimates approaching 10% in some states. However, these results represent only those herds enrolled in DHI testing and there are herds with very high mortality rates. Percentiles of mortality and transition cull rates by percentile are reported in Table 1. There were 467 herds that did not report mortality and they are not included in the mortality rate percentiles.

Table 1. Percentiles for mortality rate and transition cull rates
Percentile Mortality Rate Transition Cull Rate
10 1.9 1.4
25 2.6 3.3
50 4.5 6.3
75 7.3 9.6
90 10.5 12.8

Low survival herds that had both high mortality (> 8%) and transition cull rates (> 12%) were compared to high survival herds (< 1.4% mortality and < 2.9% transition cull rate) in order to describe herd management factors that may contribute to high mortality. Several differences were apparent and are reported in Table 2.

Table 2. Characteristics of high and low survival herd environments

Low survival High Survival
Mortality (%) 12.9 0.3
Transition cull rate (%) 15.2 1.3
Total herd turnover rate (%) 42.8 19.5
Milk yield, lactation 1 62.7 58.4
Milk yield, lactation 5 67.1 70.2
Somatic Cell Score 3.1 2.9
Fat-protein inversions (%) 9.5 5.9
Week 1 fat percent 5.2 5.0
Week 15 fat percent 3.4 3.5

Some caution is warranted when interpreting the numbers presented in Table 2, particularly for the low survival herds. Mastitis (as indicated by SCS differences) contributed to higher mortality and culling in some low survival herds. However, herds with somatic cell scores of 2.0 or less also had low survival. Other factors such as early lactation body condition loss (as indicated by a larger change in milk fat percentage from calving to 15 weeks post calving) contributed to mortality and transition culling in those herds. It is clear that cow health and well being was compromised in the low survival herds, particularly near calving. The overall herd turnover rate for the low survival herds (42.8%) would cause severe economic stress because outside replacement animals would likely need to be purchased to simply maintain the current herd size.

Mastitis and excessive body condition loss are two contributing factors to higher mortality rates. A higher proportion of cows with fat-protein inversions indicate that sub-acute acidosis is more prevalent in herds with high mortality rates. Milk production levels and fertility were different in the high and low survival herds. First lactation milk yield was higher in the low survival herds, whereas mature cow yield was lower. Poor health in later lactations likely compromised potential mature cow milk yield in the low survival herds.

While the economic costs of high cow mortality rates are serious, the impact on cow welfare and public perception of dairy production are more troubling.

It appears that Pennsylvania dairy producers generally maintain lower levels of mortality than their counterparts in other states. However, there are several potential herd management areas to investigate for those herds that struggle in these areas. Are cows losing excessive body weight due to over conditioning or long dry periods, is there a mastitis problem in the herd, are cows suffering from sub-acute rumen acidosis, are cows slipping and injuring themselves, is there a dystocia problem, etc.? Losing a cow to death or during early lactation represents a large opportunity cost to a dairy farm because little or none of that cow’s milk production potential has been realized despite a large investment in the rearing or dry period. Consultants oftentimes disagree about the optimal herd culling rate, but we all agree that when and how cows leave are important. Monitor mortality and early lactation culling rate to determine whether opportunities exist to capture more milk from healthier cows.

Chad Dechow, Assistant Professor of Dairy Cattle Genetics,  Department of Dairy and Animal Science