Total tract NDF digestibility varies considerably in cows

One of the core research objectives at Miner Institute is to increase forage utilization by dairy cows. Many of our conversations about dairy cattle nutrition at the farm and in the lab revolve around improving forage quality, and in particular, NDF digestibility. Feeding high quality, highly digestible forages can help to increase DMI and lactation performance. For example, a 1%-unit increase in NDF digestibility has been reported to increase DMI by 0.37 lbs/d and milk yield by 0.51 lbs/d.

Listed in Figure 1 is the relationship between total tract NDF digestibility (%) and DMI (lbs/day; panel A) or NDF intake (lbs/day; panel B) for lactating dairy cows. This dataset was developed from 4 recently published studies that measured lactation performance of dairy cows fed various low-starch diets. The dataset includes 161 individual cow observations. On average, the diets contained 51.9% dietary forage (range = 38.8 – 68.5) and 33.6% NDF (range = 26.5 – 38.0).

Within this dataset there was no relationship between DMI and total tract NDF digestibility (R2 = 0.02). There was a slight negative relationship between NDF intake and total tract NDF digestibility (R2 = 0.30), possibly due to increased passage rate of NDF from the rumen as NDF intake increases. The focus of this figure is not be the lack of a defined relationship between NDF digestibility and intake, but rather the large variation in NDF digestibility between cows, regardless of intake. Total tract digestibility averaged 43.0% and ranged from 16.3 to 70.5%. Certainly, some of the difference in NDF digestibility between cows can be explained by dietary effects. Benchmarks for total tract NDF digestibility are not readily available. Total tract NDF digestibility was reported to average 62.7% in grass silage based diets and 59.4% in diets containing various forages. These values can serve as starting points to benchmark herd performance (but let"s all agree to avoid telling the Miner Institute president that our research trials result in below average NDF digestibility measurements).

There are several ways to increase NDF digestibility of diets. These include replacing conventional forage hybrids with brown midrib hybrids, and replacing relatively indigestible forages, such as straw, with more digestible forages such as high quality corn silage and legume or grass dry hay or silage. Furthermore, partial replacement of forage NDF with highly digestible byproduct feeds such as soy hulls or beet pulp can improve NDF digestibility. In addition, supplementation with yeast products can increase both rumen pH and organic matter digestibility. Known negative associative effects on NDF digestibility include excessive amounts of highly ruminally fermentable starch or sugar and high levels of unsaturated fatty acids. Fermentable starch and fat levels in dairy cattle diets should be evaluated routinely.

Total tract NDF digestibility can be monitored on farms with relative ease. Most commercial feed testing laboratories can measure dietary total tract NDF digestibility. Simply collect a TMR sample and composited fecal sample from a minimum of 10 to 20 cows per pen that you are interested in monitoring and send it in. Routine monitoring is the first step and will help to identify dietary changes that can be made to improve NDF digestibility. Total tract NDF digestibility should not be the only focus of a successful nutrition program, but monitoring and improving NDF digestibility should assist in boosting both DMI and milk yield.

Figure 1. Effect of DMI (lbs/d; panel A) or NDF intake (lbs/d; panel B) on total tract NDF digestibility (%).



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