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- Dietary protein intake is the most important factor determining milk nitrogen efficiency, urinary nitrogen losses, and consequently, nitrate leaching and ammonia and nitrous oxide emissions from dairy cow manure.
- Dairy cows producing up to 90 lbs/day can be safely fed balanced diets with 16% crude protein (CP) without affecting milk production or composition. When feeding low protein diets, extra caution should be paid to balancing dietary energy and key amino acids.
- Fiber digestibility will likely be decreased for diets with CP < 16% [rumen degradable protein (RDP) ≤ 10% of DM] and diets with CP <15% [metabolizable protein (MP) deficiency of about <12%] will likely result in decreased milk yield, partially through decreased dry matter intake (DMI).
- Production losses with low-protein diets are caused by: (1) depressed feed intake due to impaired rumen function or physiological regulation of intake; (2) deficiency of RDP, which may cause decreased fiber digestion, microbial protein production, and milk fat and protein yields; and (3) insufficient supply of key amino acids limiting milk protein synthesis.
- Low protein diets may benefit from supplementation with rumen-protected amino acids, especially those found to be first-limiting for milk production. Methionine is usually considered the first limiting amino acid for dairy cows in the U.S. In some situations, most likely with high corn-based diets, lysine can be a limiting amino acid. Recent data have demonstrated that histidine may also be a limiting amino acid in MP-deficient, corn silage/alfalfa haylage-based diets. Long-term trials showed that supplementation of such diets with rumen-protected histidine increased or tended to increase milk yield and milk protein percent and yield, mainly through increasing DMI.
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Feeding dairy cows diets with lowered CP content reduces nitrogen input, improves nitrogen utilization efficiency, and reduces nitrogen losses from manure. Dairy producers are interested in not overfeeding protein primarily to reduce feed costs. When corn price is low and the price of protein supplements is high, it only makes economic sense to feed less high-protein feedstuffs. Dietary interventions that carry the risk of decreasing milk production, however, have to be carefully evaluated before being implemented.
Benefits of feeding low protein diets
If an average lactating cow diet has around 17% CP, diets with CP at or below 16% can be considered ‘low-protein’. These kinds of diets may or may not be deficient in MP, which is the unit used by the National Research Council’s Nutrient Requirements of Dairy Cattle (NRC, 2001) to define protein requirements. As an example, the two diets in Table 1 would meet the MP requirements of a dairy cow that is milking around 90 lbs/day and is not in a negative energy balance. Feeding a diet with 17% CP is likely not going to increase milk or milk protein yields, assuming dietary energy and amino acids are balanced. It is likely that this extra protein is going to contribute mostly to the RDP pool and a significant proportion of it may end up being metabolized to urinary urea and excreted with urine. A large portion of this urinary nitrogen is going to be lost during manure storage or application to soil. Therefore, over-feeding protein above the animal’s requirements in a balanced diet would increase feed costs and will have no beneficial effect on milk production or composition. A simple measure of the efficiency of use of dietary nitrogen by the cow is milk nitrogen efficiency (MNE). This is the proportion of dietary nitrogen that is secreted as milk protein. An accepted threshold for MNE is 25% and usually efficient cows will have MNE of 30% and above. Increasing MNE, however, should not come at the expense of decreased milk production.
Effect of dietary protein on dry matter intake
One way low-protein diets may have a negative effect on milk production is through decreased DMI. In a lactating dairy cow, feed intake is literally what ‘drives the train’ and any negative effects on dry matter or feed energy intake will, in most cases, result in decreased milk production. Long-term (up to 10 weeks) trials at Penn State have shown a variable effect of decreasing dietary CP or MP on DMI. For trials in which DMI decreased when feeding MP-deficient diets, milk production also decreased. On the contrary, when DMI did not decrease, milk production was also not different among diets. For example, diets that were 8 to 13% deficient in MP (according to NRC, 2001) depressed DMI and/or decreased milk production in cows producing 84 to 95 lbs day of milk. In another trial, diets that were 5 to 8% deficient in MP did not result in depressed DMI or milk production in cows producing around 95 to 99 lbs/day.
Another way decreased dietary CP may affect production is through its effects on ruminal fermentation. If dietary protein is intentionally decreased, it is usually the inclusion of common protein feeds, such as soybean or canola meals, that is decreased in the diet. This may result in deficiency of RDP, which may result in decreased fiber degradability, acetate production, and microbial protein synthesis. These effects may result in decreased milk fat and protein concentrations and yields. In all Penn State trials with low protein diets, total tract apparent digestibility of fiber fractions was decreased. Although we have not measured ruminal fiber degradability, it is likely safe to assume that a big part, if not all, of the decrease in total tract fiber digestibility occurred in the rumen. Interestingly, however, this generally did not affect milk production or milk fat content. We were not able to detect a consistent response in microbial protein synthesis in the rumen with the low CP diets. Overall, our data indicate that diets with RDP of around 9% (of dietary DM) decrease fiber digestibility (compared with diets with RDP of around 10% or higher) but do not appear to have a consistent effect on ruminal microbial protein synthesis.
Supplementation with rumen-protected amino acids that are known to limit milk production and milk protein synthesis may compensate, in some situations, for MP deficiency in dairy cow diets. Usually, methionine is the first amino acid limiting milk production in typical North American dairy diets. Methionine intake should be closely monitored, particularly in low protein diets and when milk protein content is of interest. The responses to rumen-protected methionine have been far from consistent, in big part due to excessive total protein in the diet. When cows are fed diets with MP close to or below requirements, the resulting low methionine intake will likely decrease milk protein yield. There are also some data showing positive effects of methionine hydroxy analogs (a significant portion of which is degraded in the rumen) on microbial protein synthesis. Responses to lysine have been even less consistent than the responses to supplementing with methionine. Although corn-based diets should, in theory, benefit from postruminal supplementation of lysine, this does not appear to be the case in most studies.
Amino acid supplementation research at Penn State has pointed to histidine as a limiting amino acid in dairy cows fed MP-deficient diets based on corn silage and alfalfa haylage. In such diets, supplementing with histidine may increase feed intake, which in turn, will increase milk production. Our data indicate that supplemental histidine may trigger a production response when dietary protein is low and microbial protein becomes an increasingly important source of amino acids for the cow. Requirements for histidine have not been established, but our analyses indicated that the proportion of histidine in MP should be similar to that of methionine, which, for practical purposes, is around 2.2 to 2.4% of MP
A word of caution
There is significant variability in the day-to-day ration composition on commercial dairy farms. If lowered protein feeding is being implemented, extra caution must be taken to stay as close to the formulated diet composition as possible. Changes in forage DM, quality/digestibility, or protein content, inclusion in the diet of unanalyzed by-product feeds, or deficiencies of key nutrients, particularly energy, can all have a large impact on the amount of protein that is being consumed by the cows or her protein requirements and may consequently have a negative effect on milk yield or composition. For example, diets with CP concentration of 16% or less are not uncommon on commercial dairy farms. In on-farm projects conducted by Penn State, total mixed rations from commercial farms often had (from direct analyses) below 16%, and in some cases, below 15% and even 14% CP. It is apparent that when feed prices are high, diets may reach critically low levels of CP that may limit production in some herds. Dietary protein should not be reduced in diets that do not meet the requirements of the animal for other nutrients, particularly energy. The first and most important factor for successful reduction of dietary protein close to or below the animal’s MP requirements is to keep dietary energy balance at or slightly in excess of requirements. Amino acids are inevitably used for glucose synthesis by the cow, but their role as a source of energy to sustain production becomes more important if dietary energy is deficient. Early lactation, when the cow is in a negative energy balance, is another example when dietary protein should not be reduced, and in fact, some data suggest it should be increased.
|Item||Diet formulated at 16% CP||Diet formulated at 17% CP|
|Crude protein, %||16.0||17.0|
|Rumen-degraded protein, % CP||10.0||11.0|
|Rumen-undegraded protein, % CP||6.0||6.0|
|Rumen-degraded protein supply, g/day||2,490||2,780|
|Rumen-degraded protein balance, g/day||10||296|
|Rumen-undegraded protein supply, g/day||1,500||1,540|
|Rumen-undegraded protein balance, g/day||37||30|
|Metabolizable protein supply, g/day||2,700||2,700|
|Metabolizable protein balance, g/day||30||25|
|NEL balance, Mal/day||1.3||1.6|
Alexander N. Hristov
Department of Animal Science
Pennsylvania State University