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From a dairy management perspective, such models (e.g., SHIELD; Robinson, 2000) offer a site specific way to quantify the N balance and N excretion on a dairy operation, while providing information on (animal) nutrient balances in each cattle sector of the facility, since the evaluation is based upon complete input/output balances of all nutrients to/from each defined cattle sector of the dairy facility. An example of such site-specific calculations is provided in Appendix A.
The farm sector mass balance approach will likely be the most useful way to provide information to a dairy operator on the nutrient efficiency of the various cattle sectors of the dairy, thereby identifying the sectors with the poorest efficiency. These can be targeted for improvement by the dairy.
2.3 UC Committee of Consultants Estimate of Nitrogen Excretion by Dairy Cows The UC Dairy Animal Waste Model was developed from SHIELD to estimate whole dairy manure nutrients as the sum of those of its cattle groups. Additional nutrients (e.g., P, K, Ca) and trace elements (e.g., Zn, Cu, and Se) in the manure are also estimated. To create estimates, rations used were the measured dietary intakes of high and low production strings of lactating dairy cows in four representative commercial dairies in four regions of California, being Petaluma, Modesto, Tulare and Chino. The measured feed compositions, cow characteristics and milk productions were incorporated into the evaluation.
Based upon on-farm measurements and evaluation using the UC Dairy Animal Waste Model, these lactating cows excreted 0.92 to 1.25 lb N/head/day (417 to 565 g N/head/day), depending on the rations of the cows on the specific dairies (Table 2-2). The average N excretion was 1.02 +/- 0.13 lb/head/day (462 +/- 60 g N/head/day).
These estimates are considerably higher than the 1973 Water Quality Task Force of the UC Committee of Consultants estimate of 0.61 to 0.85 lb N/head/day (average 0.69 +/- 0.18 lb N/head/day), or 277 to 386 g N/head/day (average 315 +/- 81 g N/head/day). The increases primarily reflect increases in milk production, and the resulting increase in feed intake, between 1973 and 2004.
In apparent contrast to these estimates, a white paper prepared for the National Center for Manure and Animal Waste Management (Arogo et al. 2001) estimated that the daily average N excretion of dairy cows in the US varies from 0.58 to 0.82 lb N/head/day for lactating cows weighing 992 to 1,400 lb (263 to 372 g N/head/day for lactating cows weighing 450 to 635 kg).
However milk production levels of dairy farms in California are higher than the national average, and so N excretion would also be expected to be higher than the national figures of Arogo et al.
Table 2-2: N Balance of Lactating Dairy Cows in California.
Most herds have a consistent number of lactating and dry cows, over time, unless seasonal calving is practiced (which is very rare in California). Dry cows are fed a ration that is has lower levels of most nutrients (e.g., N, metabolizable energy, fat, and most minerals) than that of lactating cows, and they eat much less of it. Arogo et al. (2001) estimated that a dry cow excreted 0.36 to 0.50 lb/head/day (163 to 227 g N/cow/day). There is no evidence that dry cows differ from the national average, and so the estimate of Arogo et al. (2001) is accepted.
Uncertainties about exact N excretion levels in dry cows have only a minor impact on dairy herd excretion estimates. Because dairy cows, on average, lactate for 305 days and are dry for 60 days per year, dry cows are a relatively small contributor to the total adult dairy herd N output on a commercial dairy. Hence, a 20% error in N excretion estimates from dry cows represents only a 1.5% error in N excretion estimates from all dry and lactating cows.
2.4 California Regional Differences that Effect Estimates of Nitrogen Excretion by Dairy Cows N excretion by dairy cows in the various dairy production regions of California will vary due to regional differences in the milk production level of the cows, but there is no evidence that these differences are, or likely will ever be, of a sufficient magnitude to require additional design criteria for dairy farms, as long as expected manure and nutrient excretion are based on actual herd data (i.e., using methods 2 or 3 above to estimate manure nutrient excretion).
2.5 Role of Diet in Nitrogen Excretion by Dairy Cows Traditionally dairy rations have been formulated based upon nutrients, rather than ingredients, to be both the lowest cost per unit of diet fed and to maximize milk production. The National Research Council Dairy Sub-Committee (NRC, 2001) provided recommendations on the nutritional requirements for dairy cows, and these are the nutritional basis used by virtually all consulting dairy nutritionists for formulating rations. While the utilization of dietary N by dairy cattle can be optimized by feeding ruminally degradable and undegradable N sources to NRC (2001) standards, there is no evidence that diet formulation principles, which are based upon nutrients, differ among regions of California that have cost effective access to different feedstuffs, and different ‘region typical’ diets.
The majority of California dairy herds (particularly the larger herds) utilize dairy cow diets that have been competently formulated to meet milk production goals. Typically, the whole farm N conversion efficiency from intake to milk is between 25 and 32% in California (Tables 2-1, 2-2).
A recent survey in Merced County indicates that the range of whole farm N conversion efficiency in that county is between 20% and 34% (Alejandro Castillo, personal communication].
The feeding and management strategies of dairy cattle will impact nutrient levels in manure. For example, feeding cows close to the nutrient needs for their actual level of production will result in maximum N efficiency, and the least amount of N excreted per unit of milk produced.
However in poorly managed dairies, cows may be fed poorly formulated diets and may not meet their production goals. In such cases, the N conversion efficiency from feed intake to milk produced will be reduced and N excretion per unit of milk production will increase proportionally (Table 2-2). At the same time, cows will eat less of the poorly formulated diet, thereby producing less urine and feces.
There are other strategies to improve feed utilization efficiencies on even the better-managed dairies. For example, administration of bovine growth hormone, extending photoperiod with artificial lights and milking three times daily have been shown to reduce nutrients in manure by 8, 5 and 6%, respectively (Kohn, 1999). N excretion of dairy cows can be minimized by appropriate management of the herd and, in this way, manure and the N excretion can be minimized to the limits of current knowledge.
2.6 Summary The recommended average excretion of N by lactating dairy cows in California is 462 g N/head/day, and by dry cows it is 195 g/head/day. However the Committee stresses that these are average numbers, that the trend will be to higher values in the future as milk production per cow increases, that actual values will vary sharply among dairies, and that there are techniques available to make very accurate site specific estimates.
Short of measuring feed intake N, which allows for highly accurate estimates of manure N excretion using, e.g., the UC Dairy Animal Waste Model, the Committee believes that expressing manure N excretion relative to milk production, rather than body weight or ‘animal units’, is the biologically most sensible assessor on within site efficiency of animal N use, as manure nutrient production is primarily a function of feed intake and feed intake is primarily a function of milk yield. There is no evidence that regional differences in California are of a sufficient magnitude to require additional region specific design criteria for dairy farms.
Dairies feeding better formulated diets will tend to have higher efficiencies (i.e., the lowest N excretion relative to milk production) while poorly managed dairies, where cows may be fed poorly formulated diets, will tend to have lower efficiencies (i.e., the highest N excretion relative to milk production). In contrast, the better managed dairies with higher per cow milk production will tend to have the highest manure N excretion per cow, thereby demonstrating the error in expressing manure N excretion on a per cow basis, rather than on the basis of milk produced.
There is no evidence that diet formulation principles that are based upon nutrients differ among regions of California, even if the cost effective access to different feedstuffs varies among regions. Hence, different ‘region typical’ diets need not be considered.
Chapter 3 - Distribution of Manure on Dairies
3.1 Introduction The design elements and physical layout of a commercial dairy will determine where manure is deposited and how it will be collected. Three basic animal housing/manure collection categories are common in California: freestalls with flush systems, corrals with flush alleys, and corrals with no flush alleys2. A small number of facilities collect manure from feed lanes via regular scraping or vacuuming. The feed apron is generally concrete, extends the length of the feed bunk, and ranges from 9 to 14’ in width. Older facilities may have feed aprons as wide as 20’.
Flushing frequency for corrals with flushed feed lanes or freestalls is variable and can occur multiple times per day or once weekly. Scraped or vacuumed systems are operated to relocate solid manure to storage or treatment areas or to open corrals. This can be done multiple times daily.
This chapter reviews our current knowledge about the distribution of manure within the various sections of a dairy (section 3.2), the partitioning of manure and nutrients between the solid and liquid phases of manure (section 3.3), and the partitioning into surface runoff (section 3.4).
3.2 Partitioning of manure on surfaces within the dairy Dairy animals excrete feces and urine during their daily activities. It is generally assumed that manure is excreted based on residence time on each surface of the dairy. No published studies exist to describe manure production in each area on a dairy. Dr. Tom Shultz provided input into observations regarding manure accumulation at dairies (Shultz, 2000; personal communication).
The overriding assumption was that animals excrete manure proportionally to the time they spend in various locations and that the excretion is uniform throughout the day for lactating cows. This assumption has been used and continues to be used by dairy design consultants who specialize in facility design, including manure storage systems (Harner, 2005; personal communication).
3.2.1 Feeding Areas
According to studies conducted in Tulare the average daily hours that cows spend on feed aprons varies by season, comfort modifications, ration form as well as the method and frequency of feeding (Shultz, 1989). Operators deliver feed from once to ten times daily and push up feed throughout the day. Some producers have environmental modifications at the feed apron, such as misters and fans, to reduce heat stress during hot weather. In general, cows remain on the feeding apron longer when these modifications are present. Total time for lactating cows on the feed apron ranges from 3 to 6 hours/day. The form of the feed can also affect time spent on the Freestalls are individual cow bedding areas where partitions orient the cow for comfort and sanitation (Stull et al., 1998). Manure deposited on the solid surface alley is collected daily (MWPS, 1985) (usually more than twice daily). Corrals are loose housing. Animal resting is not guided to any specific location. Feed bunk and water trough areas have scarified concrete areas. The concrete feed apron area can be cleaned via flush or scrape method.
feed apron. This may increase the estimated feeding time by as much as 0.5 to 1 hour/day for facilities that do not feed a total mixed ration.
Dry cows and replacement animals are housed in corrals and spend up to two to three hours a day on feed aprons. This amount of time can increase when the feed aprons are managed more intensively and/or inclement weather exists.
3.2.2 Milk Parlor Animal housing and milking facilities are designed to minimize the time that cows spend away from corrals, generally 1 to 1.5 hours/milking. Manure deposited in sprinkler pens and milk parlors is removed by power spraying or flushing to a retention pond. Cows milked twice daily spend 2 to 3 hours in sprinkler pens and milking parlor. Cows milked 3 or more times daily may spend more time in this area. Facility design, traffic patterns, string size, and parlor throughput, define the time cows are away from their housing area (Smith et al. 2003).
3.2.3 Freestall Design
Design and maintenance of the freestalls affects the amount of time that cows spend in them. The micro-climate of the freestall will effect cow comfort and residence time in freestalls compared with adjacent corrals. Management of corral access varies, with some operators confining cows to freestalls when corrals are muddy or overly hot. Other producers impose no restrictions.
Properly designed and well managed freestalls will be occupied by the majority of cows, which spend 24 hours daily on concrete surfaces. Fans, misters and rubber matting in the feeding lanes are modifications that increase the time that cows spend in freestalls and on feed aprons. Cows may spend 3 or 6 hours per day feeding included in the 8 to 16 hours per day in the freestalls, at facilities that do not have environmental modifications, with the remaining time spent in the corral area.
3.3 Partitioning of manure nutrients into solid and liquid forms Based on the observation of Shultz, the time that cows spend in various parts of the dairy can be generalized and material collected as a liquid estimated (Table 3-1). The remainder of the material would be collected in a solid form.
3.3.1 Liquid Forms