Measuring pellet quality during the feed manufacturing process is the key to producing a consistent quality pellet
By Charles Stark, Wilmer Pacheco, Adam Fahrenholz
Customers often evaluate the quality of feed based on its physical appearance. The quality of mash feed is often evaluated based on its flowability through bins or the feed system. Textured feed is evaluated on the percentage of molasses, pellets, and cracked corn present in the feed. With that said, pelleted feed is probably the most scrutinized feed based on the percentage of fines in the feeder at the farm or the fines in the bottom of the bag. Customers can easily judge the physical quality of pelleted feed based on the fines observed at the farm, which can lead to customer complaints on feed quality. The nutrient content of feed may be correct but if the physical appearance is not acceptable, customer might perceptive that feed has poor quality. Since pellet quality is very important to the customer and can affect animal performance, it’s important to have tools in place to predict pellet quality prior to packaging and delivery.
The first approved method for predicting pellet quality was developed by Harry Pfost at Kansas State University and published in the Agricultural Engineers Yearbook in 1969 (ASAE Standard S269.1), this method has often been referred to as the K-State tumble box method. The method has gone through a couple reviews, but has basically remained the same, the current method is ASAE S269.4. The official name of the method is “Cubes, Pellets, and Crumbles – Definition and Methods for Determining Specific Weight, Durability, and Moisture Content”, which has been shorten by the feed industry to Pellet Durability Index (PDI). The purpose of the method was to establish a universal method for testing the physical qualities of crumbles and pellets that would allow for comparison of data in research and operations using the standard. While the methodology has remained the same, its application in the feed industry has changed over time, integrated poultry and swine feed mills are now using it to predict the percentage of fines in the feeder on the farm. Pellet quality as defined by the PDI is combined with the percent of fines created at the feed mill prior to loading to predict the percentage of fines in the feeder. A comprehensive pellet quality model should be created for each individual production system based on the company’s standards for both PDI at the feed mill and fines at the feeder. Feed mills can then use the model to estimate PDI and percent fines at the feeder and make adjustments in the pelleting process to meet or exceed pellet quality targets at the feeder trough. Additionally, the model can provide feedback to the nutritionist and purchasing agent as to the positive or negative affect of an ingredient or formulation change had on PDI and percent fines.
Pellet quality can be determined with the standard PDI method (ASAE 5269.4) or using the Holmen Durability Tester (NHP 100). The Holmen method takes less time to perform and requires less sample (100 vs. 500 grams) compared to the ASAE method, but the initial cost of the equipment is higher. Both methods require the pellets to be sieved prior to testing. The ASAE method has sieve recommendations, which are used for both procedures based on the common pellets diameters (5/32”/4.0 mm - #6 sieve, 3/16”/4.8 mm #5 sieve), but there is no recommendation for the 11/64”/4.4 mm, which is commonly used for swine and broiler feeds, thus most companies have selected the #6 sieve to remove fines prior to testing for pellet quality. The ASAE method may need to be modified through the use of additional hex nuts that range from 5/8” to ¾” to create a model that is representative of a company’s manufacturing and delivery process (Pacheco, 2009). The Holmen test has different time settings (30, 60, 90, 120 sec), which can be adjusted by the feed mill based on the prediction of fines at the feeder level. For instance is the PDI at the feed mill is 90% with 30 seconds, but the percentage of whole pellets at the feeder is 65%, increasing the testing time from 30 to 60 seconds could help to predict better pellet quality at the feeder. In addition to the time setting, the most common difference in in the Holmen method is whether to sieve the pellets coming out of the basket after the test, typically sieving this way will lower the results by 5-10%, depending on the formulation. Kort et al. (2019) also reported that warmup time and filter type would affect the results of the Holmen test. Companies who have switched from the ASAE method to the Holmen method have tried to correlate the results, but have found it difficult due to the differences in the methods. While not impossible, it’s probably best to just select a method and move forward.
Establishing a specification for percent fines at the feeder will allow the nutritionist, purchasing agent, and feed mill managers to formulate, purchase, and manufacture feed to the specification while exploring options to lower feed cost. The consistency of pelleted feed may be more important than the actual amount of fines due to feeder management. Inconsistency in the percentage of pellet fines between feed deliveries requires more feeder management to minimize feed wastage. Measuring pellet quality during the feed manufacturing process is the key to producing a consistent quality pellet. Factors such as conditioning time and temperature, production rate, and varying the amount of water and/or fat in the mixer can be performed during the pelleting process to maintain a consistent quality when pellet quality is monitored during the pelleting process. Regardless of what method is used at the feed mill for measuring pellet quality, the operators must be committed to monitoring the process as close to real time as possible.
References ASAE Standards S269.1/4 Kort et al. 2019. Pacheco. 2009.
