Matching Forage Quality With Production
by Monty S. Kerley, Ph.D., Department of Animal Sciences, University of Missouri-Columbia
The prevailing view by most of us that own cattle is that the animal is the centerpiece of the business enterprise. Thus, the reason the title of this presentation suggests that forage quality should be matched to production needs of the cattle. In reality, the centerpiece of a grazing livestock enterprise is the land and the forage that it produces. Conditional upon economic analysis, the safe assumption is that land committed to forage production is best suited for that purpose (rather than cropping or housing development) and that the land resource will be managed to maximize high quality forage production. Other papers and speakers at this conference, and well above the scope of this paper and my ability, will address maximizing high quality forage production. While the centerpiece of the business enterprise is high quality forage production from the land resource, it is impossible not to consider the harvesting and manufacturing component, namely the livestock, as an equally important component. Maximizing productive yield from a grazing system necessitates that the nutrient requirements of the animal be matched to the nutrients supplied by the forage.
Successful production from a forage-livestock enterprise depends upon knowledge of digestible nutrient production by the land resource and digestible nutrient requirements of the grazing animal. This presentation will limit its focus on the beef cowherd. The beef herd has a myriad of digestible nutrient requirements that change dependent upon age of the animal and stage of production. Typically, beef producers manage calves, developing heifers, pregnant heifers, and two-year-old and mature cows. Pregnant animals have greatly different nutrient requirements dependent upon their stage of pregnancy with nutritional requirements increasing as term approaches. Lactating animals also have variable nutrient requirements dependent upon stage of lactation and milk production ability with nutritional requirements decreasing as lactation ensues. The science is knowing how these nutrient requirements change within the beef herd throughout the year. The art is coordinating these nutrient requirements with the production of digestible nutrients by the forage.
Forage quality is typically determined from the measurements of crude protein, neutral detergent fiber (NDP), and acid detergent fiber (ADF). Crude protein and forage quality generally move in simultaneous directions; as crude protein level decreases, forage quality declines. In most cool-season forages, our first nutritional concern is energy, not protein. The reasons for this is that the protein content of cool-season grasses more closely matches the animal’s requirement than does its energy value.
NDF and ADF are both measures of the fiber in forage. Fiber is composed primarily of cellulose, hemicellulose, and lignin. NDF measures all three components while ADF measures only cellulose and lignin. The significance of these two measurements is that NDF is correlated to forage intake and ADF is correlated to forage digestibility.
Table 1 Relative Feed Value (RFV) Calculated from Different NDF and ADF Values
There is general agreement that cattle will consume 1.2% of their body weight as NDF. Consequently forage intake (% of body weight) by cattle can be predicted if NDF is known by the equation 120/NDF. This equation is not without errors, but it is the best relationship that presently exists. ADF, because it is correlated to digestibility, can be used to predict NEm by the equation 0.9996 – 0.0112 (ADF) for grasses and 1.037 – 0.0124 (ADF) for legumes. Therefore, NDF and ADF values can be used to predict the amount of energy that cattle can consume from a forage.
A Relative Feed Value (RFV) equation was devised to allow a more rapid comparison of differing forage qualities. This is most often used when evaluating hays. RFV is calculated by combining expected intake and energy values of the forage. Dry matter intake (DMI) is calculated as 120/NDF, for reasons discussed above. Digestible dry matter (DDM) is calculated by the equation 88.9 – (ADF x .779). RFV is calculated by the equation (DMI X DDM)/1.29. Table 1 lists various NDF and ADF levels and the resultant RFV. The higher the RFV, the higher the forage quality. As an example, full bloom alfalfa would have an approximate RFV of 100.
Nutritional quality of a forage is primarily assessed by its NDF and ADF content.
Calves - 60 days of age until weaning
The nursing calf develops a functional rumen when it is approximately 60 days of age. From this time point, milk production becomes less important as a source of nutrients and the forage becomes more important. At four to five months of age, the dam may provide a fourth or less of the nutrients the calf needs for growth and development. Consequently, growth rate and, ultimately, weaning weight is dependent upon forage quality and availability.
A weaning weight of 500 lbs at seven months of age requires an approximate daily gain of 2 lbs. Forages required to support this rate of gain need to have an NDF and ADF level of 43% and 35%, respectively, or lower. The relative feed value of such a forage would be 134 or higher. This type of forage quality could exist when pasture is in a highly vegetative state. In a spring calving herd, managed on a cool-season grass-based pasture, the calves would be expected to have forage quality that could support targeted gain through June. Nutritional quality in July and August, however, would most likely not be sufficient to support adequate weight gains (Figure 1). In actuality, little gain may occur during these two months. Interseeding legumes, grazing warm-season grasses, or planting warm-season annuals can result in more than 100 lbs heavier weaning weights. To maintain a consistent weight gain in nursing calves, high quality forage needs to be made available. In the example being used in this paper, targets were set at 43% or lower for NDF and 35% or lower for ADF.
Equally dependent upon high quality forage for growth is the developing heifer. If weight gains of weaned heifers being developed for reproduction are to meet the targets of 65% mature weight at breeding, similar quality forage as that needed by the nursing calf would have to be maintained.
Pregnancy elicits additional nutritional demands upon the female. The pregnant, developing heifer has the combined demands of weight gain and fetal growth. The two-year-old has the additional demand of lactation at the initial stages of her second pregnancy. The greatest nutritional demands of pregnancy occur during the last two to three months, when the majority of fetal growth occurs.
A forage that will maintain acceptable weight gain by the pregnant heifer can have a maximum NDF level of 58% and a maximum ADF level of 22% (Figure 2). The most important point to note is that the last 60 days of pregnancy requires that energy density of the forage increase by approximately 1.5-fold. If forage quality does not match requirements of the animal, growth will be slowed, calf vigor could be critically impaired, and timely breed back would most likely be a phenomenon of chance. As shown in Figure 3,not increasing forage quality to meet the heifers nutritional requirement can substantially limit her growth. In this example, brome hay cut in late bloom with an NDF level of 70% and an ADF level of 41% is fed to heifers through the winter. Heifers fed this hay and calving in March would have to gain approximately 80 lbs (1.2 lbs ADG) prior to breeding season to achieve optimum body condition for breeding. The potential to gain this weight is made more difficult due to the demand of lactation during this time. Interestingly, the process of conception can be regulated by body condition. However, once conception has occurred the female will commit her tissue resources to maintaining the pregnancy. Consequently, the more mismatched forage quality is to nutritional requirements of pregnancy, the poorer condition the animal will be in at calving.
The nutritional requirement imposed during lactation will be effected by two factors, amount of peak milk production and the stage of the lactation cycle. Typically, calves nursing cows will have a fully functional rumen by 60 days of age. The production of milk peaks within the first weeks after birth and then declines. At six to seven months of age, the dam is contributing 20% or less to the calf’s total intake. The greatest demand for nutrients to support lactation occurs within the first three months after calving.
To support a cow producing 20 lbs of peak milk a forage diet would need to have an NDF % no greater than 50 and an ADF% no greater than 34 (Figure 4). If the cow was capable of 30 lbs milk, the maximum NDF and ADF values would be 46 and 30, respectively. If the forage available at peak lactation were 70% NDF and 41% NDF, the cows would lose body weight to maintain milk production (Figure 5). This amount of weight loss could result in losing approximately one body condition score prior to breeding. If the cow were a condition score 5.5 at calving, she would be in sub optimal body condition to ensure successful conception.
Matching Production and the Forage System
Figure 6 and 7 shows a diagrammatic representation of the maximum level of NDF and ADF that the forage diet can contain if the nutritional requirements of the cow herd are going to be met in a spring calving system. The forage quality required by the growing calf and developing heifer remain constant. The challenge is to provide the calf with high quality forage for the approximate 5 months prior to weaning. The challenge for the developing heifer is to provide high quality forage for the approximate 7 months from weaning until breeding age. The difficulty of this is apparent and why many producers use strategic supplementation to maintain adequate heifer growth. Assuming a mid-March calving date, the reproducing animals have their greatest nutritional demand from January to June. This time period covers the requirements for fetal growth and peak milk production. The period of lowest nutritional requirement occurs from September through December. This provides a window of opportunity for the cow to enhance her body condition, since cool-season grass pastures typically are much higher in nutritional quality than are the cows requirements for maintenance during this time period. If a cow is in a body condition score of 6.5 to 7 at calving, the forge quality requirements can be substantially relaxed because she can use her reserve body tissues to meet the demands of lactation and reproductive tract repair. Figure 8 shows diagrammatically the maximum forage NDF and ADF values that can occur for a fall calving system.
Figure 9 shows an approximation of NDF and ADF levels of smooth brome throughout the growing season. In April and May, when cool-season forages are vegetative, their NDF and ADF levels are sufficient to support most classes of livestock. In July and August, the quality is inferior to support most classes of livestock considered in this paper. This would be particularly true for the categories that encompass growing animals. The question becomes how can a forage system be devised that parallels nutritional requirements of the animals. One strategy would be to complement the quality of the existing cool-season grass, to offer an alternative forage of acceptable quality during summer, to ensure acceptable quality hay is made available, and to provide grazing when cool-season grasses are not existent. The outward appearance is that this would be a logistical nightmare. In actuality, it can be achieved by interseeding legumes and using two to three additional forage varieties. Figure 10 represents the NDF and ADF levels that can be achieved by using annual ryegrass and rye for early-spring grazing, a cool-season grass and legume mix for spring grazing, sorghum sudan or alfalfa for summer grazing, and a return to the cool-season grass and legume mix for fall and early winter grazing. The winter grazing assumes that the cool-season grass and legume mix was stockpiled.
It is possible and probable to devise a managed forage system that matches forage quality with the nutritional requirements of the beef herd throughout the year. With the exception of mineral supplementation, little if any supplemental feeding would be required. In addition to benefits bestowed upon the animal, the land resource and environment can also be enhanced by a managed forage system.