James F. Karn

Phosphorus nutrition of grazing cattle: a review☆

Phosphorus (P) is a widely studied element which is integral to many vital body functions. In ruminant nutrition, however, the degree of naturally occurring P deficiency in grazing cattle, the lack of uniformity in response to P supplementation, and even suggested P requirements have generated a great deal of confusion in the United States and around the world. Much of the confusion arises because animals have the ability to draw on skeletal P reserves when dietary P levels are inadequate. The mechanisms that control skeletal P withdrawal, the conditions that trigger withdrawal, and the rate and extent of withdrawal without affecting animal performance seem to be poorly understood. Another area of confusion involves the detrimental effect of P deficiency on feed intake which makes it unclear in many situations whether animal responses to P supplementation are due to P per se or simply to increased feed intake. This review attempts to discuss these and other important problems concerning the P nutrition of grazing cattle.

Cattle grazing behavior with season-long free-choice access to four forage types

This experiment investigated how season-long, free-choice grazing affected weekly cattle grazing behavior and resource use. Our objectives were to determine if known forage preferences change through the season, if feedbacks from previous grazing intensity affect current use, and if resources such as water and salt in combination with a shorter forage base increase grazing time compared to previously rested pasture. Two replicate 24.4-ha grazing enclosures contained four forage types: smooth bromegrass (Bromus inermis), crested wheatgrass (Agropyron desertorum), western wheatgrass (Pascopyrum smithii) and native range. Each forage type was divided into three sub-areas containing either: a water source, a salt block, or forage deferred from use before the experiment (rested). Resource use was tracked with analysis of aerial photos of the pastures before and after the 3-year study and with weekly direct observation of animal use of each area of the pastures. As each season progressed, use of smooth bromegrass declined and use of crested wheatgrass and western wheatgrass increased. Use of native range remained relatively constant and low throughout the study. The previously rested sub-areas of the replicate pastures had significantly and substantially more grazing than those with salt or water. Significant auto-correlation in the data was well described by an auto-regressive parameter comprised of the sum of the previous 2 weeks grazing time, which showed gradual shifts in forage preference from 1 week to the next. Aerial photo analysis supported the findings of the behavioral data; preference between forage types changed over time, and grazing use within pastures was uneven and negatively correlated with salt and water sites. Cattle can rotate themselves among various pasture types if given free-choice; and season-long grazing may be an effective system if a variety of forage types are available.