Herd cell count won’t budge de­spite culling cows

Dairy News Australia - - ANIMAL HEALTH - ROD DYSON * Leon’s name has been changed for this ar­ti­cle. • Rod Dyson is a vet­eri­nary sur­geon and mas­ti­tis ad­viser at www.dairy­fo­cus.com.au

“I CULLED the 10 high­est cell count cows and the cell count didn’t change — not at all!” Leon* was frus­trated — very frus­trated! He milks about 300 cows in North­ern Vic­to­ria with a spring/au­tumn split calv­ing sys­tem, and I could hear the frus­tra­tion in his voice. Leon sup­plies a pro­ces­sor where the pre­mium pay­ment thresh­old for Bulk Milk Cell Count (BMCC) is 250 000 cells/ml. The farm has been con­stantly in and out of the pre­mium band for a cou­ple of years now, and noth­ing he has done has solved the prob­lem. In the last month, the aver­age BMCC was just over 250 000 and a chart of Leon’s BMCC for that last month is pretty typ­i­cal of what has been hap­pen­ing through­out the last cou­ple of years. He just can’t get the BMCC be­low 250 000 and then keep it there. As Leon said, “I can’t just keep culling cows if it isn’t go­ing to make a dif­fer­ence.” This is a story we reg­u­larly hear — farms ei­ther culling or with­hold­ing high cell count cows from the vat, and the BMCC doesn’t seem to change. There are a num­ber of rea­sons why this might oc­cur, but three fac­tors are likely to be very sig­nif­i­cant. Firstly, a cow’s In­di­vid­ual Cow Cell Count (ICCC) is not a static num­ber — a cow’s ICCC can change from day to day, and even from hour to hour. A cow that has herd tested yes­ter­day may have a sim­i­lar ICCC to­day, or could have a quite dif­fer­ent ICCC to­day sim­ply as a re­sult of both nat­u­ral bi­o­log­i­cal vari­a­tion and the cow’s im­mune sys­tem re­spond­ing to the vary­ing lev­els of chal­lenge in the ud­der. If enough cows change their ICCC, then the over­all BMCC will change as a re­sult. Se­condly, Leon’s worst 10 cows in terms of ICCC were prob­a­bly not the top 10 cows in terms of con­tri­bu­tion to the over­all BMCC. A cow’s con­tri­bu­tion to the BMCC is a func­tion of both her cur­rent ICCC and also the vol­ume of milk she is con­tribut­ing to the vat. Anal­y­sis of Leon’s last herd test showed that about 15 per cent of his herd had ex­ceeded the level of con­tri­bu­tion which we con­sider to be sig­nif­i­cant, and com­bined with the pat­tern of con­tri­bu­tion, it showed that in Leon’s herd, there is a larger num­ber of cows with a mod­er­ately high ICCC rather than a small num­ber of cows with a very high ICCC cre­at­ing the prob­lem. In­ter­est­ingly, one of Leon’s cows with an ICCC over 1 000 000 cells/ml did not even make the list of sig­nif­i­cant con­trib­u­tors, sim­ply be­cause her pro­duc­tion was quite a bit lower than other cows. Hence, culling or with­hold­ing this cow (and other cows like her) is un­likely to make a sig­nif­i­cant dif­fer­ence to the BMCC. The third likely fac­tor is that mas­ti­tis in­fec­tions in a herd are a dy­namic and fluid sit­u­a­tion, con­stantly chang­ing. Over any pe­riod of time, pre­vi­ously un­in­fected cows are be­com­ing in­fected, and some in­fected cows are be­ing cured, ei­ther by treat­ment or by self-cure. It is the rate at which th­ese changes are oc­cur­ring which can cause sig­nif­i­cant changes in which cows are con­tribut­ing most to the BMCC at any given time, and if this rate of dy­namic change is high, then culling or with­hold­ing cows from the vat may seem to have lit­tle or no ef­fect, sim­ply be­cause other cows have be­come in­fected over that time and have be­come sig­nif­i­cant con­trib­u­tors to the BMCC. So where does this leave Leon? Fur­ther anal­y­sis is needed to see whether at pre­vi­ous herd tests the same cows con­stantly ap­pear in the list of sig­nif­i­cant con­trib­u­tors, mean­ing that Leon has a small group of cows which are the key to his BMCC, or whether there is a con­stant change in the sig­nif­i­cant con­trib­u­tors, mean­ing there is likely to be a rel­a­tively high rate of new in­fec­tions be­ing a ma­jor con­trib­u­tor to Leon’s BMCC. Prob­a­bly the most likely out­come is that both con­di­tions are oc­cur­ring. Ei­ther way, the key to in­ter­pret­ing and un­der­stand­ing this anal­y­sis will be milk cul­tures. Cul­ture re­sults will guide the fur­ther in­ves­ti­ga­tion of the prob­lem, the aim of which will be to iden­tify the source of in­fec­tions, the method of spread, and how to pre­vent spread. The cul­tures will also help us un­der­stand the like­li­hood of be­ing able to cure ex­ist­ing in­fec­tions. Leon’s next task will be to care­fully col­lect a set of about 25 milk sam­ples from se­lected cows im­me­di­ately after his next herd test. As hap­pens most of the time, a bit of back­ground anal­y­sis and a good set of milk cul­tures prior to launch­ing into a de­tailed in­ves­ti­ga­tion are likely to re­sult in a higher qual­ity in­ves­ti­ga­tion and a bet­ter out­come for Leon.

“I can’t just keep culling cows if it isn’t go­ing to make a dif­fer­ence.”

A cow’s con­tri­bu­tion to the BMCC is a func­tion of both her cur­rent In­di­vid­ual Cow Cell Count (ICCC) and also the vol­ume of milk she is con­tribut­ing to the vat.

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